Rear Diff Posi Fluid Question. Please help
Thread Starter
Drifting
Joined: Jan 2005
Posts: 1,389
Likes: 9
From: Seabrook Texas
St. Jude Donor '05
Rear Diff Posi Fluid Question. Please help
OK, I searched the archives and read all different opinions, I called 2 Chevy Dealers 1 said 4 bottles of fluid and 1 bottle of additive, the other said 3 bottles and 1 additive. I have read online to use 2 bottles of additive or 8oz. So I will 
and ask for my situation.
I have a 78 corvette with 87,000 miles and assume it has never been replaced. Also what is the easiest way to get the fluid out?
Thanks!
and ask for my situation.I have a 78 corvette with 87,000 miles and assume it has never been replaced. Also what is the easiest way to get the fluid out?
Thanks!
Melting Slicks
Joined: Oct 2000
Posts: 3,023
Likes: 2
From: TEXASTEXASTEXASTEXAS TEXASTEXASTEXASTEXAS
St. Jude Contributor
Easiest way to get it out is have somebody else do it. 
Actually, I'm not kidding. It stinks, it's messy, it's hard to reach without a lift. My local lube place charges $18 to suck it out and put in the new fluid that I provide. A bargain at twice the price, IMHO. You'll have to take the spare tire carrier off for them to be able to reach it.
If you do it yourself, you'll need something to pump it out with. A Mightyvac vacuum pump will do the trick, and it's a handy tool to have around anyway, so personally that's the way I'd go. Easiest access is to remove the rear passenger tire and go at it that way (though if I remember right, you still have to pull the spare tire carrier).
The accepted wisdom for the amount to put in is two bottles of posi additive, then fill with gear lube until it's level with the drain hole. At least that's what I've always understood.
JB
Actually, I'm not kidding. It stinks, it's messy, it's hard to reach without a lift. My local lube place charges $18 to suck it out and put in the new fluid that I provide. A bargain at twice the price, IMHO. You'll have to take the spare tire carrier off for them to be able to reach it.
If you do it yourself, you'll need something to pump it out with. A Mightyvac vacuum pump will do the trick, and it's a handy tool to have around anyway, so personally that's the way I'd go. Easiest access is to remove the rear passenger tire and go at it that way (though if I remember right, you still have to pull the spare tire carrier).
The accepted wisdom for the amount to put in is two bottles of posi additive, then fill with gear lube until it's level with the drain hole. At least that's what I've always understood.
JB
Last edited by JB; Apr 18, 2005 at 11:48 PM.
Le Mans Master
Joined: Oct 2002
Posts: 5,714
Likes: 6
From: Silver City NM
St. Jude Donor '05-'07
I love the smell of gear lube in the morning..it smells like....VICTORY!
Worst smelling stuff I ever whiffed was Mopar gear lube. But what the heck....be a man and DO IT YOURSELF.
Easiest way to change it is to take off the rear differential cover. Whoosh!!!! Out it comes.
You'll need a new gasket if you do it that way. No biggie. And you can give the inside a good cleaning with something else equally smelly...kerosene. When you get all done, NOBODY will want to get within 10 feet of you.
Dep
P.S. Skip the additive. Use Moroso fluid and you won't even need it.
Worst smelling stuff I ever whiffed was Mopar gear lube. But what the heck....be a man and DO IT YOURSELF.
Easiest way to change it is to take off the rear differential cover. Whoosh!!!! Out it comes.
You'll need a new gasket if you do it that way. No biggie. And you can give the inside a good cleaning with something else equally smelly...kerosene. When you get all done, NOBODY will want to get within 10 feet of you.
Dep
P.S. Skip the additive. Use Moroso fluid and you won't even need it.
Drifting
Joined: Nov 2003
Posts: 1,634
Likes: 4
From: Northern NJ
St. Jude Donor '04 & '05
I did this job last year and it can be a messy PIA. That said I was able to get the fluid out thru the filler hole using a hand suction pump. Took alot of pumping and quite a bit of effort to make sure I got all of the old fluid out, but it worked out pretty well.
For refilling, I followed what was the prevailing recommended process of using 2-4oz. bottles of GM posi additive and then filled the rest up to the filler hole with gear lube. I ended up using approx 1 and 3/4 quart bottles of lube plus of course the 8 oz of posi additive.
