Rear Differential Fluid

[tgreen]

I get a loud thump from the rear end of my 72 when I pull away from a stop. I read that it is likely from the posi-traction sticking and I should change the fluid. What is the best way to do this? Any help would be much appreciated. Thanks.
Ted

[pws69]

Originally posted by tgreen
I get a loud thump from the rear end of my 72 when I pull away from a stop. I read that it is likely from the posi-traction sticking and I should change the fluid. What is the best way to do this? Any help would be much appreciated. Thanks.
Ted

You need to get a suction pump at your local parts store as well as some Mobil 1 (obviously MY personal preference) and a bottle of GM Posi Additive.

Then go to it! Suck the old junk out through the fil hole, and squirt the new stuff in. After you squirt about a cup of the Mobil 1 in, put the additive in, then fill 'er up with Mobil 1.

[TWINRAY]

Personally , I'd use GM Rear Gear lube with their Posi additive. Been working for me for 30 years

[GTR1999]

could be the fluid but also check the front diff bushing and U joints for play. To change the fluid jack uop the rear remove the right rear tire and use a long extension ans 11/16 12 pt socket to remove the fill plug. Go to a parts store and get a siphon pump and draw out the old oil. Add a bottle or two of posi additive and then fill with gear oil until the oil weeps out the fill hole. The clutches usually knock going around corners when hot. Use only the GM additive as others don't always work. Use some teflon tape or Permatex #2 on the plug when you're done.
Look for rust stains around the Ujoints or any play in them. The rubber bushing/front of the differential shouldn't have play up/down.

[tgreen]

Thanks guys! I will jack her up and check the U-joints for play tonight.

[crause]

Good luck with the socket. I tried every socket in my tool box on my 75 and nothing would hold it tight enough to turn. I went to Sears today and bought one of those "gator grip" sockets ($10) that you see on TV. They have the pins that are suppose to grip any bolt. I will let you know how it works. Mine is making the same sound yours is and I was told the same thing. I went to my local Chevy dealer and bought a bottle of the additive.

Andy:s

[clar2001]

open end 16mm. wrench socket fits perfect and easy to ratchet off.

[Ron Miller]

If memory serves me correctly, it takes two bottles of the GM additive, not one.

rlm

[Dave L.]

Ron - It suggests(GM shop manual) to use 1 additive (GM#1052358) and 3 oil (GM# 1052271),but like you said I always put in 2 additives first and then fill with the oil.

Dave

[tgreen]

I got the fluid changed and for the most part the loud thump when pulling away from a stop is gone. She seems to run much quieter now. If I try I can get some noise from the rear end at speed by letting the car coast a bit and then stepping back on it. U-joints? It is not much of a noise, just seems to me to be the drivetrain re-engaging.

[clar2001]

Very well could be a u joint, put your foot on the brake & drop it in gear do you hear a clunk ?

Changed mine yesterday. Removed the old with a large (2.5 OZ.) syringe and a tube. Pulled almost 3.5 quarts out (dark but still "clear" in the thin tube). Added syn blend for limited slip plus a bottle of additive. WOW what a difference, no clunk, quiet and smooth.......

[Bob Chadwick]

Originally posted by clar2001
Very well could be a u joint, put your foot on the brake & drop it in gear do you hear a clunk ?

This clunk can also be the front differential bushing.