I finished my interior resto on my 76 to the point where it was ready for a ride. I backed it straight out of my garage and when I began to turn in reverse it began kind of binding and popping fairly severely. It seemed to happen much less when the car was going straight, but was very pronounced when trying to turn at a slow creep. I never got it up to any significant speed because of how bad it was at 2 to 3 mph.

I climbed underneath and saw no apparent signs of damage or broken parts. We put the rear on jack stands and gassed it in both forward and reverse. There was maybe a tiny bit of a noise once, but it seemed mostly to operate as would be expected. We even got it "up to speed" (rear tires in air) and stepped on the brakes. There was no sound and the brakes seemed to operate properly. The rear tires both seemed to draw up a bit into the wheel well when we did this, however. Thinking it was just a fluke, we dropped it and tried to take it for a ride. The problem remained. I slowly rolled to the end of the driveway before I decided the popping was too dangerous to attempt to drive, even for further diagnosis.

There seems to be no binding or "catch points" when you spend the rear tires by hand.

This may sound silly, but I could best describe the sound as a "cha-pop errr cha-pop errr cha-pop." It almost sounds like a spring being bound up and popping loose. It seems to be mostly on the drive side, but the effect can be felt touching the passenger side rear fender, and you can actually feel it inside the cockpit of the car.

I did not do anything whatsoever to the rear or front suspension during the 9 month process. This was the first time the car moved since then. The car was jacked a few times when installing the new floor pans, but I always used frame, never suspension or shafts, as my jack point. The right rear wheel was sitting on a halfway deflated tread-separated blown out tire for about 3 months. I drove the car for a couple months before beginning the interior resto, and I never had any issues like this. The car handled fine and the rear end made no noises, before now.

For what its worth, there is some wiggle in both wheels if you lift it up and try to torque each end of the tire in opposite directions. I believe they say that's a worn halfshaft issue, but it is no worse now than when the car was driving fine 9 months ago.

Thanks for the help!

 

Sounds like the posi clutches in the diff are dried out. Make sure the diff has oil then try some sharp turns or figure 8's to get it working again. You may have to change the diff oil and add the posi additive.

 

Also, do not "run" the rear suspension with the tires off the ground unless you have the control arms supported with stands which keep the outer half shaft u-joints less than 1" below the inner u-joints. mike...

 

In my dream of dreams it will be that simple to fix. Thank you for that advice.

If I pull the plug and it has plenty of gear oil in it, should I still suspect that the posi clutches are dried out? Is it like other diffs where there is no drain plug, only an add plug up top?

If it is low on oil and I changed it out, should I expect most of the problem to be gone when I back it out of the garage and cruise the neighborhood? If not, how long until that should work itself out after new fluid? I guess I'm just worried about trying to turn around in a circle at speed while this thing is still popping, thinking I'm letting itself "work the problem out" while actually causing damage.

EDIT:
If those clutches were dried out, would it have operated normally when the rear tires were in the air?

Thanks!

 

It shouldn't have operated "normally" with the rear tires off the ground, unless it was supported under the shock mounts or the trailing arms. You should never operate the rear wheels with them "hanging"...it could do significant damage to the U-joints.

You can't 'drain' the differential. There is no drain plug. There is a "fill" plug on the side where you have to remove lube, if you want to re-lube it. However, you can remove the plug...and if there is enough room, add a little bottle of the Posi-Trac additive.

If you get in there, you can decide if you want to replace the present fluid or not. Kinda depends on how long its been in there.

 

Thanks for that advice. I did not know that. Hopefully I haven't done more damage. I really just want this thing to get going right.

I'm going to syphon out the old fluid as best I can and then put some new fluid and additive. The old fluid doesn't even look like gear oil. Thinner than I thought it should be and pretty dark.

For what its worth, the fluid was only about and inch lower that the opening. Should the clutches have dried out if they were still sitting in fluid?

 

The clutches are not dry but the lube may lack the friction modifier that allows them to "slip" without chattering or binding especially when going around corners. I would recommend GM fluid and their posi additive. mike...

 

The reason it doesn't make the noise going straight is that the posi clutches aren't slipping because both tires are going the same speed. When you turn a corner one wheel has to go faster that the other, therefor the clutches have to slip, hence the noise(chatter).

 

I suspect that when you put some new gear lube (nearly 2 bottles of it) and a small bottle of that posi additive, then do some figure-8s in a parking lot, your diffy will work a bit better.

 

Thanks to you and to everyone who has helped. I thought I was really screwed.

I am still curious about a couple things, however.

As I described in the OP, its very bad right now. How long after I changed the fluid should I expect it to settle down and become normal? Should I be prepared to drive through the neighborhood several times with it popping and binding like its doing now, or should it settle down by the time I leave the driveway?

If its still acting this way when I leave the driveway, should I go ahead and bring it up to neighborhood driving speed and make loops around the culdesac?

I just want to make sure I know what to expect so, in the event the differential may have permanently failed, I don't cause excessive damage. This kind if problem is uncharted territory for me.

 

Yes. Don't overthink this.

 

If the car is making nasty sounds when it is driving straight, you may have some problems with your U-joints. If it only makes sounds from the differential when turning, head for the cul-de-sac.

 

Or the church parking lot on a weekday...........

 

I had a similar problem and sound with one of mine right after I bought it. I also thought it might be the clutches, but while changing the fluid I just happened to notice that one of the u joint retainers had come out and the bearing cup had worked half way out causing it to bind and make a popping sound once every revolution.
Just one more thing to check.

 

Sounded just like mine, pulled the plug and had hardly any fluid came out. Changed out the fluid with posi additive and maybe within a couple hours of driving the noise went away. Have never heard it since.

 

My 76 did the same thing after somewhat sitting for a year. Making the same noises mainly when I would turn the wheels going slow. I asked on here and everyone said the same thing to me as they are to you. If your diff is full, just go find a parking lot and drive it in figure 8s. a few times. should be good to go after that. Might want to go ahead and drain your diff and put some new oil and posi fluid just for GP.

 

I did drain (or rather, suction out) the old fluid and replaced it with new oil and posi additive. It was literally 95% better as soon as I started backing it out of the driveway. It still binds a bit when I am making a hard over turn at a slow creep, but I've only put 20 to 25 miles on it since the fluid change, and most of that was straight on a highway.

I'm banking on the rest of it going away soon. Thanks to everyone who helped!

 

Did you not go to a parking lot to do the right & left figure-8's as mentioned? By making some slow, full-lock turns...in both directions...you eliminate having to go many miles of normal driving to get the diffy plates "treated".