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C3 Rear End Chatter


Oct 9, 2007
San Francisco Bay Area
1978 Pace Car
Hello everyone. This is my first post. It's great to finally find an active forum!

I have a 78 Pace Car with about 31K miles. It's driven very little, which I'm told is bad news for a Vette. My car has been to numerous shops in the SF Bay Area, none of which know how to resolve this issue. In my non-technical terms, here's my explanation: More noticeably after warming up, but not always, it feels like the left rear wheel "grabs" and makes a rubbing/clunking sound. Also, the noise is more prominent when turning left or right. Shop #1 replaced the entire brake system- calipers, lines, hoses, master cylinder -except for the proportioning valve. About $2K later, I have good brakes, but still have the noise. They noticed the right rear axel was sticking out of the differential and recommended a driveline specialist. Driveline specialist replaced a clip and some rubber bushings on the right rear control arm, but said nothing about yokes or the differential needing work. After $850 more, I still have the noise. Shop #3 looked at the brakes again and said the line pressure was good. He added 2 cans of "friction modifier" to the differential and proceeded to tell me the noise had dissipated the more it was driven and was non-existent after his last test drive. I took it home and heard the noise. Needless to say, I'm at wits end. I am at the mercy of these mechanics, and I'm about ready to rid myself of this money pit. Your suggestions are most appreciated. ;help
... it feels like the left rear wheel "grabs" and makes a rubbing/clunking sound...


I wonder What the "friction modifier" additive was that was put into your diff.
I had a similar experience. It turned out to be a cheap fix. I purchased a bottle of GM posi additive fluid (available at your GM dealership).

After adding the fluid, be sure to take your car to an open area -- such as a parking lot and drive several figure-eights to get the fluid mixed-up. It worked for me, and the clunking is now gone.

Good luck... and welcome to the CAC!
Unless he drained the old diff lube first and added NEW fluid, he missed half the fix. Chances are the diff lube has been in there since 1978. The proper fix is to remove ALL the old lube first (with a suction gun - there's no drain plug), add two 4-oz. bottles of GM Posi Additive (P/N 1052358), then add GM Diff Lube (P/N 89021671) to about 1/8" below the fill plug hole, then do the low-speed figure-8 routine to get the fresh lube into the Posi clutch plates. If that doesn't take care of the noise and chattering (it usually does), the Posi clutch plates are damaged and the Posi unit needs a rebuild. Use the GM lube and additive - that's what the diff was designed to use.

"...Unless he drained the old diff lube first and added NEW fluid, he missed half the fix. Chances are the diff lube has been in there since 1978. The proper fix is to..."

Ooops, yes, JohnZ is correct (of course)... the shop did indeed drain the old fluid and replaced with new diff fluid and the GM posi additive...
try some BG LS1 rear diff additive I have had very good results with this additive in stubborn chatter problems it would probably be about 15 dollars , if the unit does not need a rebuild this additive has worked great for me Steve
If the figure eights didn’t resolve the problem then I would find a Corvette mechanic that specializes in C3 Corvettes they are out there. I would ask about half shaft U-Joints and rear wheel bearings.
I bought my 1977 in 1982 with 29,000+ actual miles on it and just after a couple of thousand miles I had the same noise your experiencing start and it was a rear wheel bearing so I wouldn’t rule out a low mileage car having these mechanical problems.
Your car is almost 30 years old now and maybe not as collectable value wise as earlier C3 but still something that will continue to increase in value if kept in its original state during repairs.
Sounds like posi chatter and very common. Typical shows up in the later model years but also found it in the earlier years. The clutches in a stock setup are usually sloppy and the fact the clutch type used in the 71-79 was weaker doesn't help.
A lot of time draining the old oil and filling with a 90 wt or 140-85 gear oil plus 1-2 bottles of GM additive will help. If it doesn't then the posi needs attention.
I have detailed this procedure on different sites and can link it if you would like more info. I set up the posi's a little differently and this has eliminated the chatter, provides better action and less wear.
Try the oil change first to see. The 78-79 diff were also prone to some other problems such as soft yokes and ring gear bolts backing out.
I have a 78 Pace Car with about 31K miles. It's driven very little, which I'm told is bad news for a Vette.

