I was just wondering if anybody could offer me some tips on how to change the fluid in my differential tomorrow. I have a 99' Z/28 Auto with the 2.73 posi disc rear, and I have never done this before. I am going to start by jacking the car up by the body and letting the rear hang down so that I can access all of the cover bolts easily, and then break the seal loose and let the oil drain into a pan. However, I am curious as to whether or not the fill plug is hard to open (the fluid probably has never been changed in the 71,000 miles it has been around), and what fluid and posi additive to buy. I will be going to a generic auto parts store (Pep Boys around here, I dont know if its all around the country) to buy all of the necessary items to do this. Any tips are greatly appreciated.

 

you are gonna need to go to a GM dealer for Limited Slip Additive... DONT PUT IT BACK TOGETHER W/O IT!

The drain bolt is really easy to get to. Get a new gasket too as well as so RTV high temp silicone. Make sure to scrape ALL of the old gasket off, if not, you will have leaks like me the first time i did it.

As for fluid, i cant remember what i put in there, just look for the mobil one synthetic rear diff fluid. Oh, fyi... its a pain to get the last 1/3 of the fluid in each bottle into the differential since the the drain plug is placed so poorly on the side of the housing.

 

Well I use Redline which has the friction modifier in it. GM recommends 4 oz of the modifier otherwise. It is not critical to use the modifier, especially on the Torsen Zexel which is in your car but it supposedly suppresses the rusty bolt noise that "might" occur. This isn't common and it isn't destructive but GM has had enough complaints that it is their recommendation to use it.

As for the access plug on the passenger high side, just use your 3/8" ratchet directly or use a short extension (3/8" again) directly into the plug. It should come out with little effort.

The real challenge is not making a huge mess putting the new fluid in. I blocked the bottle securely and then used one of those siphon pumps to install the new fluid. It will take slightly than more one bottle (a quart). Because the stuff is so thick it will have a delayed action when the rear end is full. Therefore, I recommend putting in one bottle, let it settle and then make a few pumps with the next bottle and repeat. When the fluid slowly begins to creep out the access hole you are done. Snug the plug and have some fun.

 

Thanks guys, I will take into consideration all of your tips. Scraping the gasket and filling until the fluid drips out of the hole are great tips, thanks!!!

 

i use royal purple which you don't need the friction modifier with. If your using other brands then redline or royal purple then use gl-5 fluid only, with friction modify also, which pep boys carries. I changed my fluid recently and its easy. The drain plug is easy to remove, and you will need a wrench and socket w/ratchet. The swaybar gets in the way of the lower bolts which is why you need both. You do need a new gasket and it is important to make sure you got all the old gasket off. Also inspect the fluid and gears for metal pieces and wear. I also like to pain the rear end cover with rustolem or something like that to help stop rusting when i service my rear end. Well good luck and let us know how it goes!

 

1) Lift car and support with jack stands.

2) Remove all of the bolts on the diferential cover except for the very top one. Only partly unscrew the top one.

3) Position drain pan below pumpkin, and use a flat blade screw driver to 'pop' the cover (use gradual pressure).

4) Remove top bolt and clean cover. ther will be a round magnet in there and it will seperate from the cover (this is normal).

5) Make sure that the cover and pumpkin sealing surfaces are chemically clean and dry. (brake or carb cleaner).

6) Apply regular RTV black to the cover in a 1/4" continuous bead. (don't waste time or money on paper gaskets.)

7) Re-install cover and bolts.

8) Use regular 80W-90 gear oil (just save your money)

9) Add Posi additive from local parts store.

10) The best trick for getting the fluid into the pumpkin with minimal spillage is to buy a syringe (at least that's what it looks like). They hold about a 1/2 a quart of fluid and have a 12' rubber/ plastic hose on it. (this would be a good time to use that $$$ you saved by just buying regular oil.)

11) Fill pumpkin till over flow (let excess drain) and re-install plug.

 

It might be best to add the Posi additive first and then add the gear oil.

 

I think synth is a waste of $$$ in a rear end.

Rich

 

Yea, I changed the fluid a few days ago. I couldnt get the damn second half of the bottle in the rear end, so I bought another and filled it until the fluid dripped out the rear end. I added 5 oz of posi additive as well. I didnt get to drive it until today. WOW!! I wasnt expecting a change in the way the car rides from doing this (I was just doing routine maintenance), the rear does not chatter anymore while taking turns or accelerating at a moderate/full amount. I suppose the posi additive that was in the rear previously had broken down (it could have been the original 71,000 mile stuff) and was not working properly, or just the new fluid. I can take turns while having the car feel more free, and not hear a clunking noise from the back when hitting the gas above a 1/3 throttle. Has anybody noticed this after doing their rear as well? It seriously feels like my car is running on glass after having this done.

 

I won't question your description of the results but since you have the Torsen rear end with no clutches, you shouldn't have experienced chatter. The Torsens can produce a ratcheting sound that a friction modifier would have no effect upon. Perhaps you have had your differential changed out.

When you had the cover off did it look like this?

 

Yea, don't thikn the Torsen's need the friction modifier.. its all gear driven.

For people that do high speeds for long periods of time, like on a roadcourse, synthetic may be worth it, for the extra thermal protection..

 

Yea it looked like the one in the picture. I swear it rides smoother for some reason, this can't be in my head. Even my friends say its smoother when it accelerates and that there is less bucking. Apparently I do have a Torsen unit in my car.

 

70,000 miles is a long time to go w/out a diff fluid change.. Maybe thats why you have such a drastic improvment

 

Yea I am not completely sure that it was the original fluid, but seeing as how nobody changes their fluid in the differential (it is pretty much forgotten), I assume it is the original fluid

 

If no one has added any fluid in 71000 miles it may have been low on fluid. Now it is full. That may be why you are seeing an improvement.