To ease getting the additive into the diif, I filled it with about 1/2 of the first quart of gear lube, then added the posi additive to the gear lube bottle and then finished off the first bottle. Then I topped it off with the second. FWIW.
Hope this helps. Good Luck!
For refilling, I followed what was the prevailing recommended process of using 2-4oz. bottles of GM posi additive and then filled the rest up to the filler hole with gear lube. I ended up using approx 1 and 3/4 quart bottles of lube plus of course the 8 oz of posi additive.
To ease getting the additive into the diif, I filled it with about 1/2 of the first quart of gear lube, then added the posi additive to the gear lube bottle and then finished off the first bottle. Then I topped it off with the second. FWIW.
Hope this helps. Good Luck!
Team Owner
Joined: Jul 1999
Posts: 26,545
Likes: 46
From: Huntersville NC
I didn't find access too tough once I removed the rear passenger side tire. The confusion on the # of bottles of posi additive comes from whether it's an 8 oz bottle (you only need one) or a 4 oz bottle. (you need 2). As for the number of bottles of gear lube, just find out what the capacity of the diff is (in your owners manual) and divide by the amount in each bottle!
Instructor
Joined: Jul 2003
Posts: 153
Likes: 0
From: Tecumseh, OK USA
??? Easiest Way ???
Originally Posted by DJ Dep
I love the smell of gear lube in the morning..it smells like....VICTORY!
Worst smelling stuff I ever whiffed was Mopar gear lube. But what the heck....be a man and DO IT YOURSELF.
Easiest way to change it is to take off the rear differential cover. Whoosh!!!! Out it comes.
You'll need a new gasket if you do it that way. No biggie. And you can give the inside a good cleaning with something else equally smelly...kerosene. When you get all done, NOBODY will want to get within 10 feet of you.
Dep
P.S. Skip the additive. Use Moroso fluid and you won't even need it.
Worst smelling stuff I ever whiffed was Mopar gear lube. But what the heck....be a man and DO IT YOURSELF.
Easiest way to change it is to take off the rear differential cover. Whoosh!!!! Out it comes.
You'll need a new gasket if you do it that way. No biggie. And you can give the inside a good cleaning with something else equally smelly...kerosene. When you get all done, NOBODY will want to get within 10 feet of you.
Dep
P.S. Skip the additive. Use Moroso fluid and you won't even need it.
Yeah... if you don't mind removing the ENITRE rear-end!!
You can't get the rear cover off the differential because it's bolted to the cross member from the top. You would have to remove the spring and drop the whole thing out of the car, cross-member and all.
The easiest way is to take it somewhere and let them do it. BUT, if you want to do it yourself pull the plug and use a pump (hand or electric) and suck as much out as possible. I've done mine twice. The first time I sucked it out with a hand pump and the second time I just replaced the whole rear-end (lower gears) so I pre-filled it on the bench. Mine is a 78 as well and I used 8oz of posi fluid and topped it off with Mobil gear lube.
Le Mans Master
Joined: Oct 2002
Posts: 5,714
Likes: 6
From: Silver City NM
St. Jude Donor '05-'07
Originally Posted by c3vettelover
??? EASIEST WAY ???
Yeah... if you don't mind removing the ENITRE rear-end!!
You can't get the rear cover off the differential because it's bolted to the cross member from the top. You would have to remove the spring and drop the whole thing out of the car, cross-member and all.
The easiest way is to take it somewhere and let them do it. BUT, if you want to do it yourself pull the plug and use a pump (hand or electric) and suck as much out as possible. I've done mine twice. The first time I sucked it out with a hand pump and the second time I just replaced the whole rear-end (lower gears) so I pre-filled it on the bench. Mine is a 78 as well and I used 8oz of posi fluid and topped it off with Mobil gear lube.
Yeah... if you don't mind removing the ENITRE rear-end!!
You can't get the rear cover off the differential because it's bolted to the cross member from the top. You would have to remove the spring and drop the whole thing out of the car, cross-member and all.
The easiest way is to take it somewhere and let them do it. BUT, if you want to do it yourself pull the plug and use a pump (hand or electric) and suck as much out as possible. I've done mine twice. The first time I sucked it out with a hand pump and the second time I just replaced the whole rear-end (lower gears) so I pre-filled it on the bench. Mine is a 78 as well and I used 8oz of posi fluid and topped it off with Mobil gear lube.
