Full time 4WD [Archive] - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

: Full time 4WD


citori3
02-08-2006, 10:32 PM
In full time 4WD when I pull away from a stop in the snow, the rear wheels spin and then the front wheels kick in with a "thump." It's pretty noticeable. This doesn't happen if I pull away slowly. Is this normal

Dave
02-08-2006, 10:57 PM
The transfer case could be tranferring most of the power to the front axle since your rear axle is slipping. It probably doesn't slip when you start out slowly. I know I drove around in 2hi in snow without much of a problem, in a4wd I would notice the jerking. I only use 4hi when it is deep snow or heavy slush.

A locking rear makes a big difference though. It is very easy to lose traction in a non-locking rear TB/EN. All you need is some wet leaves under one tire and you are not moving fast anywhere. The locking rear at least takes care of that problem.

Dave
02-08-2006, 10:58 PM
In full time 4WD when I pull away from a stop in the snow, the rear wheels spin and then the front wheels kick in with a "thump." It's pretty noticeable. This doesn't happen if I pull away slowly. Is this normal

Welcome to the site by the way!

citori3
02-08-2006, 11:14 PM
By the way, I love everything else about this vehical.

ieatglue
02-08-2006, 11:24 PM
I noticed that too. I think it's normal though, GM vehicles :duh:

ghoster
02-08-2006, 11:28 PM
I noticed that too. I think it's normal though, GM vehicles :duh

Can't speak for the TB, but my old chevy truck was the same way. I kept it in 2hi so it wouldn't auto shift. If I slipped, I would just throw it in 4hi. Come to think of it....I don't remember ever using the 4lo. Anyway, thats my take on it.:cool:

citori3
02-08-2006, 11:34 PM
By the way, I love everything else about this vehical.

ScarabEpic22
02-09-2006, 12:15 AM
Welcome!

I have the same thing happen too, I just leave it in 2HI unless I know it could get slippery then its A4WD, but if Im actually in snow (2+in) I will just go ahead and put it in 4HI to be safe.

Bearcat
02-09-2006, 08:52 AM
I have the locking rear, and have never noticed it. It most likely is the transfer case applying all the torque to the front set of wheels.:iagree:

ltz03
02-09-2006, 02:41 PM
I notice a small engagement noise under hard acceleration in A4WD in the snow. When the truck is in 4WDhi there I can't hear anything it should always be on. and welcome to the site, too

Dacomputernerd
02-09-2006, 03:23 PM
By the way, the trailvoys do NOT have full time 4wd (AWD).

They have a 4 wheel drive system. This means as follows.

2HI- Rear axle is powering the vehicle. Front is disengaged. If a G-80 locker is present, it will correct slip by sending power to the rear wheel with the most traction.

A4WD- Rear axle is powering the vehicle. Front axle is engaged, but no power is being sent to the front wheels. When slip is detected at the rear wheels, the front wheels engage with that "thump" sound you hear, and they help pull the vehicle until the rear gets traction.
If a G-80 locker is present, it will engage first, but if there is still not enough traction, then the front wheels will engage.

4HI- All four wheels pull the vehicle. Tight turns will result in a vibration felt, also known as "tire hop"
If a G-80 locker is present, it will help correct any slip at the rear wheels.

4LO- All four wheels pull the vehicle. Torque is multiplied by three. Tight turns will result in "tire hop". To be used at slow speeds only.
If a G-80 locker is present, it will help correct any slip at the rear wheels.



Please note that A4WD and AWD are completely different.

In an AWD system, all the wheels are providing traction, all the time. The computer monitors wheel speeds and driver input to change the amount of power going to each wheel to prevent "tire hop". There is no switch to manually control where the power is going.

In the TrailVoy's A4WD setting, the rear axle is powering the vehicle. the front axle is engaged, but no power is being sent to the front wheels. When slip is detected at the rear wheels, the front wheels engage with that "thump" sound you hear, and they help pull the vehicle until the rear gets traction. (To my knowledge, tire hop can occur in this setting if a tight turn is experienced, and the rear wheels slip, but the front still have traction)


The front axle is open, meaning that if one front wheel loses traction, the power will be sent to that wheel that IS slipping. Also the rear axle will behave the same if no G80 Locker is present.

Also note that the G80 Locker will NOT engage over a certain speed (25MPH?)

