A4WD trouble

[gallai]

Hi all

I'm writing from Switzerland and I'm new here. (Have been an active member over at Chevytruckworld for quite some time now)
Drive a '02 TB LTZ.

allow me start with a question. This thing is drving me crazy.

When in A4WD mode and accelerating, something gets very rough. I can feel/hear some shuddering and "slams" coming from the front. It's either the transfer case or the transmission or the front drive shaft.. I can't locate it. I would say it's ~1-3 hits / second.

This is intermittent ie. never happens when I try to show it to the dealer. I also have no idea what conditions bring out this trouble. 50% of the times everything is just fine. Seems totally random.

It's only when accelerating, especially going uphill. When the car is in 2wd,4hi,4lo there is no problem. A4WD is the only bad one.

Of course when the dealer drives it they always say everything was perfect. The warranty runs out in 2 weeks and although they promised to fix it even if it goes beyond, I'm still very upset and helpless. (they actually don't know what's wrong in the first place)

Please give me some tips...

thanks

GREG
from Zurich

[Blulytes]

Greg...
I would have to say it would probably be a problem with the transfer.
The vehicle is recognizing there is a slip with the rear wheels so it is engaging the transfer case for the front, and it is probably slipping until it engages, hence the fast clicking. I'm not an expert on transmissions... hell the most I know about them is how to drive a manual... maybe one of the other gearheads around here can help out.

On a side note... welcome!

[ScarabEpic22]

Cant help you there, my 02 LT doesnt have that problem in A4WD.

Again, welcome to the site!

[ylab]

Actually it sounds like the transfer case is switching back and fourth between RWD and 4WD. In A4WD if there's slippage, its supposed to jump to 4WD and once the slippage is gone return to 2WD. Mine use to shudder or bump pretty roughly when it jumped to 4WD mode, until the dealer flashed in the latest software and now the transition is nice and smooth.

I'm thinking your truck *thinks* that there is slippage and jumps to 4WD and comes out and then sees slippage again and jumps again. I'd be willing to bet that there's an issue with the speed sensor on one of the wheels causing the system to jump back and fourth between 2WD/4WD. There could also be an encoder issue on the transfer case, but you mention 4HI works fine which shouldn't be the case if the encoder was screwed up. - Have the dealer flash in the newest code and check the wheel sensors.....they sometimes get rusted or coroded and cause problems.

[Envoy Fan]

Greg, welcome to the forum. I think I have seen your posts over on tens.

In addition to what Ray mentioned, have you had your transfer case fluid changed?

Bill

[Braves299]

This is off topic, but how fast can you travel in the TB when the A4WD is engaged? I'm def. using it next time we have heavy rain...

[Blulytes]

This is off topic, but how fast can you travel in the TB when the A4WD is engaged? I'm def. using it next time we have heavy rain...

I believe there is no suggested restriction on A4WD speeds... I know I have used it on hwy at 65-75mph no issue!

[gallai]

One of the sensonrs was already replaced but no help. I also have the very latest software and the oil was changed too :(

I asked another technician in another shop and he said they already had two similar cases. In one they had to replace the entire transfer case.
I told this info to my dealer and he didn't seem happy. (Again, to this day I was not able to prove them the problem. So from their p.o.v. the problem doesn't exist!)
:mad:

[Rainman]

I've encountered a similar problem. Originally, the dealership arranged to have the transfer case replaced, until they discovered that the front tires were not exactly the same size. Apparently, 2 tires were stamped wrong and made for a different brand of vehicle. :crazy: I had never heard of such a thing. I thought a tire size was the same across the board. Regardless, his final diagnosis was that the input speed was greater then the output speed which resulted in the AWD mode thinking that slippage was occuring. In turn, the vehicle was trying to engage the AWD time after time. I'd describe the problematic sound/feel was as if I was driving over road cracks, even though the road was as smooth as silk. If anyone thinks a better explanation would be helpful, just say the word and I'll get detailed info.

[MTPockets]

Good call. Tires can be a big issue. This could be related to all 4 tires not being the same diameter (different rotational speeds). Sometimes rough-road induced wheel hop can cause the wheel speed sensors to activate the ABS or cause A4wd issues.

Joel

[Highton]

Anyone else having this problem...... bumping it to the top..... my 04 is acting very similar to this intermittently always in A4WD. Just as he said "Like going over slight cracks in the road when the road is smooth as silk"..... it does not do it at every point of accelerating, only sometimes.

I have 20" wheels with brand new Falkens, all same size... that shouldn't affect this either??

Anyone have any input on this?

