service 4wd light on- no selector switch lights [Archive] - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

: service 4wd light on- no selector switch lights

12-04-2007, 10:37 PM
I have no 4wd. The "service 4wd "light is on the IP and the selector switch is not lit up. What to do?

12-04-2007, 11:17 PM
Pull the fuse under the hood and disconnect the battery for (not sure actually) lets say 30 min

12-04-2007, 11:24 PM
I had this happen to my 03tb a week after i drove it home, and i brought it back and they did a new download to the computer and it solved my problem. They told me there are constantly new downloads coming in. They did mine for free but im not sure what they usually charge.

12-05-2007, 02:50 PM
Thanks for the replies. I have been searching the archives for more info and it seems like I'm about to try the fuse pull solution before I give in to a new TCCM. After subscribing to the TSB service at, I see that the stealerships are advised to replace all TCCMs on the 02 Envoys if the fuse pull (to wake the TCCM module) doesn't work. (see TSB 02-04-21-006E, July 20, '06) When I first bought the 02 Envoy in 02, I started having intermittent 4WD issues. I pulled the fuse for the ATC by intuition and it worked the first time. After the later warranty work to recode the TCCM at the dealer, I drove home and it acted screwy again. I called the dealer and the service tech said" Just use it more and it will get better". I paid 30 grand for this kind of machine? :sadcry:It worked on and off for the next few years until (you guessed it) the warranty expired. Now I have a "Service 4WD" light and no switch lights, no TC action at all. Maybe they downloaded "Windows98" into the TCCM. That would explain the constant need for rebooting !:duh:

12-05-2007, 04:00 PM
i know the 4x4 switch is common to fail, maybe you would want to check that first.

12-05-2007, 09:13 PM
i had that same problem it was a short in the switch on the dash it was covered under warrenty at the time good luck.

12-05-2007, 11:21 PM
I had the exact same problem on my 02 Envoy-minus the service 4wd light being illuminated. They replaced the 4wd selector switch and "reflashed" the system. It fixed the problem, however when I am in A4wd my front axle seems to be always "engaged". meaning I am in 4wd all the time. If I am in A4wd at a stand still and I switch to 2wd- I can hear/feel the front wheels disengage. This can't be right or there wouldn't be a reason to have 4HI. I hopes this helps.

12-08-2007, 06:46 PM
Thanks for the replies. I have been searching the archives for more info and it seems like I'm about to try the fuse pull solution before I give in to a new TCCM. After subscribing to the TSB service at, I see that the stealerships are advised to replace all TCCMs on the 02 Envoys if the fuse pull (to wake the TCCM module) doesn't work. (see TSB 02-04-21-006E, July 20, '06) When I first bought the 02 Envoy in 02, I started having intermittent 4WD issues. I pulled the fuse for the ATC by intuition and it worked the first time. After the later warranty work to recode the TCCM at the dealer, I drove home and it acted screwy again. I called the dealer and the service tech said" Just use it more and it will get better". I paid 30 grand for this kind of machine? :sadcry:It worked on and off for the next few years until (you guessed it) the warranty expired. Now I have a "Service 4WD" light and no switch lights, no TC action at all. Maybe they downloaded "Windows98" into the TCCM. That would explain the constant need for rebooting !:duh:

I have same issue (02 Envoy:Service 4WD light on + switch indicator light not working + not able to switch mode while driving) and am considering to buy new dash switch. Currently trying the fuse-pull/battery disconnect. I noticed I can switch the mode from 2WD to 4-Hi (and vice versa) as follows:
1. turn switch while running vehicle
2. turn off ignition
3. turn on ignition
4. mode will switch to new selection (accompanied by clunky noise)

Also notice that while turning the ignition all indicator lights turn on for a brief period, so lights are functional.

Does anyone have any more feedback based on this discription? Should I proceed to install a new dash switch in case fuse pull does not work? Can TCCM reprogram only be done at dealership? (I am not very impressed with mine)


12-18-2007, 09:51 PM
Waiting for a reply on this also.

12-18-2007, 11:59 PM
Which fuse are you pulling?

12-22-2007, 12:01 PM
Well the next day mine started working as normal again ????

01-04-2008, 10:27 PM
HELP! So today I go to leave work (truck sat almost 9 hrs in 55 degree temp) and my battery is dead. I did accidently leave my sirius sat. radio on (which is powered by cig. outlet) but then again I've done that before with no problems. I get it jump started and once started the 'Service 4wd' light comes on and hasn't gone off yet. The 4wd switch below the HVAC controls doesn't have the indicator light above 2hi illuminated either. On start up, all the lights on the switch light up and then go out. Anyone else run into this problem or have any suggestions before I head to the dealership?!?!?

01-04-2008, 10:45 PM
my friend has an 02 ls and his is doing the same thing. we scanned it and came up with code 550 which i found out on here was the module. does this mean he can have the pc reflashed and fix it?

01-04-2008, 10:45 PM
Hey mikepc2fl, It is normal for all the lights to illuminate on the 4wd switch upon starting the vehicle. It is basically a bulb check. However the selected position should remain lit there after. Sometimes a reflash is needed at the dealership. It is a computer download for your system. I had it done when I experienced the same problem. I want to say the price was between 25-40 dollars. There is also the "common" need to replace the selector switch itself. Good luck and let us know how you make out.

01-21-2008, 02:44 AM
I seem to have the same problem, But I have reset it by doing the following.
Driving a couple blocks to ensure everything is warmed up, then turning off the ignition all the way off (open and close door to kill accessory power) turning the switch past 4lo and holding it there for 10seconds then restarting everything then seems to work fine. As add assurance I put in neutral and shift thru 2hi to 4lo and back.
Have done twice in yr and half. And everything seems to be fine.

PS i have 04 envoy just passed 70k

01-31-2008, 08:34 AM
Is this an 02 issue? And does it affect TB's or Envoys or both?

01-31-2008, 10:14 AM
Is this an 02 issue? And does it affect TB's or Envoys or both?

I just got back from the stealership, and my transfer case module is shot... it needs replacing and reprogramming. It's at least a 02 issue, since the newer models only need reprogramming. There's a service report out there for reprogramming the transfer case.

02-18-2008, 07:38 PM
My 4x4 drive doesn't work in or in low. it was working in 4whi. But now it seems to drag. Nothing lights up on the dash and lights on the switch work. Please help.:suicide:

11-21-2008, 03:05 PM
My wife and I have a 2004 Trailblazer with 50,000+ miles on it and are having the same issues. It's sporadic and has only happened a couple of times over the past few months. What i have read on these forums indicates that changing fuse No.8 can help but, it looks like reprogramming the TCCM is inevitable and replace it is probable. The stealerships are soaking people pretty good on these, as it's a common problem. It would be nice if it was just a recall but, come to think of it, GM has got bigger fish to fry right now.

That's what I have seen on this topic. I would recommend that if you don't find what you need here, keep poking around this forum. There are a lot of really savvy people on here that have seen or dealt with about everything.

Best of luck!

11-21-2008, 03:10 PM
bable. The stealerships are soaking people pretty good on these, as it's a common problem. It would be nice if it was just a recall but, come to think of it, GM has got bigger fish to fry right now.

Yep, stealership got me for about $400 to replace TCM 2 yrs ago. I don't think GM is concerned with any recalls right about now. :hopeless

11-24-2008, 12:18 PM
I will just add my two bits. Saturday, I washed the envoy. Like noramal, I washed out the wheel wells and sprayed around the axles. When I got home, I noticed that the service 4WD light was on. I took it to Autozone to read the code and when the tech came out and plugged in the reader, the light was off. He used to work at a Chevy dealer in town and mentioned that the envoys had a problem with the light coming on and consequently, no 4WD.

I have done some research on the topic and it looks like I will be in the market for a new TCCM or at least an update.

