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84C47755 Rear Differential fluid change
Guru Newb
Union, Maine
12 Posts
Member since:
2008/2/28 19:00



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I am going to be changing the fluid in my rear differential. I have the limited slip additive, and two bottles of 80/90W Mobil oil. The GM additive bottle says to change the oil when it is warm. Is that necessary?
Posted on: 2008/3/7 11:51
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toptechx6 Re: Rear Differential fluid change
Senior Guru
south Florida
354 Posts
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2006/6/6 20:00



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If you change it when warm all contaminants remain in suspension, even more important if you are using a suction gun to remove the lubricant in the rear end. If you do it cold all the debris you want to remove has settled to the bottom to mix with your nice new axle lube once it warms up.
Posted on: 2008/3/7 13:09
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84C47755 Re: Rear Differential fluid change
Guru Newb
Union, Maine
12 Posts
Member since:
2008/2/28 19:00



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Gotcha! So really, rotating the gears is the important part. If I have the rear end up on jack stands and run it in gear with the wheels off the ground, would that accomplish the same thing?
Posted on: 2008/3/7 14:12
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Notorious Re: Rear Differential fluid change
Elite Guru
Downbound train, NC
2184 Posts
Member since:
2005/9/16 20:00



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Quote:

84C47755 wrote:
Gotcha! So really, rotating the gears is the important part. If I have the rear end up on jack stands and run it in gear with the wheels off the ground, would that accomplish the same thing?

It will to some extent but it will take an eternity to get any serious heat into the oil because nothing is under load. You'll see racers do this in the pits occasionally to warm the oil. And while it's better than nothing, it's not as good as a real run to bring your oil up to temp. Consider adding a real drain plug to your differential. It's a simple mod and only requires locating it properly. Although I've done this on both D36 and D44 differentials while they were out of the vehicle, I only have the locating dimensions recorded for the D44.
Manufacturers should be painfully castrated for not having proper drain plugs in place to begin with.....
Posted on: 2008/3/7 22:41
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bogus Re: Rear Differential fluid change
Grand Imperial Pooh-Bah
San Pedro, CA
18796 Posts
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2005/9/6 20:00



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This'll help!

www.corvette-guru.com - C4 Guru - Changing Differential Fluid

It's from the Tech Library.
Posted on: 2008/3/7 23:24
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toptechx6 Re: Rear Differential fluid change
Senior Guru
south Florida
354 Posts
Member since:
2006/6/6 20:00



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Quote:

Notorious wrote:
Quote:

84C47755 wrote:
Gotcha! So really, rotating the gears is the important part. If I have the rear end up on jack stands and run it in gear with the wheels off the ground, would that accomplish the same thing?

It will to some extent but it will take an eternity to get any serious heat into the oil because nothing is under load. You'll see racers do this in the pits occasionally to warm the oil. And while it's better than nothing, it's not as good as a real run to bring your oil up to temp. Consider adding a real drain plug to your differential. It's a simple mod and only requires locating it properly. Although I've done this on both D36 and D44 differentials while they were out of the vehicle, I only have the locating dimensions recorded for the D44.
Manufacturers should be painfully castrated for not having proper drain plugs in place to begin with.....


I agree completely with Notorious on this one, while Andy's procedure in Tech Tips will certainly get the fluid changed, to me the technique is similar to removing engine oil through the dipstick tube, the oil is removed but heavy contaminants including metal and water or coolant remain at the bottom.
There is a good reason for putting drain plugs at the lowest point, it is called gravity! It is not hard to install your own drain plug, as mentioned it is easier when the pumpkin is on the bench but can be done in the vehicle too.
Don't buy one of the expensive "kits" all you need is a long 5/16” drill bit, a 1/8 X 27 pipe tap and a plug the same size as the tap.
Center punch a spot on the pumpkin 1/2 inch left of the lowest cover bolt centerline and 1/2 inch forward of the batwing cover. That dimension should work for D36 or D44. Drill a small pilot hole first before the 5/16" final diameter. Use care when drilling to avoid nicking the ring gear, a depth "stop" on your drill bit at 1 inch will prevent damage. I have used tape wrapped thick enough to make a shoulder or spot weld a washer on the drill bit.
Use an extension and 12 point 1/4" drive socket on the tap to cut the threads if doing this in the car. The draining lubricant will remove any chips.
The 1/8" drain plug seems small but warm lube drains quickly and more thoroughly than any suction gun.
Posted on: 2008/3/8 4:57
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Steel_Blue_91 Re: Rear Differential fluid change
Guru
Burlington, NC 27217
76 Posts
Member since:
2005/9/15 20:00



