How does transfer case work?

[rodelco]

Hi, I am new to 4x4s with my recent purchase of an 04 TB EXT. I was wondering if anyone knows how the A4WD transfer case works. I am just curious about it, I don't have any problems (that I know of!).

I will just list some of the questions that I have been wondering about:

* I assume that the rear driveshaft is ALWAYS connected to the transmission (as long as the transfer case is not in neutral); is that correct?

* If the transmission is in park, (and again, the transfer case is NOT in neutral), is the rear driveshaft locked (can't turn); so as to keep the car from rolling?

* When in the A4WD mode, does the front driveshaft turn whenever the vehicle is moving, or only when slippage in the rear wheel is sensed?

* Apparently I have an "open" rear differential. I have been told that means that when one wheel slips, more power is transferred to the other wheel (that is not slipping). Is that right?

* Does the front differential (if that's what you call it) work the same way?

* In 4HI mode, are all four wheels being powered, or just both axles?

Sorry if these are dumb questions. I am just curious and fascinated about how this all works.

Thanks for any help or information you can give me. If anyone knows of a good resource to read about this, that would be great also.

[PROSPHOTO]

:eek: that was a hell of an intro...

Try p.2-24:
https://www.mygmlink.com/pdf/go2content/manual/US/en/chevrolet/2k04trailblazer.pdf

This should answer most of your questions :D

Enjoy your new toy :woot:

[Gearhead2012]

Basically, you have the engine which goes to the tranny, and then to the transfercase. From there, the rear drive shaft goes to the rear diff, and the front to the front diff. Now, as long as your tires are turning, both driveshafts will be moving. When you put it in 4wd, or it senses slip at the rear wheels, and automatically engages the front wheels (when in A4WD) it engages power to that front drive shaft that goes to the front diff. So, instead of it just spinning freely in the transfer case, it now has power going to it. If you have an open diff...that means only one wheel will spin when traction is lost, and no, the front isn't that way on any of the trailvoys. Now, some of the trailvoys were equiped with an electronic locking rear end, which locks both rear wheels together when it senses one wheel slipping - however this is different from a limited slip, which comes in the SS's. When you get stuck with an open diff, in 2wd, one rear wheel will spin...and when in 4wd, one front, and one back. Now a locker, locks both rear tires together when it senses slip at one wheel, while an LSD delivers power evenly to both rear wheels. Hope it helps.

[MTPockets]

..as long as your tires are turning, both driveshafts will be moving..

The front prop shaft doesn't spin in 2wd mode on 4x4 GMT360's. The front axle disconnect removes the right side half-shaft from the front diff, so the input on the [open] front diff goes dead.

Joel

[topprolmc]

Now, some of the trailvoys were equiped with an electronic locking rear end


None were. Those that are equiped with Lockers, have lockers that are mechanical.

[Gearhead2012]

I didnt' mean to say electronic, i was reading about getting one put on mine, and had it on the brain. That's great about the front drive shaft not spinning, I didn't realize that about the GMT 360s, I didn't realize that.

[MTPockets]

That's great about the front drive shaft not spinning, I didn't realize that about the GMT 360s, I didn't realize that.

Yep, most all part time 4x4 trucks with drive flanges and a front axle disconnect are like this. You should be able to spin the front prop shaft by hand when in 2wd mode (she just free-wheels).

Joel

[ghoster]

Yep, most all part time 4x4 trucks with drive flanges and a front axle disconnect are like this. You should be able to spin the front prop shaft by hand when in 2wd mode (she just free-wheels).

Joel

Is this true with other 4wd?? Like my GMC K1500?? Just kinda curious. I am planning on getting under it this week for the first time.

[MTPockets]

Is this true with other 4wd?? Like my GMC K1500?? Just kinda curious. I am planning on getting under it this week for the first time.

My 1998 Chevy K1500 was like this, as was my isuzu and IIRC, my 1997 Jeep TJ was too. I think you'll find all newer PT 4x4's like this. True AWD's are a different story. Front prop shaft spins all the time on those.

Joel

[ghoster]

My 1998 Chevy K1500 was like this, as was my isuzu and IIRC, my 1997 Jeep TJ was too. I think you'll find all newer PT 4x4's like this. True AWD's are a different story. Front prop shaft spins all the time on those.

Joel


Well that will help then. I have plans of spending the day with a grease gun. Any idea how many points there are under there? I don't know what they used that I can actually lube. I will be looking in the usual places (ie U-joints, end links, etc.) Anything to look for thats not ordinary?

[MTPockets]

My 1998 K1500 had greaseable upper and lower BJ's, zerks on the tie rod ends & links for the steering and there was a zerk on the front prop shaft for the slip-yolk. Lots'o stuff to grease on those babies!

Joel

[rodelco]

If you have an open diff...that means only one wheel will spin when traction is lost, and no, the front isn't that way on any of the trailvoys. .... When you get stuck with an open diff, in 2wd, one rear wheel will spin...and when in 4wd, one front, and one back.

