Rear End Woes Are Gone

[Bill Cody]

On Saturday, May 27, I posted the following message:

----------

" I have a question regarding a 'chunking' sound in the rear of a '75...

It only takes place at the end of coming to a stop and upon proceeding from a stop. It seems to alternate between one or the other wheel and never happens above a few miles per hour. The sound resembles more of a grinding chunk than a metallic clank or clunk and can actually cause the rear of the car to jerk. It comes and goes and only appears to do it when the brakes are hot.

I first thought it might be the rear disc brakes grabbing, but it happened last night after coming to a stop, on a steep hill, primarily by gravity. The grinding chunk sound/feel occurred upon accelerating from the stop.

It's a '75 Stingray with a 350 engine and automatic transmission. The rear brakes are disc. Before I begin turning exploratory wrenches I hope to hear what you folks think it might be. I'd appreciate any light you may shed upon my ailing 'Vette! "

--------------------

After checking out the u-joints, bearings, etc., to make sure they were OK, we added a bottle of positraction additive to the differential and within a few miles the problem completely abated. We have driven several hundred miles with no trouble at all. We even tried to replicate the exact conditions, on the same streets, that had always caused the malady. It has not returned and I'm thrilled to report that the trouble is completely gone.

I hope this will be of help to anyone else who might be having the same problem and can't seem to locate the trouble spot.

My sincere thanks to all of you who offered your thoughts and insights.

Best Wishes,

Bill Cody

[Don O'Connell #33101]

Re: Rear End Woes Are Gone

Bill,

I to have had a similar "clunk". Who's additive did ya use and how much did ya put in?

Don

[David W.]

Re: Rear End Woes Are Gone

I would also like to know what you used and was it an additive or gear lube. Along the same lines, I checked the archives earlier today for, interestingly enough, for info on trans and rear end lubes. Alot of good info in the archives. An additional question for anyone who knows, is it a problem to mix synthetic lubes to the lubricant that is already in the rear end or transmission or should they be changed completely? Thnaks, David

[Joe L.]

Re: Rear End Woes Are Gone

David and Don------

The best additive to use is the GM positraction additive. The part number is GM #1052358. Most Chevrolet dealers carry this item. It is supplied in a 4 oz plastic bottle. I prefer to use 2 bottles. In later model C4 Corvettes GM also recommends the use of 2 bottles.

As far as mixing symthetic and non-synthetic lubricants, the two are completely compatible. However, personally, I still wouldn't mix them. It just doesn't "feel right" to me.

[Duke W.]

Re: Rear End Woes Are Gone

If you have a suspect positraction clutch chatter problem, rather than add the additive, I suggest draining the old oil and refilling with fresh oil and the additive. The best mineral base oil to use is GM 1052271 (23 oz. bottle) along with the additive that Joe specified, and you'll need enough to fill the C2/C3 approximate 3.7 pint capacity. The other axle oil that has proved satisfactory is Redline 75W-90 synthetic, which has the friction modified already added. As Joe said, mineral based or sythetic based lubes are usually compatible, but I, too, would not mix them intensionally. I currently lean toward the synthetic, though it is a bit more expensive, and it's alway a good idea to change positraction rear axle oil about every 30 to 60K miles to keep the clutches from developing a chatter problem.

Duke

[Bill D.]

One more step !!

Bill and others--

Take Joe and Duke's recommendation one more step .

Now that you have cleaned the clutches, it is a good idea to change the fluid to remove the suspended contaminants. Do this after taking the car for a good run to heat up the oil. I have seen the chatter problem resurface if the oil is not changed before the clutches get recoated. You will then be good for 30K as Duke said. I too prefer synthetic,Mobil I 75w90, but I still add the GM posi additive. Just makes me feel better

Wild Bill

[Bill Cody]

Re: Rear End Woes Are Gone

Hi Don and David,

At first, I added one bottle of GM positraction additive, part # GM 1052358, in order to test it out.

When the problem disappeared, I drained the old oil from the differential and refilled it with GM 1052271 oil and two bottles of GM positraction additive (GM 1052358).

The differential has been quiet ever since (hundreds of miles).

My thanks, again, to everyone for your help...

Bill Cody