Rear Diff Clunk

[neonhomer]

Well, I noticed it first today when I had to shoot across 5 lanes of traffic. I jumped into the throttle real hard, and got a little wheelhop/axle wrap out of the back. No biggie there. I get down the road a bit, and have to make a left turn. Chirp-chirp-chrip from the tires through the corner.... the rear diff is acting like a spool. I come to a stop, go to Neutral, and back to OD. When I tap the gas to take off, I hear a clunk and everything is back to normal.

 

Same thing happend a few hours later... I was backing out of my sister's yard. It was a steep angle to get out, so I had to give it a little throttle. Heard a clunk and backed up. When I went to straighten up, I was getting the spool effect again. When I took off in OD, it stayed locked until I came to a stop to make a turn.

 

So, it sounds like the rear diff is locking up and then not wanting to unlock readily. If anyone asks, I have the G80 Positrac rear axle w/ the GU6 3.42 rear gears. Truck has about 78,000 miles on it. Was planning on changing gear lube in a couple of weeks.

 

From what I gather, the truck was used to pull a trailer early in it's life (before I got it). Shortly after my purchasing it (I bought it with 32k on it, at 34k) the tranny had to be rebuilt. Ever since then, the truck is been mostly on the highway, with occasional pulling of a 1,000 lb trailer. So I am wondering if the rear end might be wore out a little.

[05GMC4DR]

I think the locker is connected to the Traction Control and I think the locker is electronic. If you have the POSI it may need new clutches. I know that the tracloc found in Jeeps have to have the clutches replaced every so often. I would change the fluid and check to see if it has any form a grit to it.

[burnthefurniture]

the g80 is a mechanical locker that locks when it senses a difference in wheel speed. Often it locks pretty harshly, as it requires something like a 20 rpm difference in wheel rotation IIRC. That is the clunk you're heading, is the locker locking and unlocking. It is mechanical so no electronics, and its a full locker, so no posi or clutches. It may be worth changing the fluid regardless, but my truck which I bought new and don't rag on makes the same noise and acts the same way after being locked. Usually, if I just keep driving forward after it is locked, it will drag or chirp one tire. If I shift into reverse or change gears, it unlocks. My truck only has 35000 on it, and has done this from day one, so I would say its normal, but still check when you change the fluid. I would not be overly concerned though.

[2000DV4.8]

It locks when there is a 100rpm difference between rear wheels.

[neonhomer]

I just wanted to double check. Now the $64,000 dollar question is does it use just oil, or do I have to add something to it? Planned on running either Mobil 1 75w-90 or something like that.

[dlstewart01]

I just wanted to double check. Now the $64,000 dollar question is does it use just oil, or do I have to add something to it? Planned on running either Mobil 1 75w-90 or something like that.

 

Synthetic is required and GM says that NO additive is nedded for the G-80.

 

Have a good one,

 

Don

[Z45]

Clunking noise is common in G80 lockers. The fluid you use may have an affect on this but usually the more you horse it, the more the noise. That said, I use Amsoil Severe Gear 75W-90 in my truck and I don't usually hear any noise. Matter of fact there is 2 inches of heavy wet snow this am and in AUTO I heard no noises driving around this am and I spun my tires a few times.

 

 

From GM info;

The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.

[burnthefurniture]

It locks when there is a 100rpm difference between rear wheels.

 

Ah thanks for correcting me thats even harsher haha

[markw8]

I use Mobil 1 have never had any noise. Mark.

[Got V8?]

neonhomer, I was getting this type of effect when I would drive over wet leaves in a parking lot in the fall, it was almost like right after the tire spun and I hit dry pavement it wouldnt unlock unless I slammed on the brakes or backed up. I have since cahnged the rearend fluid in my G80 with Royal Purple synthetic, and have not heard any clunking at all and have not had any sticking,, and I was just driving on steep icy hills in 2WD and not having a problem, so I know that it is working fine. Mine had about 48k on it when it started doin this and I soon afterwards changed the fluid and at 58k have not had the problem since.

[burnthefurniture]

neonhomer, I was getting this type of effect when I would drive over wet leaves in a parking lot in the fall, it was almost like right after the tire spun and I hit dry pavement it wouldnt unlock unless I slammed on the brakes or backed up. I have since cahnged the rearend fluid in my G80 with Royal Purple synthetic, and have not heard any clunking at all and have not had any sticking,, and I was just driving on steep icy hills in 2WD and not having a problem, so I know that it is working fine. Mine had about 48k on it when it started doin this and I soon afterwards changed the fluid and at 58k have not had the problem since.

What weight RP did you use?