Gearhead, thanks for the information, although I still have a question if you don't mind. You said that the "front isn't that way on any of the trailvoys." I thought that meant that the on the front, that both wheels were powered when in a 4WD mode, but then you say that if you get stuck in 4WD, that only one wheel in the front spins also. Can you explain this a little further for me?


The front prop shaft doesn't spin in 2wd mode on 4x4 GMT360's. The front axle disconnect removes the right side half-shaft from the front diff, so the input on the [open] front diff goes dead.

Joel

Joel, I also want to thank you for responding. What is the "front axle disconnect" , and what is the "right side half-shaft"? I got underneath the TB this weekend and changed oil and just kind of looked things over. I saw the transfer case and the driveshafts, so I have a little better understanding of how this works, but obviously I still have a lot to learn.

Thanks again to everyone for your help (and patience)

[MTPockets]

rodelco, here's a pic of a half-shaft assy compliments of rockauto.com: http://www.rockauto.com/catalog/getimage.php?imageurl=http%3A//www.cardone.com/Imagesftp/60/601345-01.jpg

Here is a trail-voy front diff assy. The axle disconnect is the aluminum housing towards the upper left, at the end of the black shaft. There is a little 'black box' motor on it. The black tube/shaft is what runs through the engine oil pan: http://cgi.ebay.com/ebaymotors/05-TRAILBLAZER-CARRIER-ASSEMBLY-DIFFERENTIAL-1061_W0QQcmdZViewItemQQcategoryZ33731QQihZ004QQite mZ140001061671QQrdZ1QQsspagenameZWDVW

Joel

[rodelco]

MTPockets, thanks! The pictures help a lot. So, if I am finally understanding this correctly, the "half-shafts" are basically the two front axles (with the CV joints) that are "driving" the front wheels. Since the "disconnect" appears to be on the passenger side, does that mean that it is normally the "powered" wheel when in a 4WD mode? And, that if it slips, the differential in the front would then apply drive power to the driver's side?

By the way, I crawled underneath again tonight to see if the front drive shaft "free-wheels", and sure enough, it does, just as you said. This is a great forum; I really appreciate all of the information and expertise!

Thanks again.

[MTPockets]

Since the "disconnect" appears to be on the passenger side, does that mean that it is normally the "powered" wheel when in a 4WD mode?

No. Disonnecting one axle just takes the load off the differential when not in use (2wd mode). If by some reason the transfer case sent power to the front diff when the axle disconnect was open, all the power would go to the 'open' or right side and basically nothing would happen. Same principle as lifing one rear wheel off the ground with a non posi/LSD rear. The wheel with no resistance will just spin and you wont go anywhere.

Joel

[the roadie]

I thought that meant that the on the front, that both wheels were powered when in a 4WD mode, but then you say that if you get stuck in 4WD, that only one wheel in the front spins also.In 4WD HI or LO, both front wheels indeed get power (torque), but they only deliver torque to the ground if they both have traction. An open differential like we have in the front (non locking, non limited slip) means that if one of the front wheel slips, it will spin uselessly and all the torque leaks out (metaphorically). The tire on the ground will not help move the vehicle forward. This means more if you're into offroading and rock crawling - my hobby.

[Gearhead2012]

if you have an open differential...when you jack up both wheels at the same time, you can hold one wheel, and spin the other. The transfer case sends the power out, via the front drive shaft...past that, which wheels get power is dependent on the diff. like Roadie said, it also depends on whether the wheels have traction. In an open diff, if the wheels loose traction, only one wheel will spin, which ever is easier. Theoretically, if you were stuck, with one wheel on ice, and the other on dry pavement, you wouldn't move at all...as only one tire would spin. Obviously you'd move eventually, and sometimes with an open diff, power will shoot randomly between the wheels.

when you take a turn, the outside wheel must make more revolutions...so you can't lock the wheels together. in the 60's LSD's became popular, or positraction as it was called then. this meant that the engine still sent power to the differential in the same way, but the differential sent power to the wheels differently. it allowed the outside wheel to slip some, but also always gave power equally to both wheels. watch my cousin vinny...they talk about it in there a little. hope this helps.

[ScarabEpic22]

Yep, the roadie is right, but we dont have a LSD or locker available for our front diff yet. The H3 uses the same front diff, so there is hope we will see something for them that we can use too!!

[MTPockets]

A limited slip front would be a nice option, but IMO would tear up the current front drivetrain on our GMT360's. They'd have to be beefed up first.

Joel

[the roadie]

if you have an open differential...when you jack up both wheels at the same time, you can hold one wheel, and spin the other. Just one thing to add to this sentence - the G80 automatic rear locker triggers by RPM difference between the wheels. If you don't generate 100 RPM difference, it won't lock up. So it's basically impossible to detect it by manually spinning tires on a lift, where you could detect the presence of a LSD.

I posted a bit more about it last year here: http://forums.trailvoy.com/showthread.php?p=58122&highlight=g80+locker+rpm+100#post58122