11-24-2008, 02:05 PM
That's good to know. I did the same thing right when this started. I always power wash the wheel wells. Maybe that was a mistake.

Thanks for the input. Best of luck!

11-24-2008, 02:53 PM

I noticed that you are new to the site. From a fellow Iowan, welcome!!

This site has great resources and has saved my bacon more that a few times. I always wash the wells and the underbody. Force of habit with all the salt the Iowa DOT dumps on the roads each winter.

11-24-2008, 03:51 PM
Indeed! How about that 55-0 Hawk win!? I do love Iowa but, last year beat the crap out of our vehicles. My wife's 98 GMC Jimmy, (which was a great vehicle; 180K miles), just had too much and we didn't want to face another Winter with it knowing the 4WD and suspension were going to need some major work. We bought the 04TB to dodge that bullet...just goes to show, you can't outrun your demons! Heh!

This is a great forum. I have learned a lot already and am blown away by the knowledge base that camps out here. I'm no schlepp around vehicles but, some of these contributors have obviously forgotten more about all things GM than I will ever know. Love it!

12-01-2008, 02:24 PM
I just got back from the stealership, and my transfer case module is shot... it needs replacing and reprogramming. It's at least a 02 issue, since the newer models only need reprogramming. There's a service report out there for reprogramming the transfer case.

I am dealing with this issue right now as well on our 2002 TB. There is an outfit on eBay selling brand new TCCMs for $189. His description of the TCM includes the statement "don't let the dealership tell you anything about programming or that it will have to be programmed - they are full of sh*t ...these new ones are not programmable nor do they need programming. That was the problem to begin with."

So what this guy is saying is that you can swap out the new TCCM for the old and the new essentially comes "pre-programmed"...anyone know if this is true?

Also, our TB has a four position dashboard selector switch for 2WD, 4WDLO, 4WDHI, and AWD. This guy on eBay describes this TCCM replacement as "replacing the original P/N 15748800, used for all NVG-236/246 transfer cases. It works with the 4 button dash switch. Should also replace P/N 15040103, 12577412, or 15154323."

Are there two types of dash switch options in a 2002 TB? Selector switch or 4 button?

With my luck this will be the wrong TCCM for my truck, but if anyone can help with P/N verification and whether the "new" TCCMs need to be programmed, and /or whether there are two dashboard selector switch options, it would be greatly appreciated!


04-19-2010, 08:14 PM
WOW! Thanks to all your posts, I just "fixed" my 4wd problem. I have an '04 TB. The "service 4wd" light just came on a few minutes ago. When I turned my selector switch from "2wd" to "AWD", the selector switch went out. I came home, checked this forum, went out to my TB, turned and held the selector switch past 4LO for 10 seconds, then started the TB. FIXED!!!

THANKS!!!:thx :woohoo: :rotfl:

05-01-2010, 09:31 PM
I just had the same thing with my '03 today. Everything was fine when I parked it last night. This morning when I started it the light was on, and nothing lit on the selector. I tried shutting it off and restarting it, but it was the same. I made a stop, and when I started it back up everything was good again. I'll have to try that trick with the switch if/when it happens again. It's great to have a place like this to go for information, tricks, and tips.:thumbsup:

05-03-2010, 11:49 PM
It's great to have a place like this

thanks alot:thx

10-21-2010, 09:02 AM
WOW! Thanks to all your posts, I just "fixed" my 4wd problem. I have an '04 TB. The "service 4wd" light just came on a few minutes ago. When I turned my selector switch from "2wd" to "AWD", the selector switch went out. I came home, checked this forum, went out to my TB, turned and held the selector switch past 4LO for 10 seconds, then started the TB. FIXED!!!

THANKS!!!:thx :woohoo: :rotfl:

I'll try this before replacing. What should the ignition be at? Off/acc/on? Thx.

the roadie
10-21-2010, 11:15 AM
I'll try this before replacing. What should the ignition be at? Off/acc/on? Thx.It's irrelevant. What you're doing is just cleaning the switch contacts by turning it a few times. EVERY owner of a 4X4 truck should exercise their 4WD system monthly to make sure the switch, controller, actuators, and transfer case work, when you don't need them to get through slippery conditions. That way you can get familiar with the various noises it makes, and can identify changes in the noises that can pinpoint a failure if it quits working later. When you need it.

10-24-2010, 05:19 PM
roadie. i c that ur prob the most up to date with this probelm. so i do have a question. all my light were not working on the 4wd selector module. so upon reading this post i tryed pulling the number eight fuse. after doing so the service 4x4 light came on and now wont shut off. Light was not on before this. any idea y.
and on a differnt note is do u know of any where on this site i can get info about my envoy stuttering when going up hill, like when cruise control is on.

the roadie
10-24-2010, 05:32 PM
berube26x: You need a new 2003+ TCCM, and to clean your throttle body.

10-24-2010, 05:34 PM
thanks for the fast reply. will clean the throttle body and where is the best place to by the tccm. cant seem to find any on ebay. do u know of any post with install pics>

also if i were to get a used one from a junk yard would it have to be programmed. Thats the most confusing part to me.

the roadie
10-24-2010, 05:51 PM
A junkyard unit would have been used in a vehicle, so it would not need reprogramming.

Well, strictly speaking, it *should* be touched by a dealer to confirm that the proper gear ratio is programmed into it. Practically speaking, nobody has seen ANY problems using a junkyard unit. The gear ratio issue is to optimize the behavior of the A4WD function, but as regular readers of trailvoy know, that's a silly option of almost no use in the real winter world out there. If it's slippery enough to let your tires slide, use 4HI. If it's just rainy, you won't get the tire slip necessary to activate A4WD. And when A4WD engages, it's usually with a lurch that can break tires loose worse than they were already slipping, and break some internal transfer case parts like a weak design 2002-2005 bearing mount.

10-26-2010, 05:41 PM
I'm planning on replacing my transfer case tonight after work, my light came on and I did a fluid change to begin with and the fluid came out along with metal shavings... :ugh: I'm a dumbass and last winter I drove 90 miles at 75 miles an hour in 4hi... yeah, that'll wreck a transfer case.

I'm just hoping now that my TCCM isn't bad too, I went all summer without 4wd, because well, it's summer. But now the snow is already falling (damn you mother nature) and it's time to finally put the new one in.

I have the service light on, but no lights on the switch. I hope the switch is ok?

the roadie
10-26-2010, 06:05 PM
I have the service light on, but no lights on the switch. I hope the switch is ok?You probably have the dreaded 2002 TCCM failure as well. You can't even tell if the switch is OK until the TCCM wakes up and communicates with the outside world by lighting up the switch lights. The PCM is what lights up the "Service 4WD" lamp, and it does that if it loses communication with the TCCM. If the TCCM is alive, and communicating, it controls the mode switch lights, and does its little flashy dance if it's not happy with the position feedback sensor data it gets from the front axle actuator or the transfer case encoder motor.

10-26-2010, 06:10 PM
You probably have the dreaded 2002 TCCM failure as well. You can't even tell if the switch is OK until the TCCM wakes up and communicates with the outside world by lighting up the switch lights. The PCM is what lights up the "Service 4WD" lamp, and it does that if it loses communication with the TCCM. If the TCCM is alive, and communicating, it controls the mode switch lights, and does its little flashy dance if it's not happy with the position feedback sensor data it gets from the front axle actuator or the transfer case encoder motor.

So basically even after I put the new TC in tonight I still wont have 4wd?

the roadie
10-26-2010, 06:15 PM
Not unless pulling the fuse on the TCCM wakes it up enough to get it to work. It's a classic, although temporary, attempt at a fix. Sometimes works, sometimes doesn't.