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Quote:

Use care when drilling to avoid nicking the ring gear, a depth "stop" on your drill bit at 1 inch will prevent damage. I have used tape wrapped thick enough to make a shoulder or spot weld a washer on the drill bit.


I've used a piece of rubber tubing (fuel line) cut to leave a measured lenght of the bit exposed. Works like a charm.
Posted on: 2008/3/8 8:42
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CasetheCorvetteman Re: Rear Differential fluid change
Elite Guru
QLD, Australia
2071 Posts
Member since:
2006/7/17 20:00



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Sucking the fluid out through the filler plug is really quite desperate to say the least, and to me it ranks right up there with dropping $50 worth of engine oil and leaving the dirty old $4 filter in place.

The best way to do it is to remove the diff from the car and take the back cover off. And considering how easy this is, its hardly worth doing a half a job of it when for abit more effort you can do it right first go.

Standing the unit with the pinion yoke facing the ground, remove the rear cover bolts, then remove the rear end, personally id carefully remove the old sealant BEFORE you drop the oil so anything that falls in will come out with the old oil, and when youre ready, tip the diff up and out comes the oil. Piss easy.

Then after you finish cleaning it all up nice and make sure the mating surfaces are looking good, clean them up with turps or metho, get some Permatex Ultra Black (or right stuff) and run a bead around the mating surface of the diff housing, id leave it for a few minutes, then put the cover on. Nip all the bolts up to about 5 ft/lbs in a cross pattern to keep it even, then go to 10 ft/lbs after a few minutes, then 17 ft/lbs, followed by the factory spec for each bolt. Do this procedure with the lower suspension arm bracket bolts in place, or youll be trying to get things even with 4 bolts missing (not to their final spec though, tighten them to the same as the cover bolts nearest to them)

Then id leave it to cure for a while. Ive never had mine leak after that.

Forget the additive too. Get rid of it. GM no longer use or recomend it, and havent done for a few years that i know of (if they do, they shouldnt be, cause there was a TSB about recomended lubricants for aluminum housed diffs back in 97 or 98 i think). I use and recomend Castrol Synthetic limited slip rear axle lubricant, with a viscosity of 80w-140, and being of an API GL-6 rating, it exceeds the required specs by alot.
Posted on: 2008/3/8 8:52
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84C47755 Re: Rear Differential fluid change
Guru Newb
Union, Maine
12 Posts
Member since:
2008/2/28 19:00



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I wasn't under the impression that removing the rear diff from a C4 was easy. Can you give me a ball park idea of what needs to be done to remove the diff from the car?
Posted on: 2008/3/8 15:18
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toptechx6 Re: Rear Differential fluid change
Senior Guru
south Florida
354 Posts
Member since:
2006/6/6 20:00



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Quote:

84C47755 wrote:
I wasn't under the impression that removing the rear diff from a C4 was easy. Can you give me a ball park idea of what needs to be done to remove the diff from the car?



