Limited slip differentials

[BlackHawk99]

I am about the replace the leaky rear differential gasket on our 97 Tahoe. I have a new seal and 2 quarts of 80W90 gear lube. But, I'm not sure if this truck is a limited slip or not...and if it is, do I need the limited silp additive? If so, how much is necessary?

Anyone know for sure? I want to make sure I do the job right.

[MrShorty]

If you are reasonably sure it has the factory diff in there, look on your RPO code list for "G80": the RPO code for the factory "gov-lock" differential.

Either way, though, the factory doesn't call for the LS additive for either the conventional open diff or the G80. The gov-lock is not a typical clutch type LS, so it doesn't need the additive that clutch type LS's call for.

[Jimmiee]

An easy way to tell if you need the additive is to jack up the rear end. With the transmission in park or in gear if it's a stick see if you can turn one wheel. If you have an open diff one wheel will turn forward and the other will turn the opposite way. If you have a posi then the axles will be locked together and the wheels will not turn.

[MrShorty]

An easy way to tell if you need the additive is to jack up the rear end. With the transmission in park or in gear if it's a stick see if you can turn one wheel. If you have an open diff one wheel will turn forward and the other will turn the opposite way. If you have a posi then the axles will be locked together and the wheels will not turn.Except in the case of the G80. The G80 will behave just like an open diff: spin in opposite directions.

[1st Synthetics]

Either Diff you possibility have does not call for the additive. You are going to need at least 1 more quart of gear lube.

Differential, with 10.5 ring gear Dana Rear..........5.5 pints
Differential, with 10.5 ring gear EX Dana Rear..........6.3 pints
Differential, with 11 ring gear Dana Rear..........8.4 pints
Differential, with 8.5, 8-5/8 ring gear Rear..........4.2 pints
Differential, with 9.5 ring gear Rear..........5.5 pints
Differential, with 9.75 ring gear Rear..........5.5 pints

[Jimmiee]

Except in the case of the G80. The G80 will behave just like an open diff: spin in opposite directions.

Exactly! Even though it's a "posi" it doesn't require the additive because it is not locked in turns. The conventional Auburn style is always locked and will chatter in turns without the friction modifier.

[BlackHawk99]

Thanks for the help, guys!

I do have the G80 RPO code and have no reason to believe the rear end has ever been swapped out. But it sounds like I don't need the additive anyway.

I'll pick up another quart of gear lube. AutoZone told me the capacity was 1.75 quarts. LOL That's what I get for listening to those guys! I'm going to tackle the job tomorrow night

[BlackHawk99]

I swapped out the gasket. What an act of congress it was to get the 10 bolts off! 12 years of rust really made things a challenge.

So...got it all swapped and drove it about 50 miles. The leak is back. I'm so irritated. UGH. I wire wheeled the cover and removed all the gasket residual from the mating surface on the rear diff. Set the new gasket in place with RTV on both sides. Cleaned the bolts with a wire wheel and used PB Blaster on them before torquing them down. I torqued them to 165 inch lbs (13.75 ft. lbs.). Did I do something wrong? This feels like a waste of what should have been a really easy job.

[MrShorty]

Are you certain the leak is through the gasket? A lot of times other seals (especially a leaky pinion seal) will look like a leaky gasket because the oil flows down the housing and drips off the bottom: right where the gasket seal would drip.

I've had trouble sealing up a diff before because I used the wrong "stuff". Don't take offense, but are you certain you used RTV silicone? There's another product out there that calls itself a "gasket maker" but isn't RTV, and it doesn't work on gear oil.

All sealing surfaces looked clean without imperfections/scratches/pitting/etc.? Housing flat? Cover flat and not bent?

[1st Synthetics]

I swapped out the gasket. What an act of congress it was to get the 10 bolts off! 12 years of rust really made things a challenge.

So...got it all swapped and drove it about 50 miles. The leak is back. I'm so irritated. UGH. I wire wheeled the cover and removed all the gasket residual from the mating surface on the rear diff. Set the new gasket in place with RTV on both sides. Cleaned the bolts with a wire wheel and used PB Blaster on them before torquing them down. I torqued them to 165 inch lbs (13.75 ft. lbs.). Did I do something wrong? This feels like a waste of what should have been a really easy job.

Did you use a synthetic gear lube? You can not use RTV with synthetic lubricants. They react and cause seal failure.

[Jimmiee]

I swapped out the gasket. What an act of congress it was to get the 10 bolts off! 12 years of rust really made things a challenge.

