Changing rear diff fluid

[HiHoeSilver]

Correct, GM describes the unit as having locking and limited slip features. The reason GM does not recommend adding a friction modifier, the Limited Slip Additive, is because the fluid already contains it. I had old bottles that said it contained the additive and the new ones don't say anything. Same part numbers and new part numbers.

12345836, 12378261, 89021677, 88900401

The 2nd number above was called for in 2000, the 3rd in 2009 and the last one is current as of this writing.

Bingo was his name-o

 

[Vegasmarc21]

Well guys sorry for the late response, but I went ahead with the differential fluid change (mobile 1 75w90 synthetic with LS and it rolls much, much better....if brand new is 100 percent smooth, I went from it being 25% smooth to about 75% smooth. Money is very tight right now, so I did the fluid change "half baked". Yes you guessed it, I sucked the fluid out with my harbour freight suction pump, which I got out about just under 2qts. Here is the shameful part, I only bought 2 qts of the new fluid and used about .3 to .4 quarts of the fluid I sucked out to top it off...I know, I know....I'm guessing if I bought a new gasket $40 bucks, new bolts (someone said it's a good idea to use new bolts plus blue thread locker both probably $10 bucks)
The Tahoe would I'm guessing roll closer to 85% smooth....I still get a that rear passenger wheel shudder....but I rolls much better and shudder happens much less...could it be the traction control coming on and off? Anyone?

 

[swathdiver]

I bought a gasket for $5 from Amazon and didn't even need to use it. I see no need to replace the bolts unless corroded, let alone put thread locker on them. The last time ours was opened up we only used 2 bottles and skipped the little extra. Too much fluid does more harm than good.

Now addressing your shudder, you may be dropping that differential cover after all. Could be a wheel bearing. If you can put the rear axle up on jack stands and let it idle in drive and listen for any unusual noises with the shudder you might pin point the problem that way. Could even be a parking brake shoe.

 

[Vegasmarc21]

I bought a gasket for $5 from Amazon and didn't even need to use it. I see no need to replace the bolts unless corroded, let alone put thread locker on them. The last time ours was opened up we only used 2 bottles and skipped the little extra. Too much fluid does more harm than good.

Now addressing your shudder, you may be dropping that differential cover after all. Could be a wheel bearing. If you can put the rear axle up on jack stands and let it idle in drive and listen for any unusual noises with the shudder you might pin point the problem that way. Could even be a parking brake shoe.

Before I changed out fluid, I went to a reputable wheel alignment shop and they put up the rear end and just like you said spun the tires in drive...the shop owner said everything looks great, driveline, u joints suspension bushings....he said it was rear passenger tire was so bad he thought I had a bad axle, so I went to les schwab to get new tires, they jacked up the rear end for me and I stood there as the tires were spinning and then the right tire would stop...he said it was because of the traction control....? Anyway the tires were good....but I did spin the supposedly bad tire and it was hard to turn when it was in neutral. I always suspected the parking brake was the culprit...who knows...my wife has told me numerous times the back passenger tire feels like it's skipping..which I agree..but why?
1. Is it brakes?
2. Is it parking brakes?
3. Is the damn traction control thinking it needs to turn on and off?
4. Clutches in rear diff? By the way my RPO code shows it's a GU4 not a G80 diff

 

[Vegasmarc21]

You DO NOT put any limited slip additive in the gear oil. The G80, if you have it, is not your run-of-the-mill limited slip that requires friction modifiers. Use straight 75W90 synthetic.

I looked, and my RPO code is GU4 not G80

 

[HiHoeSilver]

I looked, and my RPO code is GU4 not G80

Gu4 is the 3.08 ratio. That doesn't tell you if it's posi or not.

 

[swathdiver]

Before I changed out fluid, I went to a reputable wheel alignment shop and they put up the rear end and just like you said spun the tires in drive...the shop owner said everything looks great, driveline, u joints suspension bushings....he said it was rear passenger tire was so bad he thought I had a bad axle, so I went to les schwab to get new tires, they jacked up the rear end for me and I stood there as the tires were spinning and then the right tire would stop...he said it was because of the traction control....? Anyway the tires were good....but I did spin the supposedly bad tire and it was hard to turn when it was in neutral. I always suspected the parking brake was the culprit...who knows...my wife has told me numerous times the back passenger tire feels like it's skipping..which I agree..but why?
1. Is it brakes?
2. Is it parking brakes?
3. Is the damn traction control thinking it needs to turn on and off?
4. Clutches in rear diff? By the way my RPO code shows it's a GU4 not a G80 diff

So no G80 code? Then you have an open rear. Ok, I would think that if the Traction Control or Stabil-a-Trak were activating it would trigger the light on the dash to let you know it's on. A bad caliper or worn parking brake could cause the wheel to stop you'd have to take the wheel off and pull the caliper and drum in hat rotor to inspect/adjust the shoes. Might need to replace the caliper/pads and or the parking brake shoes.

I would think that your truck should have both codes, GU4 and G80. If the differential carrier is worn, you can rebuild it, replace it, or buy an entire salvage yard axle and swap it out. The latter option is often the least expensive.

Let's see what the rest of the fellas on here think.

@gpracer1

 

[Meccanoble]

Definitely get a cover with drain plug. You’ll never have to remove the cover again to change fluid.

View attachment 205725

From what I was told, you want to remove the cover anyway. There is a magnetic piece inside the cover that holds any shavings and you wont get rid of that when you ONLY change the fluid. TAking the cover off and properly cleaning the cover and extra garbage is recommended. After you changed the fluid in the transmission, you'll enjoy the simpler process of unbolting a diff with nothing really in the way

 

[Vegasmarc21]

So no G80 code? Then you have an open rear. Ok, I would think that if the Traction Control or Stabil-a-Trak were activating it would trigger the light on the dash to let you know it's on. A bad caliper or worn parking brake could cause the wheel to stop you'd have to take the wheel off and pull the caliper and drum in hat rotor to inspect/adjust the shoes. Might need to replace the caliper/pads and or the parking brake shoes.

I would think that your truck should have both codes, GU4 and G80. If the differential carrier is worn, you can rebuild it, replace it, or buy an entire salvage yard axle and swap it out. The latter option is often the least expensive.

Let's see what the rest of the fellas on here think.

@gpracer1

I don't think the differential carrier is worn, it drives prrtty smooth until it gets to operating temperature, then once in a while when in a high gear, like 5th or 6th and 1100rpm it gets jittery, then when I give it more gas, it goes away...ive been down the torque converter shudder in my other post, it maybe that, but the dragging parking brake makes sense, because I'm thinking, the passenger rear wheel seems to skip while the jittery/shudder starts to happen as if the parking brake the the TC lockup are having a tug of war...i don't know, my 2004 z71 Tahoe i had 13 years ago gave me not 1 problem. Maybe it's the 6l80 transmission software, maybe I just need to have it updated since I read somewhere a guy had his updated and it didn't lug (lug like someone driving a manual is not smooth on the clutch and is in too high of a gear for the speed he is going) I'm just getting tired of it and spending too much time thinking about it. At least it rides smoother now that I changed diff fluid. Going to try to take the traction control stabilitrac fuse out today to see if it eliminates the skip/drag tug of war of the passenger rear wheel I'll let you guys know.

 

[Vegasmarc21]

So if the rear end is up and in in drive, then the wheel should keep spinning not stop right....and when it tried by had to turn it when it when in neutral, it was much harder to turn than I was expecting, in fact it if I spun it like the wheel on wheel of fortune, Bob Barker would tell me to try again because I couldn't spin it, not even 2 degrees out of 360 degrees.[/QUOTE]