Removal isn't that tough but I wouldn't do it just to install the drain, not that hard to do it in the car and steel_blue_91 had a great suggestion to use the fuel line as a drill stop. You need to remove the exhaust, rear spring, rear toe link assembly, unbolt the camber bars at the differential and swing them down, unbolt the driveshaft and both half shafts and remove the rear bolts from the support beam, finally the two bolts attaching the batwing to the frame. The assembly then drops out, but you will need to remove the cover bolts to separate the batwing from the pumpkin. I'm sure someone will remind me if I missed anything.
Posted on: 2008/3/8 16:03
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CasetheCorvetteman Re: Rear Differential fluid change
Elite Guru
QLD, Australia
2071 Posts
Member since:
2006/7/17 20:00



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Quote:

84C47755 wrote:
I wasn't under the impression that removing the rear diff from a C4 was easy. Can you give me a ball park idea of what needs to be done to remove the diff from the car?

It is, its quite easy infact.

TopTechx6 has a procedure above.

Considering you only change the fluid every 30,000 miles of extreme duty driving or 75,000 miles of regular driving, i wouldnt bother with a drain plug. But that's just me. Just makes another potential leak.
Posted on: 2008/3/8 17:48
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84C47755 Re: Rear Differential fluid change
Guru Newb
Union, Maine
12 Posts
Member since:
2008/2/28 19:00



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Quote:

toptechx6 wrote:
Quote:

84C47755 wrote:
I wasn't under the impression that removing the rear diff from a C4 was easy. Can you give me a ball park idea of what needs to be done to remove the diff from the car?



Removal isn't that tough but I wouldn't do it just to install the drain, not that hard to do it in the car and steel_blue_91 had a great suggestion to use the fuel line as a drill stop. You need to remove the exhaust, rear spring, rear toe link assembly, unbolt the camber bars at the differential and swing them down, unbolt the driveshaft and both half shafts and remove the rear bolts from the support beam, finally the two bolts attaching the batwing to the frame. The assembly then drops out, but you will need to remove the cover bolts to separate the batwing from the pumpkin. I'm sure someone will remind me if I missed anything.


Your definition of easy is a little more liberal than mine. I just got done replacing the rear hub/bearing units, so I know what a PIA some of that stuff was to get off, and I don't even want to think about how rusted the bolts on my exhaust must be. I am dealing with a 1984 Corvette with 180,000+ miles, and as near as I can figure, a sketchy history. I also only have jack stands to work with. It may be heresy, but I think I will take my chances with sucking out what I can after running the car for a little while. It may not be optimal, but it will sure beat doing nothing. Thanks all for the input.
Posted on: 2008/3/8 20:33
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KeyWestJack Re: Rear Differential fluid change
Guru Newb
Pittsburgh
25 Posts
Member since:
2006/8/7 20:00



Offline
Quote:

toptechx6 wrote:
Quote:

Notorious wrote:
Quote:

84C47755 wrote:
Gotcha! So really, rotating the gears is the important part. If I have the rear end up on jack stands and run it in gear with the wheels off the ground, would that accomplish the same thing?

It will to some extent but it will take an eternity to get any serious heat into the oil because nothing is under load. You'll see racers do this in the pits occasionally to warm the oil. And while it's better than nothing, it's not as good as a real run to bring your oil up to temp. Consider adding a real drain plug to your differential. It's a simple mod and only requires locating it properly. Although I've done this on both D36 and D44 differentials while they were out of the vehicle, I only have the locating dimensions recorded for the D44.
Manufacturers should be painfully castrated for not having proper drain plugs in place to begin with.....


I agree completely with Notorious on this one, while Andy's procedure in Tech Tips will certainly get the fluid changed, to me the technique is similar to removing engine oil through the dipstick tube, the oil is removed but heavy contaminants including metal and water or coolant remain at the bottom.
There is a good reason for putting drain plugs at the lowest point, it is called gravity! It is not hard to install your own drain plug, as mentioned it is easier when the pumpkin is on the bench but can be done in the vehicle too.
Don't buy one of the expensive "kits" all you need is a long 5/16” drill bit, a 1/8 X 27 pipe tap and a plug the same size as the tap.
Center punch a spot on the pumpkin 1/2 inch left of the lowest cover bolt centerline and 1/2 inch forward of the batwing cover. That dimension should work for D36 or D44. Drill a small pilot hole first before the 5/16" final diameter. Use care when drilling to avoid nicking the ring gear, a depth "stop" on your drill bit at 1 inch will prevent damage. I have used tape wrapped thick enough to make a shoulder or spot weld a washer on the drill bit.
Use an extension and 12 point 1/4" drive socket on the tap to cut the threads if doing this in the car. The draining lubricant will remove any chips.
The 1/8" drain plug seems small but warm lube drains quickly and more thoroughly than any suction gun.



