Hi Guys,

I've got a '72 4-speed that I've been pulling my hair out trying to track down the source of a drivetrain/suspension "instability".

Here's the symptoms...
- Foot off gas coasting down to idle speed (like in a parking lot)
- Car decelerates smoothly reducing speed
- When car reaches the idle speed and transitions from decelerating to accelerating slightly there is a clunk from the rear end that sounds like some slop being taken up. This can be felt as a jerk in the car as well.
- That first clunk and jerk is violent enough that it now causes movement back in the direction it just came from resulting in another clunk and jerk.
- This cycle will continue indefinitely unless you accelerate from idle or press the clutch in.

The series of clunks can also occur at higher speeds (say 3rd gear) when going from no throttle to light throttle. If I tap the gas lightly I can get a clunk-clunk-clunk that then damps out.

So far here's what I've taken a look at:
- all u-joints have been replaced / look good
- new front differential cushion that doesn't appear to allow any movement
- replaced both trailing arm bushings
- new rear shocks
- swapped differential to a rebuilt 3.06 from Vtech Corvette (with GM posi fluid)
- trans output slip yoke replaced

Nothing is jumping out at me as having excessive slop when turning things underneath the car.

Anyone have any ideas? Thanks!

 

Sorry Mosley that I can't help you with your issue but I'll be following this closely as I have almost the exact same issue and all new parts everywhere as well
.
The only things I can think of is either the umbrellas have worn on the frame for diff cross member (new cushions) or the diff is not torqued down correctly to the crossmember (4 bolts) or the two bolts which connect the front bracket to the diff are lose or have to much clearance . I just haven't got around checking it as it involves tearing down the whole rear end
Nick
PS mine's a auto

 

Did you change the diff out because is this noise? If so I would look at the 4 speed. You could have some chipped gear teeth or something internal to the trans that is causing that play. If the noise started after the diff install, I would check that all the bolts are tight and the crossmember bushings are good as Dembo said.

 

The only bushing you have left is the transmission mount. They can break in half and you won't even see it unless you take a pry bar to it.
If it's not that, find a dyno shop and put it up on the rollers. They can simulate almost any driving situation.

 

Yae, the clunk symptoms existed before the differential swap. There was also issues with differential clutches locking up when they shouldn't. Swapping the diff fixed that but not the clunk.

I'll take a look at the trans mount

 

I find pushing the clutch in to be very effective. Don't know what to say to dembo with the auto.

 

Are your wheels tight ? How are the mounting areas of the rear end system and have you checked your wheel bearings and the trailing arms attachment points.

 

When on the highway, run the car up to about 50 mph and keep steady speed. Note the vibration level/instability. Now, let off the gas and push in the clutch. While coasting at constant 50 mph, note the vibration level/instability. Now, shift to a lower gear, but keep speed up to a steady 50 mph. Note the vibration level/instability. Now, let off the gas while still in the lower gear range and note the level of vibration/instability during coast-down.

The difference in these sensations at the different conditions can provide clues as to where to look for your problem. If you have difficulty in understands what these differences mean, report back your results and maybe we can help you diagnose the problem.