'79 differential problem

[MacD-Vette]

I recently purchased a 1979 Chevy Corvette and wasn’t sure about the condition of the differential oil so I drained the old and replaced it with Quaker State SAE 75w-140 GL-5 Full Synthetic gear lubricant. My problem is that within a half hour of driving a strange noise and sensation started coming from the rear of the car, it sounds like rubbing rubber almost like something pushing against a tire, but this only happens when accelerating through a turn both Right and Left. It doesn’t happen when accelerating or cruising straight or coasting through a turn. The tag on the differential reads “use limited slip oil only” and the unit was making no noise before. I hope you can help with diagnosis, Thank you.

 

[XLR8]

Hey MacD! I'm going to relocate your thread to the C3 forum where it will receive greater exposure because this forum is for testing posts, features, signatures, etc.

Jane Ann

 

[muddywaters]

Use ONLY GM lubricant with their additive, drive vehicle in tight circles, both directions and your problem should be resolved.

 

[gmjunkie]

Use ONLY GM lubricant with their additive, drive vehicle in tight circles, both directions and your problem should be resolved.

 

[Hib Halverson]

With a 30-year old rear axle, it's very possible that the limited slip is somewhat worn and sensitive to lubricant.

Generally, GM's lubricants are good choices but...it is pure BS that you must use only a GM lubricant and limited slip additive in a Corvette rear axle.

The recommended lubricant for a 79 axle was an SAE90 GL5-rated gear lube. You can get that from any source. Lubricant upgrades, such as the multi-vis synthetic you got from Castrol, provided they're compatible with limited slip diffs., can be a good upgrade.

As for the additive...more properly a "friction modifier"...

Some limited slips will make noise or will chatter. I'd expect that from the axle in a 79 which may never have had a lubricant change and may have had some friction modifier in the lube you drained out. A friction modifier works by reducing the friction between the limited slip clutch plates in the interest of reducing noise or chatter. The problem with this can be that you are actually reducing the very characteristic (friction) that makes the limited slip work. There is fine line between using too much additive and degrading limited slip action and using just enough to stop the noise.

The first step here is to determine of the Castrol gear lube you put in the axle is designed for limited slip differentials. If it is, I'd go to GM and buy one bottle of its limited slip additive. Add half the bottle's contents then road test. If it still makes the noise, add the rest and road test, again.

If the Castrol is not for use with limited slips, you'll have to drain it. Then, I'd look for any name brand GL5-rated gear lube designed for use with clutch-type limited slip axles. It doesn't need to be synthetic unless you are racing the car. Put the new lube in and road test.

If it's noisy, you need a bottle of GM's limited slip additive. Again, add half the bottle and road test, then add the rest of it's still noisy. If the noise persists after adding a full bottle, you can use more if you want, but what you really will need is a rear axle overhaul.

 

[CarolinaC4]

When I change the oil in my differentials I will add 2 bottles of the additive. I have done 4-5 this way without any problems. Older rearends usually just need a fluid change. Easy way to do it without dropping the rear cover is to use a suck gun, this will get about 90% out.
*Also one other thing to check is the side yokes for play, 79's are bad for excessive wear on their side yokes due to incorrect heat treatment when they were manufactured. Every one of the 79's I've checked had excessive wear except one. My own with only 32,000 miles on it is worn out so bad that the U-Bolts holding the Half shafts to the yokes are rubbing the sides of the differential housing. In this case all that metal is running through the gears and bearings which will require a complete rebuild soon. :ugh As I don't drive the car every week or so I have been putting it off for now.

 

[Hib Halverson]

Many people use two bottles of additive, but for a limited slip in good condition, that's way too much and will degrade the limited slip's function.

The problem is that most do not understand what these "additives" do...which is trade limited slip action for less noise. "Posi" or limited slip additives are friction modifiers and, thus, by definition, a "necessary" evil in a limited slip diff. Yeah, you may want some to eliminate the noise, but you get that quiet through slightly degraded limited slip action.

When I add friction modifier I use the minimum and actually accept a low level of noise---a sort of "subdued squeal"---from my rear ends when driving in tight circles.

In my experience this has required 1/2 to 3/4ths of a bottle of additive.

Also, in my experience, when it takes more than 1.5 bottles to quiet or subdue the noise, that usually means the limited slip needs repair rather than an additive.

If noise is the main issue and limited slip action is secondary or unnecessary, put three bottles in.

 

[GTR1999]

I suspect that the oil is part of the issue, the 79 diff parts the other.
I have a 79 apart now in fact to build a super 10 bolt out of it. The 79's have issues with the lousy clutches used,weaker RG bolts, and soft axles faces.
This one came with "snowflake" clutches and everyone was broken.

Your problem most likey is posi hammering caused by the oil. I have tried several oils that had LS additive in them and never had luck with them. Since I custom build these I can't have issue with oil, so I only use Lucas 85-140 gear oil (no syn) & 1 bottle of GM additive, I never have hammering.

Suck out the oil and add the Lucas and GM additive, if you still have hammering then the posi needs attention.