99 GMC Rear End Issue

[Anonymous]

I own a '99 GMC 1500.
Sometimes the rear-end locks in positive traction and stays that way for a while. When I make turns I can feel and hear that one tire is turning slower than the other.
It eventually returns to normal after driving for a day or so.
I do not want to put any more $$ into this truck-will ignoring this issue hurt the truck?
Any advice will be appreciated.

 

[Earl Thigpen]

Mr. Sugarman,

A P-Trac has clutches that preload the spider gears in the rearend. The P-Trac is therefore, "on" all the time.

When cornering, one wheel wants to travel faster (or slower) than the other and will overcome the preload on the spider gears allowing the wheels to turn at different speeds. This is the way its supposed to work.

To keep the preload at the proper setting and everything operating smoothly you must run an oil in the rearend specifically designed for P-trac's. The oil has special friction modifiers that allows the clutches to slip while cornering and yet not self destruct. Used to be the oil or the additive was ONLY available at GM but now I think the formula has gotten out and it is probably available at your nearest Jiffy Lube. I would check with you GM dealer to find out what the spec number is and make sure the place you go has oil that conforms to that spec.

Oh yea, one more thing. With P-trac you must run wheels and tires of the same size across the rearend. If you don't the clutch pack will be in the slip mode all of the time and will quickly wear out.

Long winded but hope this helps.

 

[Bullbuyer]

I think this is a simple fix. We have been throught this with several Chevy pickups. Change the oil in the rear end and that will take care of it. All you have to do is remove the cover (why there isn't a drain plug is beyond me) let the old gear oil out, put some gasket maker on the cover and put back in place. It will take about 2 quarts or so of 80 w gear oil to refill and the secret is to get a small bottle of gear oil treatment for locking differential. This is located right beside the gear oil at your local auto supply place. This additive is what keeps the differential from clacking, smacking and just generally making alot of noise. A word to the wise - this additive stinks to high heavens so be careful.
It has been a year or so since I've changed mine so there may be a new gear oil WITH the additive. Anyways, the first time we had one do this was back in '84 with a brand new one - changed the oil once and never had another problem. We do it once with any truck we buy and never really worry about it again - until we find ourselves driving in a creek or other deep water and then we try to get that old oil out as soon as possible.
Not a real high-tech job!

 

[Anonymous]

Thanks guys for the input.
Sounds like this may be a cheap fix after all.

 

[gerardplauche]

Sugarman - I have another cheap suggestion. Quit tearing your truck up. Try learning how to drive like you have sense and stop with the burn-outs and candy canes.

 

[dyates]

Like some of the other replies said, sounds like you got water in your diff. Clean it out and change the oil.

 

[Bullbuyer]

So, did we get 'er fixed or what?

 

[Anonymous]

I guess the new oil (Royal Purple 75w90) is working.
It has not been making the noise since.

gerardplauche,
I must admit that I could not resist powerbraking and burning both back tires in the O'reilly Auto Parts parking lot after I bought my new gear oil. I don't ordinarily tear up my truck- but this was an exception.

 

[Brute 23]

Change the fluid or just check the fluid, it might be just low.

NO one has a posi-trac from factory. Its a limited slip. If you keep running it like that it will lock up for good and then it will only be a matter of time before you break teeth.