[bullethole]

I had the exact same problem you described at approx. 50k miles. It did fine in 2wd and 4wd. Of course I was not going as fast in 4wd to notice any problems plus I was off road. I went for two months without using the awd at all hoping it might go away. Tried it again and got the same result, shuddering etc. It almost felt like the engine was missing.
Question? Have you replaced the transfer case fluid? I replaced mine and the problem completely disappeared. That was 45k miles ago and I used it today just to make sure it was OK and has not given any touble since the service. Something to consider.

[MTPockets]

Anyone else having this problem...... bumping it to the top..... my 04 is acting very similar to this intermittently always in A4WD..

Are you guys continuously driving in A4WD mode? Even on dry pavement? This cant be good for the long term. It's not an AWD system.

Joel

[Highton]

Are you guys continuously driving in A4WD mode? Even on dry pavement? This cant be good for the long term. It's not an AWD system.

Joel

Yes, I leave it in A4WD because I have noticed (I know this seems wrong) that I get better gas mileage in that mode. I know it doesn't make sense but I figure... maybe it transfers the torque while accelerating better in A4WD making my gas mileage higher due to it getting better grip?

I have just over 40K and I have never changed the transfer case fluid.... got it used at 35K and do not know if it was changed prior either.

[gmcman]

Unless your are encountering slippery conditions or going to in a short distance, DON'T leave it in A4WD. You are adding unnecessary wear to your drivetrain. A4WD is helpful on snowy or slippery roads that have alot of dry patches and for turning into parking spaces allowing the front axle to turn freely so it doesn't bind up on turns. When you are in the A4WD mode your transfer case is engaged but no power is sent to the front driveshaft until slippage is sensed, this reduces the time to switch from 2WD to 4WD because it is partially engaged. You want the front to grab as quickly as possible and A4WD puts it in a "ready" mode if you will.
I honestly hope you haven't been in A4WD mode for 40K miles:no: if so don't tell the dealer. Not to sound harsh but you may have serious transfer case issues now and hopefully a fluid change or the dealer can replace under warranty. Stick to your guns to get it replaced if need be, good luck.

I just re-read your last post and see you have had it for 5K miles. NOT a good thing to leave in A4WD though for normal driving on dry pavement.

[MTPockets]

..I figure... maybe it transfers the torque while accelerating better in A4WD making my gas mileage higher due to it getting better grip?..

This system does not work that way. This isn't an 'all wheel drive' system, where infinite amounts of power can be sent independantly to any wheel. We have *automatic* 4-wheel drive where the front axle basically slams full on or full off (no in-between) when wheel spin is detected. The way I see it, A4WD is more suited for when the missus is out and about on a snowy day.

Joel

[snwplow]

I left mine in 4AWD after coming out of the snowy mountains once and was running it at 65mph on dry pavement. All of a sudden the "Service 4WD" light came on. I looked down and the 4Hi light was on, yet the switch was pointed at 4AWD. Go figure. It's done that on other occasions since then, yet the 4Hi still works just fine normally. I've just learned not to use the auto mode too much. Has anyone else encountered a light illuminating something other than what your switch is pointing to?

[gmcman]

Forgot to touch on the tires, you should keep the same type of tire all around on any 4WD,AWD vehicle. The diameter of the tire can differ between manufacturer even if it's the same size. Don't mix and match brands and keep them rotated, I would say 5k miles max.

[the roadie]

...the front axle basically slams full on or full off (no in-between) when wheel spin is detected. The shop manual explains that the transfer case has a clutch pack, and the encoder motor can vary the amount of torque shared with the front axle. In 4HI and 4LO, the motor is sent to the full-share position, which is essentially locked (although I think I've managed to slip this clutch on nasty hills). In A4WD, the clutch is engaged by the encoder motor at some rate that is set by the computer, and varies by driving speed (presumably to avoid surprising grabbiness) and it doesn't have to go to full-torque-share position if the wheel slippage goes away early.

I suppose if you have two rear wheels on ice, and the front wheels on pavement, it will appear to slam full on because that's what it will take (maybe not even then) to get rid of slippage.

[gmcman]

I try to use my 4WD on a straight section of road maybe once a month for about 1/2 mile just to keep all the parts coated with fluid and the CV joints from being stationary too long. When I do this and accelerate I get a shudder but only initially...then all is good.

[the roadie]

I try to use my 4WD on a straight section of road maybe once a month for about 1/2 mile just to keep all the parts coated with fluid and the CV joints from being stationary too long.
Not a bad idea, but not precisely for the reason you think. The CV joints and front drive axles are always rotating on our platform. There is no disconnect at the front wheel hubs. (Wish there were).

In 2WD, the transfer case isn't spinning the front driveshaft, and there is a disconnect spline where the driveshaft would otherwise engage the front differential. So really in 2WD, the only thing not spinning and being exercised is the front driveshaft and its seals.