Maybe I am just being a little to particular but what material plug, brass, stainless, black iron? BTW, your instructions are a major help. Thank you.
Posted on: 2008/7/5 18:50
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CentralCoaster Re: Rear Differential fluid change
Senior Guru
San Diego, CA
9160 Posts
Member since:
2007/10/27 20:00



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Quote:

84C47755 wrote:
I wasn't under the impression that removing the rear diff from a C4 was easy. Can you give me a ball park idea of what needs to be done to remove the diff from the car?


I wouldn't bother doing all that, rear diff fluid is about the least critical fluid on the car.

But if you want to, you've gotta pull the exhaust, disconect two half-shaft straps, disconnect the lower camber arms, remove the rear spring, disconnect driveshaft and cbeam, then drop it out. It's not easy, but it's much easier than you'd think. You don't have to pull the rear tires either.

Click to see original Image in a new window
Posted on: 2008/7/5 21:17
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pianoguy Re: Rear Differential fluid change
Elite Guru
Apple Valley, MN
10866 Posts
Member since:
2007/12/28 19:00



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Quote:

CentralCoaster wrote:

I wouldn't bother doing all that, rear diff fluid is about the least critical fluid on the car.

But if you want to, you've gotta pull the exhaust, disconect two half-shaft straps, disconnect the lower camber arms, remove the rear spring, disconnect driveshaft and cbeam, then drop it out. It's not easy, but it's much easier than you'd think. You don't have to pull the rear tires either.

Click to see original Image in a new window


Well, that certainly looks easy ;-)
Posted on: 2008/7/5 23:11
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bogus Re: Rear Differential fluid change
Grand Imperial Pooh-Bah
San Pedro, CA
18796 Posts
Member since:
2005/9/6 20:00



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Time elapsed photography has effectively take a 3 month project and distilled it down to 8 seconds. Impressive.

I agree with Kevin here. Anything in suspension isn't going to "contaminate" the rear end lube THAT much. Think about this for a minute: What contaminates motor oil? GASOLINE!!! There isn't any combustion or other items that can enter into the diff and ruin the fluid.

IIRC, GM never had a maintenance cycle for it! Whatever numbers that are being suggested are coming from Dana, or, experience.

This is a heavy lube, 80wt, and isn't exposed to the heat that exists in the engine or even the transmission.

Sure, there will be wear and debris, but I don't think it's going to be all that fatal if some of that stays in the bottom of the diff. Gee, the gears will eat that crap for lunch, really. And if anything THAT big is in there, you have more problems than we are gonna fix here.
Posted on: 2008/7/6 16:00
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The single biggest problem with communication is the illusion that it has taken place. - George Bernard Shaw

Education is the best tool to overcome irrational fear. - me

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CentralCoaster Re: Rear Differential fluid change
Senior Guru
San Diego, CA
9160 Posts
Member since:
2007/10/27 20:00



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It can get water in there from the fluid heating and cooling and drawing moisture in through the vent, or some dust, or lots of water if the car was in a flood. But my point was more that you could probably run 100K on it with mississippi mud in there.

BTW, 80wt gear lube is about the same thickness as 30wt motor oil.
Posted on: 2008/7/6 20:05
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KeyWestJack Re: Rear Differential fluid change
Guru Newb
Pittsburgh
25 Posts
Member since:
2006/8/7 20:00



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Hey Toptechx6, I followed your instructions (above)and it worked out great. Thank you!
Posted on: 2008/7/8 23:28
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