10-26-2010, 06:16 PM
Not unless pulling the fuse on the TCCM wakes it up enough to get it to work. It's a classic, although temporary, attempt at a fix. Sometimes works, sometimes doesn't.

Ok, well we'll see what tonight brings then, that's fuse 8 correct? Thanks very much for your help!

10-27-2010, 02:07 AM
SUCCESS!! Transfer case replaced and we have 4WD again! Switch works just fine, didn't have to pull the fuse. I just plugged it all in bolted the transfer case back in place (which was fun because I didn't have a lift so I basically had to bench it into place), turned the key and ran it through the motions.

Thank you all for the great info! :thx:thx

the roadie
10-27-2010, 11:48 AM
Thanks for the report, and good luck with it! I didn't think that would be all you needed to do, but can't complain about success. :thumbsup:

10-28-2010, 12:35 PM
roadie. just swaped out the tccm for another unit. still no selector lights and wont go into 4x4 or awd. Took it to a garage and he telling me the code is a hard code. meaning it wont let him clear it. says he tried twice and still comes up. any ideas what i can do. trying to save as much money as possiable and snow is on the way.

the roadie
10-28-2010, 01:21 PM
Where did you get your new unit, and was it proven working before installation? Does it have power? If your mechanic can't talk to it, then it's either a bad TCCM, no power to the TCCM, or a bad communication path to the TCCM. If the data communication bus in the vehicle was preventing other modules from communicating, you'd think your mechanic would see that.

12-04-2010, 03:36 PM
I am having the same problem with my 2002 TB.
I just replaced the starter and after having getting the new one in and reconnecting the battery I now have the lights on the switch Illuminating briefly and the service 4wd light on at all times.
I also noticed that the "N" light on the selector switch is always on and it is red.
I never really noticed this before so I am not sure if this is normal or not.
I tried the disconnecting the battery to reset the eccm, no luck
Anyway, I have read through all the posts it sounds like it is either the eccm or the encoder.
I plan on doing the repairs myself just wondering if someone might have some info on where to start, what manuals, should I get a reader etc.
Thanks, Greg

12-04-2010, 04:21 PM
my problem is now fixed. Turns out the first tccm i got from the junkyard was just that. JUNK. Got one from a 2003 trailblazer and popped it in today and the lights work and no more service 4x4 light.

the roadie
12-04-2010, 07:07 PM
I am having the same problem with my 2002 TB.Classic symptoms of a bad 2002 early design TCCM. Try to get one out of a 2003+ for better reliability.

my problem is now fixed. Turns out the first tccm i got from the junkyard was just that. JUNK. Glad to hear I advised you accurately.

12-05-2010, 10:34 AM
Hello all. This IS a great site, it's been awhile since i've been here, so greetings.:) My 02 TB has Service 4WD light on,however the 2WD light is illuminated on the selector switch. I will try the "fuse pull", then the "turn past 4LO for 10 seconds" options and will report back in a bit. I live near Cleveland, so i kind of need my 4WD right about now as we are ecpected to get 1-2' of snow in the next day. :eek:

the roadie
12-05-2010, 11:01 AM
The 2WD light being on ALONG WITH the Service 4WD light is usually caused by a TCCM that wakes up enough to control the mode selector lights, but won't communicate to the PCM. It's the PCM that lights up the Service 4WD lamp. It's saying: "I know this vehicle is 4WD and I should be able to communicate with a TCCM, but it's not talking back when I holler."

This was the second layer design flaw of the 2002 TCCMs, but it sometimes responds to a dealer's software update. If you can find a dealer to charge a fair price for ten minutes work, see if you can get the TCCM wake-up communication update.

Very often, the dealers want $100+ for that service, and it's cheaper to look for a 2003+ TCCM.

Good news is, that this communication problem has a workaround, and it usually wakes up using the fuse pull trick. If you're stuck in 2WD mode and the Service 4WD light is on, put the mode selector switch in 4HI, for instance, pull the fuse for 30 seconds with the ignition on, then put it back in. The TCCM will probably wake up and get you into 4HI to get you home. Small chance you could be stuck in 4HI if you try to get back into 2HI and can't. But try the fuse pull again if this happens.

12-06-2010, 07:02 AM
Well, i tried the "turn past" method but i didn't have the ignition on, and it didn't work. I'll try again in a bit WITH the ignition on, should it be idleing to when i do this, or just the key turned to accessory??? One thing that i remembered is that even in summer months while drivin at highway speeds it would transfer back and forth from 2WD to A4WD on it's own occasionally. Sometimes on dry days too. I don't drive the TB every day (wife does) so i'm trying to rememeber the symptoms it has had. Then this morning on the way to work with the service 4WD light on (as usual) at highway speed using 2WD (light on) the A4WD light began flashing for a bit. Just thought i'd throw that out there. I'll let you know how the "Turn Past" method works in a bit.

the roadie
12-06-2010, 10:40 AM
Any time you also have the symptom of changing modes without your moving the switch, it's usually a bad switch or dirty contacts. You can try turning it CW and CCW a hundred times or so, or just going to the dealer and getting the redesigned switch. They updated it (I forget when, maybe 2005?) and dropped the price and it's not a real ripoff. Probably under $40.

12-06-2010, 07:05 PM
I have a 2005 TB LT 4.2l with it sounds like a similar problem.

A few months ago, I was attempting to switch into 4hi and the process made a different noise than normal, then all the lights went off. After looking on the forum here for a few min I saw that the #8 fuse was blown. After pulling it and getting a new one, the selector lights came back on, but the service4wd light was still on. I attempted to switch it again, this time no noise at all, just blew the fuse. I have taken the cover off the motor underneath to check the wires and cleanliness of everything, and it looks good down there. I have pulled the selector switch out, it is well lubricated and nothing smells or looks fried. The dealerships have quoted me over 100$ just to diagnose it. eff that, im poor.


the roadie
12-06-2010, 07:10 PM
Look for a junkyard TCCM. If you're good with a meter and tracing the schematic, you might be able to localize it to something else, but the used TCCM will cost you less than the dealer's diagnostic charge.

12-06-2010, 10:15 PM
Hi all, just signed up. I have an 03 TB. just hit 100k. Last winter I had this problem a couple times, always in the snow btw, never when it's dry. Seemed to clear itseslf. Used 4wd HI a couple times last week, no problem. This morning was first big snow to drive to work in and I live out on back roads now, so used it in 4wd until I hit some clear pavement then 2wd the rest of the way. No problem. Coming home I hit some tricky parts where it drifted so switched on the fly a couple times. The last time I switched I got the dreaded service 4wd light and stuck in 2wd. Later tonight I checked it hoping it would be cleared, but now, for the first time, not even the mode lights come on. I've been reading all through this, seems like issues with the module needing firmware update? Looks like only 02 needs new module. Is this all the latest and greatest info so far? Roadie, seems like you know this issue fairly well, is it time to talk to Mr Chevy dealer? I'll check fuse tomorrow. thanks a bunch. Bill

the roadie
12-06-2010, 10:58 PM
Check the fuse first. Then suspect the TCCM.

12-07-2010, 09:26 AM
So.... I was looking through the forum and saw that someone suggested trying to reset the system by holding the switch all the way over to the right for 10 seconds. Well... what ended up happening is the 4wheel engaged..... but now its STUCK in 4low. fml. :suicide:

the roadie
12-07-2010, 11:40 AM
Sorry that happened. I don't know who posted that idea, but I'm sure it wasn't me. A risky move, as you found out.