So...got it all swapped and drove it about 50 miles. The leak is back. I'm so irritated. UGH. I wire wheeled the cover and removed all the gasket residual from the mating surface on the rear diff. Set the new gasket in place with RTV on both sides. Cleaned the bolts with a wire wheel and used PB Blaster on them before torquing them down. I torqued them to 165 inch lbs (13.75 ft. lbs.). Did I do something wrong? This feels like a waste of what should have been a really easy job.

Did you use a synthetic gear lube? You can not use RTV with synthetic lubricants. They react and cause seal failure.

I disagree with the above statement. Many new vehicles have synthetic oil and use RTV instead of a traditional gasket. I can't speak for Amsoil. Maybe it's not compatible?

[1st Synthetics]

I disagree with the above statement. Many new vehicles have synthetic oil and use RTV instead of a traditional gasket. I can't speak for Amsoil. Maybe it's not compatible?

It is a fact with any synthetic oil. The synthetic base stocks will chemically attack RTV sealants. If installed properly with no RTV spill over to the inside it can be done with good results. If any RTV is in contact with the synthetic it will cause a chemical reaction. As far as I know most Gm axles after 1999 have gaskets and not RTV. This is due to Gm using synthetic gear lubes.



When using RTV sealants, it is recommended to make sure the RTV has cured completely before installing the gear oil for optimum results.

[Jimmiee]

It is a fact with any synthetic oil. The synthetic base stocks will chemically attack RTV sealants. If installed properly with no RTV spill over to the inside it can be done with good results. If any RTV is in contact with the synthetic it will cause a chemical reaction. As far as I know most Gm axles after 1999 have gaskets and not RTV. This is due to Gm using synthetic gear lubes.



When using RTV sealants, it is recommended to make sure the RTV has cured completely before installing the gear oil for optimum results.

I'm no engineer but I do know it's a fact that most if not all manufacturers use RTV sealant in their engines, transmissions, transfer cases and differentials.The reason is it's much more coat effective and easier to lay a bead of sealer with a robot than try to fit a paper or composite gasket. Permatex makes a synthetic gear oil safe RTV that is advertized to seal both synthetic and friction modifie limited slip chemicals. GM also sells an RTV that is advertized as Synthetic Safe.
Since manufacturers are selling maintenence free vehicles they have changed their lubricants to last the advertized span which in many cases is 50K miles or longer. For this reason many new vehicles come from the factory with synthetic based oils.

Here's a link to the GM Synthetic safe RTV. Click Here (http://paceperformance.com/index.asp?PageAction=VIEWPROD&ProdID=127532)

[BlackHawk99]

I did not use synthetic gear lube. Just convetional Valvoline 80W90 for about $3.75 a quart.

I used blue RTV. It was definitely RTV. I would have used black, except that I had none around the garage and figured the blue would work just as well.

I think I may have figured this out though.... Shortly after I posted this I went back and retorqued all the bolts to 205 inch lbs since I figured it couldn't hurt at that point. I also rewiped the exterior cover and the bottom of the diff. The cover was wet, and so was the bottom of the diff.

Yesterday morning I checked it and everything was still nice and dry. I then drove it about eight miles...got out and rechecked it and the cover itself was wet again, and there was a drip at the bottom of the diff. The wetness on the cover started at about 1.5" down from the top of the diff. Nothing above it was wet (not leaking from the vent). It's seeming to me that I may have a pin hole in the cover.

When I originally cleaned up the cover, the rust was peeling off it in sheets. I could get a 3"x 1" strip of rust off of it with no problems...and didn't even have to work at it. So, I'm thinking the extensive delamination caused at least one pin hole leak. It's only evident after driving...when the fluid is being sloshed around.

This may not be a gasket/sealing issue at all. I'm going to pick up a new cover, gasket, BLACK RTV, and more fluid and give this a shot again....

[1st Synthetics]

Let us know your result with the new cover.

[Jimmiee]

I did not use synthetic gear lube. Just convetional Valvoline 80W90 for about $3.75 a quart.

I used blue RTV. It was definitely RTV. I would have used black, except that I had none around the garage and figured the blue would work just as well.

I think I may have figured this out though.... Shortly after I posted this I went back and retorqued all the bolts to 205 inch lbs since I figured it couldn't hurt at that point. I also rewiped the exterior cover and the bottom of the diff. The cover was wet, and so was the bottom of the diff.