The transfer case is open, meaning that at least one of the rear wheels MUST have traction for the front wheels to grip. Also there is NO fixed torque split



Hope everyone learned something!:yes:
(Man that was a lot of typing!)

Dacomputernerd
02-09-2006, 05:31 PM
-Bump-:x

the roadie
02-09-2006, 05:45 PM
If a G-80 locker is present, it will correct slip by sending power to the rear wheel with the most traction.

With the Roadie being compelled to add: The G80 doesn't activate instantly since it's not electronic. It senses the RPM difference between the two axle shafts mechanically, and activates a clutch to lock them together. The RPM limit is 100 RPM, which means you actually have to allow a bit of wheel spin to get it to activate. HOWEVER: You don't want to just floor it with one wheel in the air or on ice, since that wheel can get up a lot of rotational momentum, and when the locker locks up, it might shear off some tangs on a clutch plate that's designed to fail before something else breaks.

When I end up balanced on three wheels on a rutty trail, I have to be careful how I engage the locker. Too tentative on the gas, and the delta never gets to 100RPM and the locker ignores me.

A4WD- Rear axle is powering the vehicle. Front axle is engaged, but no power is being sent to the front wheels. When slip is detected at the rear wheels, the front wheels engage with that "thump" sound you hear, and they help pull the vehicle until the rear gets traction.
If a G-80 locker is present, it will engage first, but if there is still not enough traction, then the front wheels will engage.

The slip that's being detected is in the transfer case, between the front output shaft and the rear. If an RPM delta is detected there, the transfer case clutch is commanded to share torque more between the two shafts. This is electronic, and the rear G80 may or may not engage first depending on if its trip point is met.

Yes, subtle difference, but it's all in the factory manual. :sleepy:

Also note that the G80 Locker will NOT engage over a certain speed (25MPH?)

Correct. And also note that if engaged at low speed, it will disengage as you go over 25MPH.

Dacomputernerd
02-09-2006, 06:10 PM
Now everyone knows the details of the 4x4 system on the trailvoy!


Thanks for elaborating :thumbsup: I never knew that the rear locker could be damaged if too much throttle is used. Good to know :yes:

BTW... The Roadie has the best envoy for offroading :hail: :yes: :D :thumbsup:

bmm354
02-09-2006, 06:29 PM
I notice a small engagement noise under hard acceleration in A4WD in the snow.

Same here - in some cases I feel a thump too. In 4HI it doesn't do anything like that as the front axle is always engaged. In my Envoy it's not bad by any means, but it can be noticeable. I think it's pretty normal for any torque-on-demand (automatic) 4WD drive system. The front axle engagement was quite noticeable on a '95 Explorer I once drove also.

ScarabEpic22
02-09-2006, 10:46 PM
DaComputerNerd basically said what I would have said, but he beat me to it this time.:laugh:

But, I thought the locker locked at 20, not 25? Whatever, not really that important to me, but it sure does help in snow and rain.:yes:

the roadie
02-10-2006, 12:12 PM
But, I thought the locker locked at 20, not 25? Whatever, not really that important to me, but it sure does help in snow and rain.:yes:

For the archives, you're correct. I typed from memory but just checked the shop manual. If locked, the G80 will unlock at 20 MPH. If unlocked over 20 MPH, differential wheel spin won't lock it, and it acts like a mild limited slip.

Dacomputernerd
02-10-2006, 03:52 PM
For the archives, you're correct. I typed from memory but just checked the shop manual. If locked, the G80 will unlock at 20 MPH. If unlocked over 20 MPH, differential wheel spin won't lock it, and it acts like a mild limited slip.

:thumbsup: Sweet! thanks for the facts :yes:

johnner1999
02-16-2006, 09:34 PM
The transfer case is open, meaning that at least one of the rear wheels MUST have traction for the front wheels to grip. Also there is NO fixed torque split



Hope everyone learned something!:yes:


this is a great wealth of knowledge !!!!

Does the above quote mean that when in a4wd mode the system can transfer all if not most of the torque to the front wheels? which if true is pretty cool; but seems very odd for a "old-school" system.


I have to say though for such a heavy suv (I use to have a Isuzu Rodeo) it makes it up my short yet steap drive way in snow with no issue. my rodeo would always need a a running start or use 4lo. I also had the same Mich CT tires???

oh well


love my fake envoy :-)