12-09-2010, 06:42 PM
Well I went to see Mr Chevy man today. That was this afternoon. Wouldn't you know, this morning while backing out of a parking spot it felt like I was pushing hard against something. I wound up noticing that the service light was out and that I was now in 4LO! I switched to 2HI and A-OK. Chevyman ran the scan on it and it showed an encoder circuit failure, which probably indicated an encoder motor replacement. He said that he didn't see a service bulletin about the firmware on my truck. He did say there were a couple bulletins regarding the wiring to the motor. I had been wonder about the weather because this problem has only happened in slushy, sandy road conditions. I guess it's exposed to weather since it's on the side of the transfer case. I think I'm gonna have a look underneath and see what the thing looks like and if maybe the connectors mate good and such. Has anybody else seen anything like this? the repair quote for a new motor was like $650, over $400 for the part alone. I'd like to look into this a bit more before biting down. It just seems funny that in the wet snowy conditions it starts happening and then clears up when the roads dry up. I'll keep ya posted.


12-12-2010, 11:06 AM
Well, i also tried the "turn past" method and it didn't do anything. I pulled the number 8 fuse under the hood for 30 seconds then put it back in and still nothing. So i called around many,many junkyards looking for a TCCM and most don't even have a TB in stock?!? WTF??? The dealers are no help either as all they want to do is sell me one for between 270 and 450$$! So, anyone have a lead on used TCCM's online??? I will also pull the selector switch and check the contacts and possibly look at replacing that too. Oh, By the way, when i called the dealers and asked how much a re-flash would be ,none of them would give me a straight answer how much it would be. (But it obviously wouldn't be free) So that's where i stand, stuck in 2WD. :mad:

12-13-2010, 08:18 PM
I walked out of the Chevy place $1000 poorer a couple of days ago (see above for issue). It was the encoder motor and the tccm. They told me that what made it short was.. get this.. the Sirius radio (that they called an 'aftermarket install' -- no?) They told me they had to pull the panel off, disconnect the radio, then install and program everything.

Try not to eff up your 4x4 eh?

the roadie
12-13-2010, 09:07 PM
I've posted this before, but I swear the GM techs must have their own private forum we'll never see, where they have contests to just make stuff up, and see who can get a victimized owner to believe it.

Did they GIVE you the encoder motor and TCCM?

12-15-2010, 08:33 AM
Hi everyone. Great info on this board. I have an 03 Envoy with 100K. Just joined to find out some ideas for the dreaded "service 4wd light" with drive switch disabled. Mine fixed itself after 5 restarts, then failed again. At least when the service light comes on it stays in the last mode selected, in this case we have it in 4HI. I will try the fuse reset but from the forum I see it might be the switch, tccm or worse the encoder motor. Thanks for all the ideas, I'll report back.


12-27-2010, 01:11 PM
I too have the dreaded "Service 4WD" light. It happened after I jump started my wife's 2002 trailblazer. I checked the #8 fuse and it was good and also rotated the switch a few times to see if it was a bad connection, none of that worked. So off to the dealership I went and the verdict was that I fried my TCCM when I performed the batter jump start. The tried to re-flash the TCCM but it didn't take, now they want $550 to replace and reprogram.

If I buy a module online (i've seen these for $200 or less) do I have to take it to the dealer to reprogram or is it good out of the box? If I go to a junkyard and find a 2003 module, will it have to be reprogrammed?

Sucks to know that you could destroy an expensive part by simply jump starting your car.......should be a big warning sign on the battery.

the roadie
12-27-2010, 01:56 PM
I think the dealer's guessing and you just had a coincidental failure. If the dealer doesn't INSTANTLY recognize the high failure rate of the 2002 TCCM, they've been living in a cave for 7 years and aren't qualified to change your oil.

If you buy a used TCCM, it should be plug and play. If you buy a new one from a discount place like, and dealer will have to program it. :yes:

Discharging or swapping a battery is risky to the HVAC actuators, but not to the TCCM, unless it's one of the 2002 TCCMs that is a ticking time bomb waiting to fail anyway. I guarantee it. Don't feel guilty.

12-27-2010, 03:06 PM
Well my problem fixed itself. The Service 4WD light is off and I have put the selector in A4WD mode. It can now stay there for the winter. If and when the light comes on again I'll read the codes and go from there. I did rotate the selector control back and forth a number of times, not sure if that was the problem, we'll see. I have a pretty smart local guy I can work with without taking it to a dealer. My gut feel is it's a feedback signal problem from one of the mechanical 4WD units or bad connection.


12-27-2010, 03:12 PM
Are the newer TCCM's cured from the disease? I've read on other posts that people are using a 2003 or later TCCM, will this work in my 2002? Any specifics I should look out for?


the roadie
12-27-2010, 03:28 PM
2003+ TCCMs will work no problem in a 2002. Just make sure it's from a 4WD Trailblazer or Envoy, not one of the AWD variants like the Bravada. Those had very small market share, so there aren't that many of them in the junkyards, either.

12-28-2010, 03:11 PM
Is 12577412 the correct replacement TCCM part number that I should be looking for?

the roadie
12-28-2010, 03:21 PM
I think that's the redesigned TCCM for the old GMT330 platform (S10 Blazer / Jimmy, etc.). There was a "Trailblazer" trim level of that platform, but it wasn't our GMT360.

You want the 12590220.

12-28-2010, 03:46 PM
thanks roadie, is that part number specifically for the 4x4 or both the 4x4 and the AWD models? I've found one pretty cheap, but don't know if those different models used the same module with a different program or different modules all together.

the roadie
12-28-2010, 04:02 PM
I think they all used the same module, but the programming was different. But the shop manual isn't crystal clear on that level of detain, being aimed at dealer service staff first. They have access to the Tech II tool for the reflash. Buy one that has been used in a TB or Envoy if at all possible.

12-29-2010, 03:34 AM
Thanks to you Roadie and many others with helpful info on this topic. I too have replaced the bad TCCM, which:
-turned off the "service 4WD" light"
-brought back the orange lights on the selector switch
-restored the functionality of the 4WD (and now can "hear" the switching sounds among the different settings too!)

plug and play! installed it in the parking lot at the salvage yard, drove off the lot with problem solved.

$75 for the used TCCM at the salvage yard! I can't recall being happier about spending $75.:yes:

next task: :offtopic: replacing the spare tire carrier I broke! :hissy:

the roadie
12-29-2010, 10:26 AM
I'm extremely interested in getting a dead TCCM to play with. Could you PM me if you're willing to ship it at my expense for analysis?

12-30-2010, 08:52 AM
I have a 2002 Trailblazer LS and I just ran into this problem this morning. I started my vehicle to leave for work, and had no lights on the selector switch. I tried switching to A4WD and 4HI with no luck. Usually can hear when it engages front end into gear. Is it possible my TCCM is junk? I have read this whole thread and from what it sounds like the 2002 TCCM is a junk design. Should I try to pull the #8 fuse first to see if it wakes my TCCM? :suicide:

the roadie
12-30-2010, 09:00 AM
Yes, and yes.

12-30-2010, 09:07 AM
If I have to replace the TCCM, I found one for $84 used. It was previously installed, unsure if it was a Trailblazer. Should be plug and play since it was already installed? Has the part number of 12590220....which I believe was installed in the 2003's and up?

12-30-2010, 01:12 PM
The fuse pull worked. But still makes me uneasy that it is working intermittently. I hope it lasts through the winter. :ugh:
I will be replacing soon if it keeps happening. I'm at 138k right now.