Yesterday morning I checked it and everything was still nice and dry. I then drove it about eight miles...got out and rechecked it and the cover itself was wet again, and there was a drip at the bottom of the diff. The wetness on the cover started at about 1.5" down from the top of the diff. Nothing above it was wet (not leaking from the vent). It's seeming to me that I may have a pin hole in the cover.

When I originally cleaned up the cover, the rust was peeling off it in sheets. I could get a 3"x 1" strip of rust off of it with no problems...and didn't even have to work at it. So, I'm thinking the extensive delamination caused at least one pin hole leak. It's only evident after driving...when the fluid is being sloshed around.

This may not be a gasket/sealing issue at all. I'm going to pick up a new cover, gasket, BLACK RTV, and more fluid and give this a shot again....

Glad you got it fixed. Next time do not use silicon sealer. RTV should always be used in place of a gasket and never with a gasket. If you are going to use or reuse a gasket then use a gasket sealer made for gaskets. Also the blue silicone is not the correct product to use with synthetic oil. Like 1st Synthetics said synthetic oil is not compatible with regular silicone sealer like the blue sealer. Permatex makes a special silicone for gear oil.

[BlackHawk99]

Is all 80W90 synthetic? Unless it is, I did not use synthetic fluid. When I redo this job, it won't be synthetic either.

Also, when I redo it, what's the best method - only black RTV and no gasket, only a gasket and no RTV, or RTV and a gasket?

[Jimmiee]

Is all 80W90 synthetic? Unless it is, I did not use synthetic fluid. When I redo this job, it won't be synthetic either.

Also, when I redo it, what's the best method - only black RTV and no gasket, only a gasket and no RTV, or RTV and a gasket?
You can get conventional 75-90 gear lube or synthetic. It's your choice. Usually conventional gear oil will last 60K in an open differential and still look like new providing there are no problems and the axle was set up correctly at the factory. I have serviced many differentials because of a gasket leak and usually the fluid looked good other than the leaky gasket.
As far as which method to use, gasket or RTV, it's up to you. If you have an open differential the odds are you won't ever have to service it again unless it leaks so I would recommend RTV.The reason is RTV sticks to both surfaces and doesn't shrink. It is much harder to remove however.On the other hand if you have a posi then most likely you will need to service it in the future. In this case I would opt for a gasket. If you use a gasket the best product I have found is the Permatex Red spray on gasket sealant. It sets up to a high tack quickly and holds the gasket in place. The reason a gasket starts seeping is because the bolts hold their original torque and position while the gasket will shrink causing the gasket to weep. Usually you can snug it up and it will be OK for awhile and it comes off very easily.
If you ask 10 people which is better RTV or Gasket you will get a 50 / 50 split on which is best. It's purely a matter of preference. Kind of like leveling kits, throttle bodies and cold air intakes. <G>

[1st Synthetics]

I switched out my rear Diff at 15K miles and the Gm synthetic that came in it was toast. I plow with my truck and haul a trailer. It was the worst looking fluid I have ever seen( silver and like water). I may have a problem in there and am going to put on a few more miles and send a sample in for fluid analysis. This way I can tell what is going on. It may just be from plowing snow or just junk fluid. I used 75W-110 for the replacement.

[Maxx]

Howdy, All.
I have a '96 Silverado 4x4 with 195k.
I had the leaking rear cover also. It is common up here in Michigan for the covers to rust pin holes in them.
I sandblasted mine looked it over then brazed the effected area.
I then used a rust preventing primer and paint.
When your low on funds and need it on the road now, You do what you got to.

BTW, The local Napa only stocked the 10 bolt and not the 14 so I opted to braze instead of waiting.

[Jimmiee]

I switched out my rear Diff at 15K miles and the Gm synthetic that came in it was toast. I plow with my truck and haul a trailer. It was the worst looking fluid I have ever seen( silver and like water). I may have a problem in there and am going to put on a few more miles and send a sample in for fluid analysis. This way I can tell what is going on. It may just be from plowing snow or just junk fluid. I used 75W-110 for the replacement.
Wow, that's not good. Silver oil means something is making metal and your rear end is going to fail. I would tear it down now before it takes out you ring and pinion.

[1st Synthetics]

Wow, that's not good. Silver oil means something is making metal and your rear end is going to fail. I would tear it down now before it takes out you ring and pinion.


It's under warranty still. I am going to check the new fluid and if I see a problem I will make the stealer fix it.

[TUTONE]

Very informative thread. Thanks guys.