With the amount of work you've already done and still need to do it's possible that your actually car has 131,000 miles. No matter.

The old adage of 'not driving it is bad' is by and large false. Neglecting it while stored is a much bigger factor.
I had the same problem with my recently purchased '75. I siphoned out all the old differential fluid (which was black and about 1/2 quart low) and replaced it with Royal Purple Max Gear 85w140. It has completely eliminated the problem.
Been there... done that... and with a 1978 Silver Anniversary.

The sound you hear is the POSI trac clutches. (They expand slightly when they warm up.) If the lubricant has broken down and no longer retains it's chemical properties, the clutches will start to grab... Do what had been stated here, drain the fluid, add two bottles of additive and top up with gear lube.

You may wish to drive in a figure 8 in reverse when its cold to help circulate the fluid better. Do not despair... it is fixable!
Thanks everyone for your suggestions. I doubt either shop drained the diff fluid completely and replaced it with the right products.

Worst case scenario - If, after the fluid is replaced and the chatter is still there, what would I expect to pay to replace the posi trac clutches? Also, does anyone know of a reputable Corvette mechanic in the SF Bay Area? Thanks again.
C3 Corvettes are unique cars and as such they must be serviced by pros. Ask here and at local Corvette Clubs for competent mechanics. That will be the best thing you can do or you will risk the inexperienced mechanic replacing parts until they stumble upon a solution or you run out of money and patience.

I am on the right coast so I can't be of help to you there in Frisco but hopefully someone will chime in with help. You can also post on the Corvette Forum at :
They have a lot of members in Ca.
Thanks everyone for your suggestions. I doubt either shop drained the diff fluid completely and replaced it with the right products.

Worst case scenario - If, after the fluid is replaced and the chatter is still there, what would I expect to pay to replace the posi trac clutches? Also, does anyone know of a reputable Corvette mechanic in the SF Bay Area? Thanks again.

I notice that you have 2 posts...
Firstly; Welcome to CAC. Post a picture of your car when you have time.

Second; A 1978 isn't rocket science. If you have a General Mechanic whom you trust... they should be able to figure it out. The only thing different between a Corvette and the rest of the Chevrolet family is that a 1978 has a different rear end, (all the joys and pains of an independent suspension, but not as bad as a Jaguar ) and a complicated vacuum system for the headlights. Everything else is fairly standard GM parts. A 1978 is a regular Carburated 350 with HEI . The Transmission is a th350. The brakes are standard discs. The heater, wipers, and electrical are fairly straightforward.

This is why I love this car. I get all of the styling and performance of a sport car, yet have all of the reliability of a standard chevy. It don't get better than this in my books.

I would suggest 2 things for you:

1) Contact your local corvette club. They will give you all the local info you could want. Also ask in the regional area of the forums. http://www.corvetteactioncenter.com/forums/forumdisplay.php?f=78

2) I would suggest getting a couple of catalogs.. Corvette Central, Ecklers, Corvette America, and my favorite catalog is ZIP products (www.zip-products.com/ ). In these catalogs you will find ALL the parts you could ever need. As well, many of them show you where these parts fit. This will help demistify the parts that make up a corvette.

Worst case senario... you buy a new part and have it installed. That is a pretty reasonable worst case. Hope this helps.
Without seeing this car, but understanding it's a 78 which has not received periodic changes of rear axle lubricant; I'd say the ultimate solution is going to be to overhaul the rear axle.

Now...let me address the sometimes-flimflam of "posi additives".

Virtually all of these additives, including the "GM additive" are friction modifiers and the modification they make is to reduce the friction between the plates of the clutch-type limited-slip units which have been used on Corvette since the 1950s. Since clutch-type limited-slips rely, in-part, on the friction between those plates to work, reducing that friction reduces the effectiveness of the limited-slip unit.

While it is true that few limited slips will work chatter-free without some percentage of a friction modifier added to the lubricant, because these additives do their magic by degrading limited slip action, for best performance; you want only the minimum friction modifier required to eliminate chatter and noise.