01-04-2011, 04:17 PM
Guys I am stumped! I have an 05 TB with the 4.2 and the 4wd stopped working, dash light shows up, and the selector switch only lights up when starting the rig then goes out. I have replaced TCCM with new GM one from dealership and had it programmed. Then they say that the encoder motor is bad. So I buy one and install it myself. While on jack stands I decide to check to see if all is well. I start the vehicle, the light on the dash is no longer on and the selector switch shows it in 2wd. I then shift between 2wd and 4hi and I hear and feel it shift at the transfer case. "Yeah", all is well! Not so fast, after I got the TB back to the ground I start the TB and everything is back to the original no 4wd, light is on in the dash and the selector lights only work upon starting then go out. I am sick of this FAILBLAZER!! It has had a ton of problems which have all resulted after the warranty has expired.
I used to work for a dealership for years in and out of the service dept and parts dept and have never seen a bigger POS than this thing! Even my rear hatch liftgate and glass will not open. I cannot hear the lock mechanism operate with the key fob like it used to. I have pulled #8 fuse even before replacing any parts and also after having these problems and all fuses appear to be ok.
I constantly have SES and check engine lights on and have replaced thermostat, fuel cap, 2 different emission control sensors, cracked exhaust manifold, throttle body AFU, right headlight burned out multiple times and fried wiring harness. Whats next? I am a loyal GM guy and currently own 2 other GM vehicles which work flawlessly but this one needs to go! Where is GM when u need help to fix their obviously poorly designed and built FAILBLAZER?
Signed; " Sick of it in Seattle"

01-05-2011, 12:51 PM
I have an '02 Envoy that started having T-case related problems similar to many of the posts on this thread. I tried fuse pulls etc as suggested, but got no joy. I figured that it was worth the hour diagnostic at the dealship to see if they could pinpoint the problem. I just got word back that while doing the diagnostic, the dealer noticed a "hole" in the case itself. They've roughly quoted $2700 to replace the T-case alone and I figure my rig was only worth 5 - 6k before I found out about this. So it seems that I need to get ahold of a new / rebuilt / remanufactured T-case (and maybe a TTCM / selector switch etc) but for now, could anyone suggest a supplier for a T-case. I live in Calgary Canada, and would like to find something reasonably close if possible.


the roadie
01-05-2011, 01:17 PM
Ebay or junkyards. might work in Canada.

01-05-2011, 02:49 PM
Thanks! I've already found a bunch on E-bay. And the website found me some cases right here in Calgary. I'll post again when I get the replacement case installed.

Thanks again.

01-05-2011, 03:06 PM
I have seen numerous replies from you on many of these issues, and have seen several remedied because of you. Any suggestions for my 4X4 dilemma? As you can see, I have replaced TCCM and Encoder. I have removed and checked the switch on the dash but it does not seem to be the issue. Any help would be sweet.


the roadie
01-05-2011, 03:19 PM
"Fried wiring harness" said it all for me. You need to get an electrical troubleshooter to look at the TCCM connector while it's sitting in the vehicle, and see which of many signals it's missing. If the dealer can't help you, you need a competent independent mechanic with access to schematics and the theory of operation.

Or I'll buy it for $500 and then you have no more problems. :thumbsup:

01-06-2011, 12:24 AM
Thanks Roadie,
I am wondering if it has anything to do with the aftermarket remote starter that we had installed a few years back? Otherwise I am wondering why I would have a fried wiring harness all of a sudden? I guess its time to have a good mechanic take a look see since wiring and electornics are not my strong suit.
Actually, $500.00 is souding pretty good right now. I would feel like I was taking advantage of you though.
Thanks for your advice.

the roadie
01-06-2011, 11:01 AM
I used to think dealers were wussies when they refused to work on vehicles with aftermarket remote start systems. But with the factory harnesses and security systems and data communications and interconnectedness of the over a dozen intelligent modules in the vehicle, I'd tend to blame the aftermarket stuff first as well. When vendors like Scosche can't even get a head unit chime adapter right, foggetaboudit. All bets are off when the already flaky electronic design gets added on to. At least for me. YMMV.

01-09-2011, 02:50 PM
I have the same problem on my 02 TB after jump starting my sisters car.After pulling the #8 fuse, do i need to remove both the positive and neg battery connection?
Sorry I dont know crap about cars

Thanks for all you great info

the roadie
01-09-2011, 03:47 PM
After pulling the #8 fuse, you don't need to EVER touch the battery cables. Doing so could kill one of the five HVAC actuators.

With a 2002, it's almost always the TCCM, and you should be able to predict what my advice will be. It's always the same.

In case you have something different, please describe EXACTLY what you see and hear when you change 4WD modes now after resetting the system by pulling the fuse.

Bob Albrecht
01-11-2011, 09:12 PM
My wife's 2002 Trailblazer EXT with 80K miles had no lights on the 4WD switch.

I pulled out the #8 fuse for 10 seconds and put it back in for the first time today (snow on the ground) and all is well.


01-12-2011, 01:27 PM
I like your idea of a fried harness. Well not so much like, more like agree! What do you think the chances are of me having a burnt out front diff actuator, or splined disconnect? I see some other posts on here about those issues and it seems that if I had one of those issues that it would signal my TCCM not to communicate with the 4 WD system, therefore illuminating my dash light, and keep my dash switch from lighting up. I am going to pull the actuator this weekend and check it out and if it seems to be operating, then its off to the dealership to get a better diagnosis.
So far I have replaced the TCCM and had it programmed. Replaced the Encoder Motor and replaced the dash switch. When I replaced the Encoder Motor my 4 WD actually engaged briefly and then quit. All of my fuses are good and have tried to pull the #8 fuse trick and that did nothing. My last resort is to pull the actuator and check it out to the best of my ability. I could always replace the stinkin part since thats all I have left, but I would hate to replace the actuator and have it be a harness or something in the drivetrain itself. Just a thought!

01-14-2011, 10:13 PM
I seem to be having the same problem most everyone else is having. Service 4wd light on, no lights on switch indicator, and trans. seems to be sticking in 4wd. Should I try all the "tricks" listed in the forum first or just dive in and get a replacement tccm? This is the 1st major problem I have had with the TB and can't stand the thoughts of getting rid of it for what seems like a not so expensive fix. Need help!

the roadie
01-14-2011, 11:46 PM
TCCM death seems to be localized to 2002 vehicles. Your 2003 may respond to a dealer reflash.

Sticking in 4WD (crow-hopping, I assume) may be transfer case fluid issues. How many miles, and have you been changing TC fluid religiously every 50K?

01-15-2011, 12:27 AM
New here, been reading all about these 4 wheel drive problems. I've got the dreaded '02 also. Guess I will be hitting the wrecking yards next week for a used '03 TCCM. How new can I look for?
Roadie, could you tell me what the Encoder should read with a meter hooked up to it while rotating by hand. I am reading across channel P encoder signal (brown/white wire) to encoder signal ground (black/white wire), mine starts odd at infinate ohms as I begin to rotate the inner portion of the encoder, the meter displays 1100 ohms. I have to rotate it another 90 degrees or so before the resitance starts climbing. Resistance will continue to climb linearly as I rotate it around until I'm almost back to where I started, at that point the resistance is at 2350 ohms. Then once I return to where I start the meter will go to infinite ohms. When I read across 8V ref (light green/black) to encoder signal ground (black/white) I get 1480 ohms. Does this sound normal to you? Maybe this info will help out some others. I just hate to buy parts that don't need replacing. I have verified the encoder motor and electric brake operate correctly. To bad the schematic doesn't mention that the encoder motor has a brake on it. I have read the service manual section for the transfer case a 100 times no mention of it.


the roadie
01-15-2011, 08:23 AM
The factory manual mentions the brake, as does the schematic. Which one are you using? Haynes and the like are much thinner for good reason - they leave out a lot of stuff the average DIYer will never need.

All newer TCCMs should work fine. Check the part number if you are worried about what the junkyard gives you.

Anyway, I don't know the exact resistance of the pot, but I may be able to get to my spare this weekend to check it. Yours sounds like a dirty wiper, but you might be able to clean it.

