Rear diff fluid change

[Aurora40]

It seems like the only way to drain this is to take the casing apart. I'm not sure I'm up for that. So what I'm thinking is to suction out as much fluid as I can with some plastic hose through the fill hole.

I was then planning on filling up with the 4 oz GM rear diff additive, and then Mobil 1 75w-90 gear oil.

Is this an ok thing to do? Is there a better way? Am I making another classic ZR-1 blunder? Thanks!

[Patton]

There should be a 13mm drain plug in the back. Just unscrew and get a coffee.

I dont know how accurate this is because I dont have a shop manual. However I just checked my Chilton repair manual and it says to use the 4floz. of GM limited slip additive part no. 1052358 and (doesnt say how much) SAE 80W-90 GL-5 gear lubircant. Sounds like that Mobil1 stuff would do just fine. BUT WAIT UNTILL SOMEBODY WITH A SHOP MANUAL CONFIRMS ALL OF THIS.

Hope this is some help! Good luck!

[Aurora40]

I've a shop manual. I think the drain plug was only certain years? My manual doesn't show one in the schematic. Also, the capacity is like the tranny, you pour it in until it comes out the fill plug. So you can't overfill it.

I'll take a closer look, I'm not in a rush to do it, and see if there's a drain plug. But it doesn't seem like there is from the manual. Thanks! :)

[blackjack]

i use amsoil severe duty gear oil,75w90...amsoil says you dont need
the diff additive...my gm service manager says that most new cars
dont need the additive anymore,with the synt lube...they only brought it back by popular demand...cant say if it is true or not
blackjack
91 ,1735

[Patton]

Well, hell. It looks like you know what you are doing.
Go on now,... Git Er DUNN :thumbsup:

[ZR1 MK]

No factory drain plug. You could install an aftermarket unit for easy fluid changes. I change my fluids more often because of track usage, so this mod is far more important to me than the average owner.

[Tom]

The 90 does not have a drain plug on the differential, but there is one on the transmission. I went to Autozone and bought a suction gun that looks similar to a grease gun. It can be used to suction out the old oil and then used to put in the new oil. I used the additive and Redline gear oil in the differential. The suction gun can also be used to refill the transmission and makes it a very easy job to refill the differential or the transmission.

[Aurora40]

Hey Tom, I've got a suction gun just like you are talking about. The only problem with it is fluid will drip out the weep hole on the back. But other than that it's great. I am always skeptical of using it to fill stuff up, though, because I use it to suck out disgusto stuff. I guess I worry about it contaminating the new stuff. Hopefully the tall narrow bottles that gear oil comes in will be narrow enough to pour right in the diff. But if not, I'll do what I did with the tranny and use a long length of hose that runs out to the side of the car, and pour it into the hose.

Thanks for all the replies! Glad I wasn't over or underthinking this one! :)

[Tom]

I've never had a problem with mine dripping out the back, I guess mine just seals better around the plunger. It has been a while since I have used mine so I don't remember if it comes apart easily like a grease gun so you can disassemble it and clean it out, but you could flush and clean it using clean oil or a solvent that evaporates readily.

[HAWAIIZR-1]

I have had great success to use this hand pump from MityVac:

http://www.northerntool.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=6970&productId=200008074&R=200008074

I think I bought mine from Sears, but they sell them at a lot of the auto parts stores. It is very reliable and easy to clean out. I use it to drain the differential and replace the gear lube with the opposite flow.

Craig

[Aurora40]

Well, I got it done tonight. I jacked up the passenger rear, pulled the tire, and sucked out the old stuff. Then I pulled the jackstand out and lowered the car on the jack until it was level and filled it up. I used about an 18" length of hose to fill the diff. Then I capped it up, jacked it back up, and put the tire on.

I have a question, though. I used about 1.5 liters of Mobil 1, plus the 4 ounces of rear diff lube. How much fluid does the diff hold total? Was I pretty close at getting it all out?

As an aside, man does gear oil stink!

[ZR1 MK]

Two tips.
1. Mobil 1 doesnt require the posi additive.
2. Install a magnetic fill plug while your there. Standard GM size.

[Aurora40]

Well it's already back together. But for a magnetic one, you mean standard rear diff size? Or what? GM sells this?

Does it matter if I put the additive in anyway? I don't even know what is in the additive that makes it different.

Any idea on the total fluid capacity of the diff? :)

[ZR1 MK]

Additive is for the clutch. It wont damage anything if used in the Mobil 1. The plug should be a standard size (diameter and thread pitch). Look for one at a local auto parts store.

[Tom]

I believe the differential fill plug has tapered pipe threads, a standard oil pan drain plug will not fit since they use straight threads. You fill the differential to the bottom of the fill hole so a magnetic fill plug would provide very little benefit since it is above the fluid level.

[ZR1 MK]

I believe the differential fill plug has tapered pipe threads, a standard oil pan drain plug will not fit since they use straight threads. You fill the differential to the bottom of the fill hole so a magnetic fill plug would provide very little benefit since it is above the fluid level.
Who mentioned an oil pan drain plug? I wouldnt have mentioned a magnetic fill plug if it didnt work. The fluid gets thrown around when under way. The plug collects plenty of debris, so your wrong.

[Tom]

When you mentioned standard size (diameter and pitch) I thought you were referring to a standard straight thread magnetic oil pan drain plug since I have not noticed any magnetic pipe thread plugs at my local part stores. What part stores have you seen them at?

[ZR1 MK]

I got the plug many years ago from a local auto parts store. It was originally in my 1970 Z28 Camaro and was transfered to my 1990 ZR1 Corvette. Ask for a diff plug, but one thats magnetic.
Since the engine and trans didnt have magnetic plugs thats where readily available at the time, I use glued on magnets. And these also collect debris. The trans accumulates plenty of non-magnetic particles, so a proper drain interval is important.

[ZR1 MK]

I did the drain plug shortly after my last, many moons ago, post.

Fluid changes are much easier and all the fluid gets drained. Helps get the particles out too, for those not using a magnetic fill/drain plug.