If you have fulltime AWD (like in a Buick), the front diff doesn't have this disconnect, and everything spins all the time, and the only thing that changes when front to back slippage is detected is the position of the encoder motor in the transfer case for torque control.

[gmcman]

I thought about that after I posted but the reason I thunk it was what releases in the front diff? I know the U-joints need some working time but i'n kinda curious how the drivetrain seperates when in 2WD?

[MTPockets]

..what releases in the front diff? I know the U-joints need some working time but i'n kinda curious how the drivetrain seperates when in 2WD?

There's an electric front axle disconnect that 'un-splines' one half of the front axle. Since only one side is live in 2wd mode and the front diff is open as opposed to limited slip or locked, the input on the front diff wont spin in 2wd mode. I'm not positive what side the disconnect is on, but I *think* I saw it on the pass side when I was doing my oil change the other day?!?!:undecided

Joel

[the roadie]

...I *think* I saw it on the pass side when I was doing my oil change the other day You got it!

[RichD]

Hi all,

Brand new to this forum and to trailblazers with my 2002 LTZ. I have a question related to this A4WD topic.

If the truck is in A4WD, and you make sharp slow turns (parking lot maneuvers) should the front tires "shudder" as if the vehicle is in 4WD??

I'm of the opinion that since the rear wheels are not slipping, no power should be going to the front wheels and the truck should not be shuddering in tight turns. Right now it behaves exactly the same way that it does in 4HI.

Is this normal or should I bring it in for a warranty repair?

Thanks - Great site!!

[the roadie]

If the truck is in A4WD, and you make sharp slow turns (parking lot maneuvers) should the front tires "shudder" as if the vehicle is in 4WD??

I'm of the opinion that since the rear wheels are not slipping, no power should be going to the front wheels and the truck should not be shuddering in tight turns. Right now it behaves exactly the same way that it does in 4HI.If your tires don't shudder at all in 2WD, then you can be sure it's not just steering geometry. A4WD should bias the transfer case so there is little torque transfer back to front, but the clutch pack is close to a fuller engagement. This calibration might be off a bit, or the transfer case fluid might need changing, or it could be your current tires have more traction than the stock ones the software was designed for. You could also have a failure of the transfer case clutch where it transfers more torque than designed, or it detects wheel slippage where it shouldn't. But those failures should be setting codes to give you a hint that something's more wrong. I'll try A4WD myself tonight in the parking lot and let you know what I hear.

I was curious a while back about why the vehicle shudders in tight turns when in 4HI or 4LO. Presumably the rear differential isn't locking up because of the 100 RPM threshold to activate the G80, so it's not shuddering because of left/right tire skid. The front differential is fully open, and can't cause left/right skidding. The answer turned out to be that the point the vehicle turns about is in line with the rear axle, so the front differential is farther from that point than the rear. The average RPM of the front axles has to be higher than the rear because the front differential describes a longer arc during the turn. But the transfer case tends to try to lock (transfer the most torque) the front driveshaft to the rear driveshaft so something has to slip. And it seems the tires have less traction than the transfer case is sending their way, so the tires slip or shudder, in tight turns. I drew this up before. (Weiner dog long wheelbase vehicles have this effect even worse.)

http://www.roadie.org/envoyturning.jpg

[RichD]

Thanks for the detailed reply.

There is definitely no shudder in 2wd.

The first thing I did was replace all four tires from the originals to a slightly more aggressive all-terrain tire in the same size. - No difference.

-Rich

[RichD]

I do not believe him.

Please send me some data. Do your trucks behave like this in A4WD???

[sanger35]

I had the same issue for the past 2 years. Only in AWD while accelerating. Not in 4HI or 2WD. Felt like the front wheels were engaging and disengaging.
Had the transfer case fluid changed and the problem dissapeared. (61K Mi). Note I found this online, and am trying to find the original note so I can thank them.

Just changed a left front wheel bearing too. Its not too hard to do. Now my truck is quiet again. Haven't heard the rattle from the roof rack when it gets cold. My last complaint is when the driver's window freezes shut like this AM. But all in all I love my SUV.

[Gunz]

Sanger 35,
What is involved in changing the front wheel bearings? You have to remove the entire brake assembly? Did you hear a specific noise on bearing failure, or did you do it because the wheel was pulling? I may be doing it on mine. How many miles was on you truck before you changed the bearing?

[kguyot]

I had this same issue and thought it was the actuator that enages the front Diff to the front drive shaft. Seems I let it go to long and now it doesn't even engage.

You hear the transfer case shift into A4WD or 4HI or 4LO and you hear the actuator enagage the front diff but the front tires don't move. You just hear a ticking/clicking noise as the drive shaft spins and the front tires don't. Its as if the diff just isn't quite locking into the drive shaft.

I'm taking it into the dealer Monday hope they can figure out what it is.