Locking rear in 2001 GMC

[Chris Jones]

I've got what GMC refered to as a locking rear in my 2001 Sierra 2WD. I had expected a switch to lock it but I have nothing still I know it locks. I've tried but have never been able to find any information on how it works. It definitely behaves like it's locked not limited slip. Take for instance the day I backed off a paved drive into a rough area to have mulch loaded. I hit a slick spot and one wheel spun for a second then I get that clunk and back it goes. I got loaded and pulled forward unto the pavement turned hard to leave and it was actually slidding the front wheels a bit as they tried to turn the truck. It did turn but it was a fight. The inner back wheel scuffed and scraped the whole time. And I was just crawling. That's the way this rear works. It requires a spin of a certain speed or time and I hear a clunk and either it moves out or both start spinning. Once locked it remains locked until it is driven straight for a bit. Another time it locked backing onto a paved parking space off of grass at work. Later that day I went to leave not having to get on the grass again but as I turned out of the space it was locked. Being unloaded this time the rear didn't put up much of a fight--just a hint of push and the inner rear chirped away. I remember asking someone at the dealership about how it worked and he said like the old posi-traction rears, but personally I don't think so as I think they were limited slip.

Anyone here know what's in there and how it goes about locking it up?

 

[buickanddeere]

Sounds like it's working correctly. There is a centrifugal weight that flings out when one wheel out spins the other wheel by several mph. This engages the friction clutch pack that limits the slippage between the rear wheels.
This system wears the clutch plates less while turning normal corners. And engages tighter than the "full time" posi clutches.
I put a Detroit Locker in my Grand National. Now that's an interesting system to drive. Only the inside wheel drives on turns, unless it's slipping on the ground.
For most applications instead of repairing or installing a factory style posi. The Tru-Trac system is the hands down winner. Used by the US armed forces in the Hummer etc.

http://www.ringpinion.com/ProductDetails.aspx?ProdID=971

 

[L.C.Gray]

Have you had the fluid changed in the differential sometime in the past? If you put regular grease in a locking differential thats got clutches they'll get sticky on you.

Had that same type rear end in an earlier model GMC, but my problem was after many highway miles the rear would be locked up like a spool and would be hard to steer and chirp the rear tires on turns.

I changed the fluid and made sure it had the additive for "posi" units and had no more problems. You can also buy the posi additive (anti-foaming agent) by itself and add to your existing fluid. I suppose it is possible for the additives properties to be spent and need replenishing.

My experience with the GM locking rear end is that it is a good unit. I've had them in the last several trucks I've had and they've been reliable and work very very good. I wouldn't make any drastic moves before just trying the fluid change.

 

[Anonymous-0]

It sounds like it's working to me also. A locking diff. is better than a limited slip for maximum traction. Not the best for corners. Posi-trac is GM's term for a limited slip diff. Dave

 

[Chris Jones]

Your explanation sounds right to me. I've just never see anything like that explained in how differentials work and it's been puzzling me. I guess I should have noted I don't think anything is wrong with it--it's been this way since new.

P.S. I don't drive my truck much--especially with gas prices as they are now. I got it in November 2000 and currently it has about 17,000 miles on it. I wanted an old truck for truck stuff and in 1999 I bought a 1977 F100 only to find it was a money pit mostly because someone had tinkered with it trying to make it a hot rod. So I sold it and purchased this one new and intend to keep it until the wheels fall off for whatever I need to haul or tow. This way I will know everything that has happened to it during it's life and it will have been cared for. Only down side is It's harder to pitch bricks into it--course I don't really do that even with the old F100 I remember telling my father just because it was old didn't mean it needed to be beat up any worse as he threw something up against the side of the bed.

While I don't miss the hot summer in the F100 without A/C, I do strangely kind of miss the 3 in a tree trans and that rough old monster look and feel. I had quite a few guys ask me in parking lots, "Dude, what year is that truck?" Heck even one lady at the office told me when she saw me get out of that truck her whole image of me changed--though I don't think it was for the better. Still I used her comment to convince some of my coworkers who were teasing me that it was a chick magnet.

 

[glennster]

stop at yer local chevy-gmc dealer and ask parts dept for the limited slip differential additve. comes in about a 4 oz bottle or so, pull the fill plug and add to differential.

 

[Doug N]

Chris, I'm not 100% sure how they work, I've gotten two different stories. But your's is working fine. I have an '02 with locking rear. I love that thing. I had a '94 GMC 4WD with 6.5 turbo, and I'll tell you, in the snow my 2WD '02 went more places than that '94 did. Love that rear.