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940 clunk in the rear suspension when jerking forward

rokaskey

Member
Joined
Mar 10, 2017
Location
Lithuania
Hello,

After a misguided drive through a very bumpy road, the rear suspension has developed a very particular clunk. It seems to appear only when the car jerks forward at low speed. For example when rolling backwards and then accelerating forward the rear clunks. It sounds and feels that something is hitting the body in the rear. Also on tighter corners it feels as if the rear axle is loose from the body and moves from side to side independantly.

All bushings except the rear cross-member (the frame that mounts torque rods, don't know the actual name) ones that bolt to the body have been recently replaced. Pan-hard is adjustable with poly bushings. The sound only appears on lateral movement (bumps don't produce any sound)

I went under the car to try checking bushings for play with a crowbar, but couldn't find any play.

Does anybody have any suggestions where the issue could be?

Car is 1990 940 sedan with G80 locker, slightly lowered. Driveshaft center bearing has been recently replaced and is raised to even out the driveshaft from lowering the car.
 
A blown out (damaged/failed) rear damper is a suspect, especially with your rough road trip.

If truly a torque reaction issue under full accel, then your torque arms which connect to the steel subframe must have broken bushes. You can remove and have new rubber bushes pressed in.
 
A blown out (damaged/failed) rear damper is a suspect, especially with your rough road trip.

If truly a torque reaction issue under full accel, then your torque arms which connect to the steel subframe must have broken bushes. You can remove and have new rubber bushes pressed in.

Full accel is not really needed I'd say.
I had a blown front damper and the clunkiness was more reactive to bumps. Although can't really dismiss it, since rear shock age and operation is questionable.

Also torque rod bushings have been replaced roughly 2000kms ago. I doubt they are the culprit. The subframe bushings on the other hand are old
 
I kind of started to have the same problem. Plus a little shudder right on take off.
Can the rear subframe move in it's bushings? I guess it would convert forward/back movement of the torque rods into up/down? My torque rods are OK with poly bushings on the big ends (axle mount side).
 
A little update on the problem. I drove around and noticed a couple more things. The clunking can be heard and felt when the car jerk either forward or backward. Seems as if something shifts front or back in the rear suspension, because the clunks only repeat once per direction of jerk without variation.
Almost seems as if the whole axle shifts forward or back, but that could be my imagination
 
Front to back with clunk I would suspect the trailing arms.

But, maybe check the torque/re-torque the torque arms. You don't mention any vibrations while driving but I recently had to re-torque and apply threadlocker to the 4 nuts and bolts for the 2 torque arms as they loosened up over time.

Just a few thoughts anyway
 
Are you sure it isn't the center support bearing moving and making noise in the trans tunnel?

I have a similar issue on my 945 that has solid (heim) torque rods and I have suspected this was my sound. I might get around to mounting my GoPro under and watch for movement.
 
The center support bearing has also been recently replaced
There is some driveline vibration, but I only feel when accelerating from a stop in first and second gear. I tried chasing down the vibration, but got fed up and settled it being from misalignment between the driveshaft halves
The trailing arm bushings are also relatively new (about 1000km), but could something cause premature failure so quickly?
 
Conclusion:

The Torque rod bolts at the axle were loose. Thanks a lot moustacio for the suggestion!
Although they were loose at most half a turn, seems that was enough to decrease the clamping force, and allow the bushings to deflect in unnecessary ways.
Another bonus is that driveline vibration when accelerating seems to also have decreased. I am not sure whether this is from less articulation in axle angle or it's just a placebo effect.
 
Good fix. Also, those adjustable panhard rods are very difficult to cinch up due to lack of working room. Make sure that locknut is tight.
 
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