reidclif

I am new to Corvettes and not much of a mechanic. If fact I have been the proud owner of a 1979 Vette owner for exactly four weeks tomorrow.

Anyway, my car makes a pretty loud grinding (or maybe more of a crunching) sound on turns or up hills only after it is warmed up and only after applying the brakes. The brakes do not grab when being applied.

I thought the problem might be with the rear end but my mechanic said the rear end was fine. He said the passenger side rear bearings needed replacing. He did that but the grinding is still there.

What else could it be?

VettePower

Sounds like you need to drain the rear fluid and refill with new posi-lube and gear oil.......And get a new mechanic!!!

0Willcox Corvette

Cliff,

You should have run this by me when you were at the store the other day.

The rear end, if a posi traction unit will make a popping sound if the clutches are sticking and you are turning around a corner. When you try to turn a corner the clutches should not bind at all, but if they are sticking on the car you will hear a loud popping noise from the rear end tracking around the corner at the same radius. “I’ve always called this Duck Waddle”. It feels like the rear end is trying to scoot around with the popping noise and you should be able to feel it as well as hear it if they are really bound.

The posi unit will allow the right wheel to turn a different radius than the left wheel when they are working properly. When the clutches stick they are in a dead one to one drive and the outer wheel is bound trying to turn the same radius as the inner wheel. . . thus the popping noise.

You can change the fluid and additive in the rear end, but this is not a guarantee it will solve the problem. It should clean up the clutches, and it may take a couple of changes to work correctly again, but it’s better than tearing down the rear end and replacing them.

If you want to bring the car by after the 2nd I’ll have someone in the shop look at this for you.

Willcox Inc.

VettePower

I'm sorry. I guess you were the mechanic he referred to in his first post.....

0Willcox Corvette

No I was not the mechanic! Had I been the mechanic I would have found the problem!

reidclif

Thanks again for your help. And I think I will look for a new mechanic.

reidclif

The guys at Willcox have been very helpful. Willcox is a parts supplier, not a repair shop but they are extremely knowledgeable. I learned more about cars in an hour with them than I thought possilbe. Their prices are great and they are all owners/builders of Corvettes. I highly recommend them.

VettePower

Again, I am sorry!!! For real!!!

0Willcox Corvette

No problem VettePower,

Cliff, we do have full service shop! They were on vacation last week and this coming week.

Call Joe and schedule an appointment and well figure out what is going on with your noise.

Willcox Inc.

VettePower

You wouldn't by chance be in Houston?? I need some work done myself!!!

DaveP52462

This is what is called posi clutch chatter. Very common in these old vettes. Usually happens after the rear is all warmed up from a cruise and then you come into town and have stops/turns, parking lots. Can be embarrassing at times as it sounds like a piece of **** pulling into a parking space.

If memory serve me correctly the reason for this is that the output spindle in the rear gets worn at the center near the c-clip which allows the shafts to have excessive in/out play which in turn causes the posi unit to bind or release a stored torque on turns as the shaft moves side to side on turns.

Usually veh with this problem will have excessive in/out play on the rear wheels. I've had a veh that had so much play the rear wheels were angled in / \ and a wheel alignment would not bring them to specs. I've heard of situations where the output shaft can grind away so much that the c-clip falls off and causes major harm.

Depends on how bad yours is. I have babied Veh like this for many many years before a rebuild. The rebuild can be expensive. Beware.. Another symptom is while you doing a high g turn the rears kicks out on you.

Good luck
Dave..

RunningMan373

I'd say you don't have any real problem,, jot yourself down as "locationally fortunate".

GTR1999

Try the oil change. Go with Lucas 85-140 and add 2 bottle of GM additive.
The 79's use the cheap clutches that break, unless someone replaced them. The fibe clutches on the market aren't much better and I never use them.
The problem is more likely the result of a sloppy stock setup. If the oil doesn't work you either live with it or drop the diff. If you do there are several ways to set up the posi. I've never had any clutch hammer in the posi's I built and I never use the springs.
When you have it in the air look at the side yokes as well be sure they are not hitting and grinding the housing, that could wreck the whole deal. The other issue with the 79 diff are the ring gear bolts. A lot of them backed out in the past.
Good luck

dave_roads

I've been using these guys in the woodlands. http://www.dqcc.net/photo.htm
the blue 71 (also seen in the frame-off) is one of the owners cars.

ENG40

The GM additive is the only thing that keeps mine quiet. Without it the sound is obvious.

Peterbuilt

I'm going to change the rear end fluid soon, I bought "Trans X" form the auto parts store, it says posi-trac limited slip oil additive.
Is this ok or must I get GM additive?
After I put 8 oz of this stuff in, what is the correct oil to top it up with? PG.

reidclif

Thanks to everyone for the tip on adding fluid/additive to the rear end. I had the fluid and additive changed today and the rear end noise stopped!

Again, thanks!!!!

original76

I'm having grinding problems on my 76 with 204000 miles. I've tried to get the differential filler nut off a couple of times in the past, but found I couldn't get a wrench on the filler nut and turn it. What am I doing wrong?

wombvette

I am supprized a 76 still has a rear end with 200K miles, they were so bad about eating stub axles. You can get a square socket, really 8 point but fits the square nut. Makes it much easier with a socket wrench.

David Ey

USE THE GM ADDITIVE.....or Royal Purple