1978BackToLife

Hi all, thanks again for all the previous help with this 78.

I have noticed a popping that's grown into serious banging coming from the rear end - it happens only after about 30 minutes of driving, and only happens at slow speeds when turning. I've done some research on the forum ahead of time, and there has been some discussion of a common problem with later 70s "rear differential chatter." I haven't seen a video or audio describing the problem, so I'm hoping this video might help as an aid to identify it as the problem:


My question is - some people have advised using 2 x 4oz GM post additive fluid. Does this sound like it could be cured with additive, or is it loud enough that you recommend I take off the cover and start inspecting for wear etc. towards a rebuild?

I've checked u joints which appear fine visually, brakes have no affect on noise or not. As a note, the previous seller was a general bubba and said he had changed the differential fluid in his list of service done... meaning he may not have used additive at all. Thanks for any advice.

DUB

You can not check the U-joints by just looking at them.

I have heard this SAME SOUND many times and it can be universal joints...rear wheel bearings....or brakes grabbing.

I am NOT 'saying' that you do not need to check your rear differential fluid...but I doubt that that is it. But I have been wrong and the additive and fluid is cheap.

DUB

Wee

DUB knows way more about these things than I do so take what I say with a grain of salt....I don't really see why turning the front wheels would make the u-joints sound like this....I would think they would do it all the time.

What does it sound like going down the road....?

When my differential started making the noise while turning it sounded like a hand full of gravel was tossed in there with the gear and you could feel it in the seat of your pants. 2 bottles of additive fixed my issue.

Brian

C3 4ME

You can't remove the cover off and inspect for wear like other cars. You'd have to drop the rear crossmember with the diff attached to open it up, and that's a major job. I'd start with draining a replacing the gear oil and refill with new, to include posi additive. It's cheap enough to try and if it doesn't help, you're not out much money, but it might help.

resdoggie

Change out the lube.

Then go do a few figure 8's. Might try a few figure 8's beforehand and see if that fixes it.

super charged

Change the Oil and add the additive.

drwet

I can't see how this can be anything but clutch chatter in the differential. Worst I've ever heard, but still has to be the clutches. Question for me is whether they are so far gone they need to be replaced, or will proper lubrication solve your problem. Given the relative cost of the two possibilities, try replacing the lube with the correct additive and see what happens. If that doesn't solve your problem, I think you're looking at a diff rebuild. I'm betting the lube will do it.

1978BackToLife

Very much appreciated, sounds like my weekend is ahead of me! will report back and thanks

REELAV8R

Sounds like something binding and releasing to me. I appears in your video that you wheel maybe has a lot of postitive camber, hard to tell for sure. This can be due to the stub axle ends wearing down and allowing the half shaft to receed into the rear diff.
I would check to see if you have a stub axle dust shield grinding away at the rear diff case.
Alternately I would check the play in the rear axle bearings. Jack it up and see how much play you have in the bearing. Bearing could also be dry.

Check the ujoints for grease, if the greasable type.

DUB

I will wait for some feed back IF he checks the rear wheel bearing and U-joints.

I am still feeling that it is NOT the differential fluid.

DUB

REELAV8R

I'm leaning that way as well.

rebel542

It's the clutch pack grabbing. Change fluid and add additive and it's good for 10K or so.

drwet

Its 5 to 2 for changing the fluid and adding the additive.

bazza77

my 77 diff sounded same as in the regular knock noise but instead of the knock it was a metal crunching sound .

I didn't think we would make it home , but I nursed it and got home . Changed the fluid(which was black) and two bottles of additive and amazingly it stopped doing it ,I was already for a renewal and all but .....


Just as a side note the fluid in it was about two years old ,not too many miles done in that time .

REELAV8R

Not a bad idea to start with the fluid, just saying there are a few other things to check as well as it could be one or more items that need addressed.

BLUE1972

Change the fluid and add GM posi additive. Use a quality oil.


Then drive it and go to an open parking lot and do a few circles at very low speed, 10 - 20 each way.

DUB

I am still going with an issue at the wheel area (bearing or universal joints).

I guess we all will soon find out. Hopefully.

DUB

Spoticus

Hi. I am interested as well to the solution for this problem. My 78 makes a similar noise though not as bad and not very often. Maybe once every 2 months and only after it has warmed up. It only started a little while after my diff had the yokes fixed and the rear bearings were replaced.

I am hoping it is only an extra bottle of diff additive needed.

BLUE1972

With DUB

I am hoping it is only an extra bottle of diff additive needed.[/QUOTE]

Just a few thoughts:

A quick PIA check - I would drive until the noise starts then pull over somewhere safe and feel the universals - they may be very warm.... if one is red hot - that one is bad. It's harder with the hub bearings due to the heat from the brakes, but if one is much hotter than the other I would then check that hub.

If you have or can get an inferred thermometer check the temps it will be more accurate.

An outside chance is a binding shock / caliper that heats up and grabs / binds internally.

corner1

I have this exact problem with my 78. What was the final solution?