My rule of thumb is to add limited slip additive on a trial-and-error basis. Admittedly, this makes the process of adding the additive time-consuming and a bit tedious, but it enables you to add only just enough additive.

On a newly overhauled limited-slip axle or one which is in good condition but undergoing a lubricant change, I fill the axle with lube, add one ounce of additive then go for a road test. This test must be at least 10 miles in length to ensure the lubricant reaches nominal operating temp.

Next, I find a safe parking lot and drive in tight circles at slow speeds. If I feel chatter or hear loud squealing from the axle, I go back to the shop and add a second ounce of additive. I road test again. If there is still squeal, I'll add a third ounce but after that, if the axle still chatters or squeals loudly, I usually recommend an overhaul or a redo if the axle is freshly rebuilt.

Lastly, my personal taste is to trade noise for effectiveness, so I'll add enough additive to eliminate chatter, but I do not add enough additive to eliminate all noise. I add enough additive such that in the tight circles I might hear a subdued squeal.
The brakes are standard discs.

Gotta disagree with you there. C2-C3 Corvette brakes are unique. Most other cars have a single piston floating caliper. Corvettes have a four piston fixed caliper. Floating calipers have far more tolerance to rotor runout.

Corvettes were built with a rotor and hub that were final machined as a set to minimise runout on the rotor surface, meaning that you may have to re-machine the set when replacing a rotor. Other cars had hubs and rotor that were finish machined separately and can be field replaced without concern for runout.

The two factors above mean that the average mechanic would not have a clue on the care and feeding of C3 brakes. This is one main reason why we see dozens of posts from people who have chronic loss of brake problems.
100% right about the brakes and lack of common runout knowledge
Take a few more minutes to evaluate the condition of your differential (or find a shop that will work better with you to checkout the current condition). At 43,000 miles my 1979 was displaying characteristics similiar to what you appear to be describing.
Once the rearend fluid got warm, the positraction unit would "chatter" when making a 90 degree turn & accelerating from a stop (like turning out of a driveway or turning from a stop sign). Feels like a jerking, or grabbing sensation from the rear axle.
The problem can be caused by several wear factors combining together, or a single part that has worn prematurely. The C3 corvettes are notorious for having soft OEM differential side yokes.
To check for excessive side yoke movement, get the rear wheels of the ground. It is best if the tire/wheel is close to normal ride height, but it can also merely hang free.
The side yoke movement is best checked by a 2 person process. One person lays under the car and observes the movement of the side yoke in & out of the center diferential housing while the other person forcefully push the tire/wheel/axle assembly towards the center of the car, and then pulls it towards the outside of the car. You are trying to get an idea of how much movement the side yoke has in & out of the center differential.
GM does not have published specifications for what acceptable side yoke movement should be. The less movement the better. If you've got 1/16" or less on each side your yokes are probably OK. (On my 43,000 mile 1979, I had nearly a 1/4" of movement on one side, and a little over an 1/8" on the other. These specs were excesive). Besure to check both sides. It's not uncommon for one side to wear more than the other. If you have too much movement either the ends of the side yokes are worn excessively, or your positraction clutch pack has worn excessively (or broken some clutches), or both conditions are present. If you have excessive side yoke movement the only way to isolate the worn parts & fix the problem is to remove the center differential & inspect the internals.
To simply check the condition of the positraction clutch pack, pull a tire/wheel, attach a torqueing fixture to the lug nuts, use a torque wrench and check the amount torque required to turn one wheel while the other is held stationary. No resistance, or too little resistance, indicates a worn positraction clutch pack.

After replacing both side yokes (with hardened afteermarket pieces) and replacing the positraction clutch pack (cheap insurance-might as well do it while your there) my side yokes movement was .010" on each side, and all the chatter was gone. The car also drives/handles alot better.

I had a similar problem with my 76 that I bought last year. The chatter was horrible and got worse as it warmed up. I changed out the fluid and added the limited-slip additive and the problem seems to have done away completely. If it wasn't for this message board, I was almost ready to take apart the entire rear brakes. Thanks!

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