01-15-2011, 12:06 PM
So I relented yesterday and took my wifes TB to the dealership to diagnose 4wd not working. Since I had relaced the TCCM, encoder motor and the dash switch and none of it worked. They called me and stated that there is a low voltage read out on the return wire from the Encoder Motor. When they called the GM tech line they were told that my transfer case was most likely the culprit. The dealership basically told me that I have worn out clutches in the Tcase and that infected my fluid. Every time my Encoder Motor tries to engage, it meets resistance in the Tcase and then shuts itself down and initiates the light on the dash. Of course the dealership wants $1900.00 for re-man Tcase plus labor. I can probably find one much cheaper of course but now my wife wants the factory unit installed by them to get the full parts and labor warranty. I also just found a complete 2003 TB online that was rolled and it is in otherwise great running order with all the bells an whistles (LT) option for $1500.00! I would love to just buy that thing and R&R the Tcase, assuming its the same as my 2005, and part out the rest online, but wife does not sound too excited about that. What to do?

the roadie
01-15-2011, 12:50 PM
Interesting diagnosis. A 2003 won't have the updated TC bearing support, but if you follow my advice to not use A4WD mode that may not hurt you much. I'd buy 2-3 transfer cases from Ebay before attempting to part out an entire vehicle, because getting rid of what's left is going to cost real money unless you have your own landfill.

Assuming everybody keeps their TC fluid fresh and topped off, I believe the main source of clutch plate wear is unnecessary use of A4WD mode. YMMV.

01-16-2011, 02:04 AM
Today it's happend to me too.
Light pops up in the cluster and the lights on the selection switch
only working when i start the truck.
i read the complete thread and tried out the "10 second" thing
with the switch and pulled breaker #8 (wasn't smoked).
i disconnected the battery and will wait for 30 minutes, we will see if this works.
if not, i have to go to a dealership to get it fixed. Thanks GM
The only suspicious thing is, i received my monthly Onstar check this morning
and it shows a prob with the drive train. The funny thing is, as i went home
yesterday night, everything was working fine. i drove all the time on 4hi
without any issue. after i read the Onstar report i went to my truck
and fired it up and the light popped up in the cluster. :undecided

Anyways, it needs to be fixed because it is not funny at all to drive around
only with 2WD in the snow.

Oh, one question : What is a TCCM ?

01-16-2011, 02:56 AM
Connect the battery back on but nothing changed.

Bill W
01-16-2011, 04:07 AM
Oh, one question : What is a TCCM ?

Transfer Case Control Module

01-16-2011, 07:27 AM
My 4 wheel drive didn't work for about 6 months. Then last week we got some snow so i decided to go get it reprogrammed after reading all the post in this forum. The reprogramming took.....Got home and it worked great getting me up my driveway. The next morning I jumped in to go to work and the selecter lights where off and the service 4x4 light was on. I made it to work with no changes. I came out of work to come home and everything was working just fine. Then everyday since thats the way it works....It works fine in the evening and nothing in the morning. Any one else having this problem or have anything to suggest on what needs to be done next. could it be a bad TCCM? Thanks everyone!

the roadie
01-16-2011, 09:04 AM
Some 2002 TCCMs supposedly can be improved by a reflash. Many early model year ones cannot. And the issue is temp related - some TCCMs just didn't wake up fast enough and communicate to the PCM after ignition on. Some electronic components (such as capacitors) change value drastically by temperature (and age), and a good designer takes that into account, plus the natural variation (in spec) of components. But this design team messed up, and that's why the 2002 TCCM has such a high failure rate.

Did you get charged a fair price for the reflash?

01-16-2011, 09:14 AM
Thanks Roadie! Not really a fair price they said the reflash was free.....But charged me 80 bucks to run the diagnostics. I explained to them that I only wanted the reflash. I am going to be on the lookout for a TCCM so I don't need to worry if the temp is high enough for the 4x4 to work. lol Thanks again

the roadie
01-16-2011, 09:28 AM
... they said the reflash was free.....But charged me 80 bucks to run the diagnostics....As they say, a distinction without a difference. :no:

And if their diagnostic failed to predict that the reflash was going to be ultimately unsuccessful as a fix, then it was a failed diagnostic that you might be able to dispute.

Unless they have weasel words in their service paperwork that says: "We will take your money, but nothing we say is guaranteed to be right." Heck, except for the money thing, that describes trailvoy! :bonk: :crackup:

01-19-2011, 01:10 AM
Went to the recycle yard today, picked up a TCCM out of an '04 Envoy. Brought it home slapped everthing back together and MAJIC! The 4 Wheel Drive System is fully functional agian. Thanks for all the great info fellas (guys and gals). The bad part is the recycle yard is steep pricewise ($150) but still way cheaper than buying a new unit and havin the Stealership program it.


01-20-2011, 09:43 PM
I have two questions.
First, where is the TCCM located and second,
Is it a easy swap ?
Maybe somebody as any pics ?

Thank you

the roadie
01-20-2011, 09:55 PM
A member did a how-to, but he used Photobucket and the pics are now gone. Sigh.....

It's above the driver's left knee. Get better access by opening the little door on the side of the dash that's only visible with the door open. Needs a knife blade to pry it out.

01-21-2011, 01:57 PM
Thank you :thumbsup:

01-26-2011, 03:31 PM
So, yesterday i dropped off my TB at Chevy.
Got a call in the afternoon and they found out that my
transfer case is gone. The mechanic said, they are metal parts
floating in the oil.
Congratulation, please pay $2500 and everything will be good.:sadcry:

Hope i get him back tomorrow.

01-29-2011, 06:05 PM
Selector lights on my 02 trailblazer 4wd switch went out yesterday, and no
4wd at all. I pulled the number 8 fuse for 30 seconds and re-installed,and bing I got lights and 4wd again! Amazing!
My question is, should I start looking for a used TCCM to replace this one ? As I'm not sure if this is just a temporary fix? I'm leaning towards yes. What do you guys think?
This forum is great, especially when you own an 02 with a 160K :yes:

the roadie
01-29-2011, 08:25 PM
For a 2002, there's no guarantee that the fuse pull trick will wake it up forever. If you need 4WD often, or even once, IMHO, start looking for a used TCCM. Under $100 is a good price. $50 is fantastic.

01-30-2011, 12:14 AM
So I pulled my T-Case a week ago and had it rebuilt by a mechanic at a dealership who did it on the side. I paid 1/3rd the cost of having the dealer install a remanned unit. After installing I had some trouble getting it into gear. The encoder motor was in a different position than what it was supposed to be. There is a little trick that the mechanic did with a 9volt to be able to unlock the brake and then turn the motor until it's in the proper position. I believe I am still off by a tooth or so because I now have 2WD but when I switch into AWD it is actually in 4WD. When I switch into 4WD it starts to flash for about 10 seconds and then reverts back to AWD. It is definately not in AWD, but atleast it is driveable. I am now afraid to even switch it into 4lo since I did this earlier and it was stuck there for a while until I pulled the encoder back off, pulled the brake off of it and rotated the motor by hand and reinstalled. This has truly been challenging to say the least. I am wondering now if I should leave it in 2WD and pull the motor one more time to see if is actually lined up 100% correctly? This might be my problem again since last time I pulled it off there was some pressure on it and as soon as I pulled it, I was not able to reinstall it without tearing off the brake and rotating the motor a bit. I hope this helps those of you who decide to tackle this. If you pull the encoder motor to replace it, make sure the new one is in the proper position. Also there are two steel sleeves that ususally stay in the bolt holes of the old motor when you pull it. Those are crucial to pull out and put in the new one since there is so much torque on this unit. I actually missed this step and had to go back to the dealer where I turned in the old one for a core charge and remove the 2 sleeves and then go home and reinstall. If you buy a remanned unit from GM, be ready to pay $ 1500.00 and it will come with a new encoder. I saved a ton of money but did quite a bit of work, but also learned a ton. Good luck

02-21-2011, 05:41 PM
Have an 02 Envoy with lights on switch not illuminated. Not sure how long it's been going on as I only use 4WD when it snows (3 or 4 times a year). I know it was working fine last winter. Never had a "service 4WD" light come on. My brother thinks it's the 4WD accuator. He is a mechanic and fixed an 02 TB with same problem a few weeks ago, replaced accuator and had no issues since. How will my DT be affected without fixing the problem right away if it is the TCCM?

