Trebor

I was not going to change the gear oil on my '96 CE LT4 because the mileage is low (16,530) but I thought it was almost 14 years old and I have replaced all the other fluids. So I changed it today. The stuff that I siphoned out of the rear end was putrid. It was thick, black and smelled horrible. I researched the forum and found that what I needed was PN 89021671 and 2 bottles of limited slip additive PN 1052358. The dealership did not have PN 89021671 80W90 they only had PN 89021677 75W90. I bought two 32 oz bottles. After I got home I called another dealership and they said they had the 80W90. So I returned the 75W90 (it probably would have been ok) and bought the 80W90 which came in 23 oz bottles and did not say synthetic on the bottle (odd). So I used my floor jack and jacked up the rear one side at a time and slid my rhino ramps under each side (wasn't comforatble trying to back up onto the ramps). I slid under the car and found the hex plug very accessible on the side of the differential. I used a 3 eights inch hex allen wrench and found the plug was easy to remove. Some gear oil dribbled out. But I had to use my siphon pump and I removed about 1.3 quarts of the crud. I cleaned the pump out and placed one 23 oz bottle of the oil plus the (2) 4 oz bottles of limited slip additive into a clean tupperware bowl (more stable than trying to place the end of the siphon hose into the top of the bottle). I pumped in the bottle of the gold colored honey like substance into the diff. Then I got almost half but not quite of all the liquid gold of the other bottle in until it started running out of the hole. I then placed the plug back in and tightened it down. So even if you C4 has low miles like mine (or not) it might be worth doing. It was satisfying getting that mud out and replacing it with nice clean oil.

Trebor

pletzvet

I'm envious...there is no drain plug in my '85 and I'm thinking I should be doing the same, although I spoke to one previous owner who confirmed that he had all fluids changed about 8 years ago when he owned it. It's a job on my 'to do' list...just not looking forward to all the siphoning....

pappy.72

Did you open the drain plug to drain the fluid out or did you just pump it out the fill hole? For anyone doing this, make sure to open the fill hole first before the drain hole. If you can't open the fill hole when the fluid has drained out the drain hole you are screwed (pun).

Trebor

Actually there is no drain hole just a fill hole. There is a kit that you can buy but you have to drill into the differential and place a threaded dowl into the hole (something like that). I did not want to drill into the rear for obvious reasons. But the siphon pump worked well. And yes yo are right. I have heard of horror stories of people removing the drain plug emptying the oil only to find out the fill hole plug was not removable.

trebor

94z07fx3

I think pappy is giving sage advice for ZF transmissions whereas Trebor is discussing the differential.

For the diff, I've only done it a couple of times but I always add the two bottles of LSD additive. I have wondered if it was needed or not with the Mobil 1 gear oil I use and also if Gordon K. was just giving an off-the-cuff answer ("two bottles") or if someone has actually done an afternoon of figure eights to test the ammount of LSD additive that will prevent clutch grabbiness while not preventing the LSD from grabbing when it is supposed to.

Mr. Peabody

Even with Amsoil rear diff fluid I still add the the 2 little bottles of the aditive first before putting in the amsoil.

For my car it did make a difference. 1st time I had the diff fluid changed we didn't put in the additive and within 50 miles the tires were chirping when doing sharp turns into the driveway. Sucked some out ... put in the additive and then no problems

The gear oil is for the gears .... the additive is for the clutch pack.

PLRX

You can make you own drain hole Use the search tool for instructions or send me a PM.

kenv

FWIW- gear oil has a putrid smell from the getgo.

Trebor

Update for anyone thinking of doing this: Well in my typical fashion to get something done I screwed up. Although I researched the forum before doing the drain and fill, I missed the part about GL-4 versus GL-5. So I filled up with GM PN 89021671 80W90 which is GL-4. :o I checked my Owners manual and it says use 12345977 GL-5 (which has been replaced by 80921669 - which I could not find). So I went to Autozone and bought two quarts of Royal Purple Synthetic 75W90 GL-5 compliant and 7 oz of Trans-X Posi Trac Limited Slip Gear Oil Additive (which said it was a suitable replacement for GM#1052358). Even though the RP said it had the additive in it. I read it still was a good idea to add some (I hope). So under the car I go again and siphoned out the 89021671 and added 1.3 qts of RP. Buttoned it up and took it for a ride. Did a bunch of slow figure 8s. Everything sounded quiet. So this time I hope I did it right.

Trebor