If you are diddling around with ratchets and breaker bars, then you might as well take it in and have someone do it. Taking the rear cover off lets you do a THOROUGH cleaning of the inside and get all the gear lube and any other nasty stuff that may be lying at the bottom of the differential case OUT.
BTW...this is just one more reason why I'm looking to replace that Mickey Mouse IRS in my Vette with a solid axle.
Dep
Racer
Joined: Mar 2005
Posts: 294
Likes: 1
From: Racine Wisconsin
I had the diff rebuilt and was disappointed when the posi chatter (slow to disengage) that was not there in the first place appeared. Using advice from CF, I drained the diff with a flexible tube siphon and turkey baster to start the flow. I refilled with Red Line heavy diff fluid and 8-oz of GM additive. No problems in 4 years.
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Melting Slicks
Joined: May 1999
Posts: 2,897
Likes: 0
From: Moon Township PA
Originally Posted by PRNDL
I didn't find access too tough once I removed the rear passenger side tire. The confusion on the # of bottles of posi additive comes from whether it's an 8 oz bottle (you only need one) or a 4 oz bottle. (you need 2). As for the number of bottles of gear lube, just find out what the capacity of the diff is (in your owners manual) and divide by the amount in each bottle!
I am sorry! You have to remove this post. It is too clear, too straight to the point, and too sensible!
Seriously, this isn't that bad if you use a suction gun and don't mind taking off the rear wheel. Pay someone to do a diff lube change? Don't think so! Back to the point - two bottles of GM brand posi additive and 1+ bottles of commercially available Mobil 1 diff lube for my car.
Gary
Safety Car
Joined: Jun 2004
Posts: 4,652
Likes: 80
From: Lakeland Florida
Hello,
I changed my diff. fluid last weekend. It was so much easier to remove the rear tire carrier. Then I had enough room to get to the bolt on the diff. cover. I did this on a 1972 so I did not have a drain plug. I bought a small pump dispenser at the parts store, put the tube in the cover, and pumped out the old fluid into an oil pan. I added the 4oz tube of anti slip fluid, and 1.5 bottles of Mobil 1 75-90W gear oil. The Mobil 1 comes in a bottle small enough to hold slightly higher than horitzontal, and allows you to squeeze/squirt the new gear oil into the opening. When it started to dribble out of the hole it was done.
Here is the differential area with the rear tire carrier removed.
kdf
I changed my diff. fluid last weekend. It was so much easier to remove the rear tire carrier. Then I had enough room to get to the bolt on the diff. cover. I did this on a 1972 so I did not have a drain plug. I bought a small pump dispenser at the parts store, put the tube in the cover, and pumped out the old fluid into an oil pan. I added the 4oz tube of anti slip fluid, and 1.5 bottles of Mobil 1 75-90W gear oil. The Mobil 1 comes in a bottle small enough to hold slightly higher than horitzontal, and allows you to squeeze/squirt the new gear oil into the opening. When it started to dribble out of the hole it was done.
Here is the differential area with the rear tire carrier removed.
kdf
Team Owner
Joined: Jun 2004
Posts: 20,216
Likes: 4
From: Smile, it's just the internet.
What the heck are you guys using to get the side plug out? I use my small butterfly impact, a wobble extension, and an eight point socket. No need for the tire or carrier to come out...
Do yourself a favor and put a magnetic drain plug in the bottom of the pumkin.
Do yourself a favor and put a magnetic drain plug in the bottom of the pumkin.
Racer
Joined: Jan 2005
Posts: 492
Likes: 0
From: Birmingham AL
Originally Posted by DJ Dep
BTW...this is just one more reason why I'm looking to replace that Mickey Mouse IRS in my Vette with a solid axle.
Dep
Dep
Le Mans Master
Joined: Oct 2002
Posts: 5,714
Likes: 6
From: Silver City NM
St. Jude Donor '05-'07
Originally Posted by steves_77vette
That's the HARD way to change the diff fluid don't ya think? Replace the IRS with a solid axle?
What everyone is missing is that any shavings or contaminants that are in the differential case are probably NOT being removed by sucking the fluid out. Would you change your, oil but NOT the filter?
Dep