UPDATE: Pulled fuse 8, now 'Service 4wd' light is on. Found used TCCM from 03 TB for $75. Even cheaper than the 4wd actuator...should I do both to be on the safe side??

02-21-2011, 06:13 PM
There is a fuse panel under the hood, pull out the fuse labeled ATC and make sure it is not blown. If it's good reinstall. This is a problem that GM knows about. Most likely the "Service 4 Wheel Drive" light will illuminate on the dash. If it does, very good bet you need a new Transfer Case Control Module (TCCM). You can get one at the wrecking yard fairly cheap and install yourself and save bucks!


the roadie
02-21-2011, 09:42 PM
With no lights on the mode switch, you have to start with the TCCM. It's not waking up and won't do anything you ask.

Once you get the 2003+ TCCM in there, then try each of the mode transitions. The destination mode (the one you're changing TO) should blink a bit, the motors underneath should make the familiar noises, and the end result should be a steady light on the destination mode.

If the destination mode light is blinking, that's a fault in the encoder motor OR the front axle actuator. It would be premature to declare the actuator at fault this soon. There are other ways to tell if that needs changing, after you get a good TCCM in there.

03-13-2011, 12:18 AM
Just got done reading through the thread, but I haven't seen anyone talking about long-term results. I had my first experience today with the Service-4WD light coming on, and the mode switches all turning off. After letting the truck sit for about an hour, it worked fine again the rest of the day.

So what's the likely long-term result of this? Is it an early indication that either the TCCM or the TC is failing? Is there something that either myself or the dealership can easily check to determine if there will be an imminent problem?

I've been using my 4WD on and off all Winter without any noticeable problems, however I'm still within my drive-train warranty from the dealership, and as expensive as it was, I want to make sure I get every dime out of it... If something is worn out, I'm more than happy to let the dealership replace it at their cost. :) But if this symptom is just something that is common with the platform and not an indication of an actual impending mechanical failure, there's no point is searching for ghosts.

03-15-2011, 04:57 PM
Well I had my tccm reprogrammed today and did the fuse pull a couple days back and no luck. so far. $30 for the programming. Just called the yard and they have a envoy with a selector switch that he is going to pull and he is going to check on the actuator for me also. No tccm thou, someone already got it. Well once i change the switch out, and if that doesn't work, what does everyone suggest on doing next? Thanks everyone

11-30-2011, 11:27 PM
So last year at this time(when it snowed) we attempted to use the 4wd and it FAILED. I pulled the fuse, replaced the TCCM, replaced the switch, replaced the encoder motor and pulled the front diff actuator and cleaned it. NONE of it worked. I ended up pulling the T-case, had it rebuilt and reinstalled it. After all of that, it made some grinding noises when trying to select 4wd. After messing with it for a while, I ended up with AWD and 2wd only and settled for that because I had enough of working on it.
Fast forward one year and it has snowed again. We tried 4wd and only got AWD. Then my wife attempted to go back to 2wd last week and the switch stays in AWD and when selecting any other mode, the lights just flash and revert back to AWD. But the T-case remains in 2wd.
So today I returned the encoder motor to the dealership under a parts warranty and installed the new one. Now I have 2wd and AWD when selected, but 4WD and 4LO still only flash and the encoder makes a different noise and it reverts back to AWD or to 2wd if selected. So to make a long story short, I do believe that the rebuilt T-case has an issue still and needs to go back to the guy that rebuilt it. If anyone has any other idea's to try before I do this, please chime in. Otherwise the guy that rebuilt it (who also does this for a living at the dealership) agreed to pull it apart and check it out for free. I just have to pull it from the vehicle again :(

12-03-2011, 09:18 PM
Pulled fuse #8, disconnected battery for 10 minutes. Reversed process and it worked like a charm. I love this forum.

12-04-2011, 09:39 AM
My "Service 4WD" light came on in the spring and all I could think of was hundreds of dollars. I found and it saved me those hundreds of dollars. I had the same symptoms listed in this thread, Service 4WD light, no lights on the 4WD selector switch. I pulled the fuses and all were good. With the thought of snow coming and no 4WD concerned me so I decided to get at it. I followed the instructions here, Pictures were great, found the old TCCM up behind the headlight switch, pulled it out and went to a junkyard. They had an old Trailblazer with a smashed front end and drivers door missing, that helped. I pulled out the TCCM from the trailblazer and took it inside. The guy at the counter said, how much do you think that's worth, I said I don't know, even though I did, and said $50? He said, I'll do it for $40. I paid, walked out, plugged it in and started my truck with anticipation. The lights on the switch flashed like always and low and behold, the 2HI light stayed lit. I drove out of the junkyard and switched to 4HI and back to 2HI with no problems.
THANK YOU TRAILVOY.COM members for saving me hundreds of dollars. I'm still smiling about fixing my 4WD for $40.:D I will surely recommend this page to my friends.

02-09-2012, 08:21 AM
So i have read the whole thread. I tore apart the dash. I was able to find the TCCM. My lights have not worked for quite a while. Once i got the TCCM down i was playing with the wires, I could get the lights to come on if i held the wires a certain way. I was able to get the car into A4D, 4HI and 4Lo by holding these wires. I could hear it engage and was able to drive down my street holding the wires.

Has anyone else seen this issue. I don;t know if it is a bad TCCM or if i just need a new wiring harness. I cant help but think it is the wiring harness due to the fact i could get everything to engage. I just need to know if everyone else experienced this when their TCCM went out. If it is the wiring harness i have a whole new problem of where those wires go. Any help is greatly appreciated

PS. What did we do before the internet.

02-24-2012, 03:57 PM
My 02 trailblazer works fine in 4hi, awd, and 4wd hi but when I slect 4wd lo the light blinks but the truck won't engage. I do this by the book: I drive only 2mp when I select 4wd lo. This happened out of the blue and I never get a 4wd error message.

I'd really appreciate some advice and I thank you in advance.


02-25-2012, 08:31 AM
In order to shift to 4Lo you need to be stopped and put the transmission in neutral. This is to prevent you from blowing up the transfer case by going too fast when you shift to 4Lo.

12-21-2013, 04:49 AM
i got the same probleme today. wad service ligt up and no slector switch light, now we are in the christmas time most garage are closed, is their anyway to force the 4wd? im living in eastern canada and we have several snow coming in the next day and i need my 4wd!

12-21-2013, 04:59 AM
Pulled fuse #8, disconnected battery for 10 minutes. Reversed process and it worked like a charm. I love this forum.

where is the fuse number 8?

12-21-2013, 06:16 AM
Check the fuse first. Then suspect the TCCM.
which fuse and where is this fuse located? Everyone talking about fuse but dont tell which one and where this fuse is located :s

12-21-2013, 07:06 AM
So I have changed the front actuator the 4x4 switch and just bought a TCCM and installed it. I bought the TCCM at Auto Zone. Nothing has changed. I guess if I change the Encoder motor I have replaced about everything this forumn has suggested. So does this TCCM have to be reprogramed or Flashed from the dealer? Any suggestions.

12-21-2013, 09:12 AM
If you bought a new TCCM, then I'd say yes it needs to be reprogrammed. I bought used one and worked great.

12-21-2013, 11:48 AM
Scott - The 2002 early models had a known programming flaw. Don't get rid of your old TCCM just yet, you may simply need a flash update.

