1500 Hd 4wd System

[TyeeMan]

Gentlemen,

 

I posted this in the heavy duty truck forum but I'm not getting too far so I'll try here.

 

I have an 02 GMC 1500 HD with the push button 4WD. I noticed that I'm getting some gear oil coming out of the vent on the front differential, this was after a 2 hour drive at say 65 mph, I engaged the auto 4WD for just a few miles then disengaged it, this has not happened in previous winters when driving in 4 WD. What could be causing This?

 

How can I tell if the front end is "unlocking" when the truck is in 2 WD mode?

 

When the truck is parked in 2WD if I grab the frond drive shaft should I be able to turn it by hand?

 

What exactly does the actuator on the front differential do? How can I tell if it is "deactivating"?

 

For my own curiosity could you please go into some detail and explain what happens in what order when I push the 4WD hi button? I know there is a slight difference in the transfer case between 4 hi and Auto 4 WD. I just like to know how things work.

 

Thank you in advance for your time and information, it is greatly appreciated.

[gmtech4]

alot of questions I'll work downwards lol

 

1--the frt diff vent may be stuck,it is a aluminum cover that swings and covers the vent tube leading to the rubber hose ,there was a problem with them and a change up is available at the dealer ,it is located above and forward of the the left frt axle shaft ,wrench it out .DO NOT crossthread the new one.

 

2--in 4wd the truck will buck turning slow on pavement,and won't in 2wd

 

3--if you have an auto 4wd (sounds like you do)because of the clutch pack in the t-case you may be able to turn it in 2wd,there was some yrs that you cannot,don't worry about it

 

4-- checking the donkey dink can be done by reading the 1st answer or take it out and hit the switch ,it will go out and hit the switch again,it will retract,

 

5--hitting the 4wd button moves the shiter fork engaging the syncro'd gear and chain to the frt output,at the same time the detent plate inside (connected to shiter shaft) closes the ball switch and powers up the donkey dink that moves a collar to lock the right frt diff axle shaft to the right spider gear,,I think the frt diff actuator also turns on the dash light,,,in 4lo the same thing happenes and another shift fork engages the plantary gearset giving the required gear reduction.

 

Auto 4wd has the clutch pack I mentioned earlier,when you hit auto 4wd the frt diff actuator engages the frt diff ,the t-case shift fork ramps up almost engaging the clutch ,,when wheel slip is detected thru the ABS system,the encoder motor moves the fork that little bit and poof you have 4wd .

 

if I'm off in anything someone will tell me ,i think I got it though lol

[TyeeMan]

gmtech4, thanks a bunch for the in-depth information!! That's what I was looking for.

 

I thought I read somewhere about a problem with the front axle vent but I couldn't find any data bulletins in alldata or anything. I will persue the axel vent replacement.

 

Hope you don't mind but I have a few more questions. Again, just want to totally understand how it works.

 

Please correct me if I'm wrong, so in 2WD going down the road, all that is spinning up front would be the two drive shafts to each wheel right? The ring/pinion are not moving at all.?

 

Would I know just by driving down the road that the donkey dink was not retracting?

 

Would I get some kind of trouble code if either the front diff or the transfer case was not dissingaging?

 

Sorry for all the questions, I'm the curious type and want to ensure all things are in perfect working order.

 

Thanks again for the great info!

[gmtech4]

2wd ,the left frt axle is connected to the left spider gear which turns the right one thru the ring gear ,the break is the right axle shaft is not connected(hard to explain ,but nothing is being transmitted from the t-case)frt diff is just turning inside ,it needs pinion torque to drive the diff

 

t case with a dash switch has a computor for the electronic shifting of the encoder motor at the t-case,and in auto mode to comunicate with LAN as mentioned.

 

a scanner (GMs TECH 2) shows data for frt diff actuator but driving down the road ?,no ,but as mentioned full turning slow ,yes(they buck and scuff and complain)so do not drive on hard pack in 4WD as mentioned in owners manual

 

and I'm spent ,to Timmies I go (canadians will know what I mean lol)

if the t-case can not complete a shift ,it will go back to previous mode and store a code and turn on the SES light thru the LAN.

 

doesn't the alldata show description and operation? I think it does or should .

 

I'm spent so off to Timmies I go (canadians will know what I mean lol)

[TyeeMan]

And I'm spent, , HAA!! Sorry I spent ya

 

Thanks again, I fully understand what you explained.

 

Perhaps I didn't look hard enough on alldata but a swear I couldn't find a description of how the system worked, I'll try again just for fun.

 

Have a good day!

[gmtech4]

you too