FYI this site was deserted a couple years ago. All of the experts are at the new site now, but mentioning that site by name here tends to get people banned...

02-08-2014, 07:16 PM
Ttiger, did pulling the fuse correct your problem? I had the same issues and as of now I am able to cycle from 2hi to 4lo. Just wondering if yours is still working or did you replace the tccm?

02-15-2014, 06:44 AM
I just had this similar situation occur and apparently fix itself, hopefully just a fluke incident. I'll start by saying that I have an 03 TB LT EXT, and the 4WD has been used a LOT this winter. I run volunteer fire/rescue, and we had a run this morning around 4:30 with about 3 fresh inches of snow on the ground. Ignition on, switch to A4WD, drive comfortably to the firehouse. Call gets canceled halfway there, so I pull in anyway and chit chat with the other guys for a few...ignition still on, still in A4WD, in park. When I come back outside about 10 minutes later, I get in and notice the Service 4WD light is on. I scratch my head and drive home, still in A4WD, with no issues.

When I get back to my driveway, I try switching back to 2HI...and nothing happens. Stays in A4WD, no flashing indicator light, no noise, no change at all. It's like I may as well have never even turned the knob. Same for 4HI. Shift to neutral...same for 4LO. Whatever...shut off ignition and retry. When I turn the ignition back on it's still in A4WD. I turn to 2HI and the light immediately comes on, but there is no precursory "blinking" to show that the TC is in the process of transitioning, and I can't hear it actually doing anything. I tried cycling to 4HI and the TC actually switched to 4HI but it kinda clunked when it did it. So I flipped back to 2HI and shut off the ignition and got on the forums.

It sat for about an hour and I went out to try the #8 fuse pull I started the ignition Service 4WD light. Odd. :undecided I try switching to A4WD...the switch seems to work normally and I can hear the TC shifting. Same for 4HI. Drop to neutral...same for 4LO. Cycled through a couple different times and everything seemed to work without a hiccup.

I'll update if it seems to have any more problems in the coming days, but I'm hoping it was just a coincidental occurrence.

02-17-2014, 10:35 AM
I have an 2002 and what I've read, the 02's are doomed to fail sooner or later because of a flaw. Just bought a TCCM from an 05 Envoy with 118,000mi for my TB. Found on Ebay for $85. Figured I'd try it first. There's also the switch on the dash, another vacuum switch on the Transfer case and the transfer case motor that could be at fault. I need to crawl under and check the vacuum hoses to the switch are not cracked, but I figured no harm, no foul on the TCCM replacement since I have an 02.....

02-24-2014, 11:39 AM
Replaced the TCCM today and 4WD is working like a charm. That sucker was jammed way back under the headlight switch. Here's what I did to replace. Remove the lower dash panel. Consist of two pieces and 6 hex head screws. Then pry off the side panel (it's right next to the headlights in between the dash and the door). This will give you a better view of what your dealing with. From underneath, remove the TCCM mount and all. Plugs included. (The mount just clips into a bracket and the TCCM just clips into the mount.) Next, remove the plugs from the old TCCM. This will allow you to remove the TCCM for the bracket. Install in the reverse order. Here's a little helpful hint. Once I had the TCCM and mount close to where it needed to snap on the bracket underneath the dash, I was able to reach under my dash with my right arm, while viewing what I was doing through the side panel.

03-15-2014, 07:18 PM
maryy, that is BS...... I'd be pissed if I was given that answer....If it is under warranty, they need to repair it! There are other things than just the TCCM that can be causing your problem. There is a solenoid on the transfer case and vacuum hoses.. Did they check all this or just reprogram the TCCM?

03-15-2014, 07:20 PM
Generally you'll only find the TCCM problem on the 2002 model. Problem was addressed after then...

03-19-2014, 08:16 PM
My issue was no lights on the selector switch, not even during the system test at start up, and no "service 4wd" light. I usually start with the least expensive possibility and work my way up. I called a couple local salvage yards and found one that would sell a TCCM for $25 if I pulled the part myself. Done and done. The TCCM was the problem. Thanks to all for the info.

03-19-2014, 10:00 PM
It's always great when you can do it yourself and save some money. Unfortunately my daughter is off at college recently and what I could of fixed for $80 in an hour wound up costing me $300. A $90 diagnostic fee after I told them what the problem was... Really? Felt like getting my teeth pulled..!

11-13-2014, 05:59 PM
I am new to the forum.
I have an 05 Envoy and the service 4 wheel drive indicator light is on.
The selector switch lights work for a few seconds when you start it up then they go off.
Do not hear anything when selecting AWD or 4hi.
I read on the forum some have replaced their TCCM or pulled #8 fuse to correct the problem, but they have an 03 or earlier Envoy. Do I try the same thing for an 05?

Thanks in advance:)

11-13-2014, 06:47 PM
If I recall it is a 25 amp fuse(in the middle of the box towards the engine). I believe #10 under the hood. Could be a number of things. The switch, the TCCM, the actuator, the disengage or the encoder motor. Good luck! I've replaced the TCCM, the actuator and had the disengage rebuilt. And I still keep my fingers cross when I turn the switch!

11-13-2014, 06:48 PM
maybe it was the #8...

11-13-2014, 06:51 PM
And yes, the fuse should be the same on the 05 model

11-13-2014, 08:21 PM
I would suggest you find any garage with a Tech II computer, does not have to be the dealer. If your Service 4WD light is on, it will pinpoint the problem.

11-25-2014, 12:48 PM
Hi I have a 2003 tb the service 4wd light is on .When I select a4wd or 4wd hi the lights blink but will not engage but will engage in 4wdlo .Changed transfercase oil .I tried to wake up tccm by pulling fuse removing bat cable.Can hear noise when switching selector switch
. Also replaced front actuator . I have read a lot of threads but most say selector lights are out I have not read about lights blinking and not engaging. Does it sound like tccm or something else? thanks jim

11-25-2014, 01:48 PM
Kind of does sound like the TCCM or maybe the switch? Does your Service 4WD light come on? If so, I'd call around and find a garage with a Tech II computer. They won't charge much and it will pin point the location of your problem. Without that it is a crap shoot. I have replaced my actuator, tccm and have the disengage(the device the actuator seats in). I can tell with the front actuator and disengage issues, the AWD or 4Hi light would blink for about 10 seconds and trip the Service 4wd light while the indicator switch would go back to the solid 2Hi light disabling the 4wd .

11-25-2014, 01:50 PM
Kind of odd how it will go into Low? Low/Hi/AWD all require action from the actuator and disengage. That is why I am thinking switch or tccm.

11-25-2014, 06:16 PM
Yes the 4wd light is on. HOW DO YOU TEST SELECTOR SWITCH?

11-25-2014, 06:25 PM
Yes the 4wd light is on

11-25-2014, 06:28 PM
Not too sure. Rock Auto has one for $15, dorman... You can start swapping parts, but if the light is on, save yourself the headache. Most all mechanics have the tech II that will tell you exactly where the problem is.....

11-25-2014, 06:28 PM
You don't have to take it to the stealership!

11-26-2014, 11:44 AM
Thanks will try to find someone besides stealership to scan

12-02-2014, 07:28 PM
check for a broken wire by the transfer case control module

03-30-2015, 05:15 PM
Yep mine came on today out of the blue. Funny it worked all winter and now all of a sudden on a 50 degree day it craps out. 161k miles. Ideas to check?

03-30-2015, 06:18 PM
Yep mine came on today out of the blue. Funny it worked all winter and now all of a sudden on a 50 degree day it craps out. 161k miles. Ideas to check?

Went back out to check it out two hours after I got home from work and the light won't come on now. And all the 4WD selections work. hmm . . reminiscent of that crazy intermittent air bag and seat belt warnings I used to get a few years back. They cleared up on their own too.