The dreaded Vibration Monster has taken up residence in my '93 960 wagon, and it appears as though he doesn't want to leave. I keep finding worn and/or damaged parts to replace, often thinking that I've finally got it figured out, but at the last minute, the monster sees me coming at it with various implements of destruction, and he jumps to another part of the car while laughing at me and shooting me the bird.

This is the situation I'm having right now: from a standstill up until about 40mph, it's as smooth as can be. Around 40, I start to feel a slight thrumming vibration coming from the drivetrain. From about 45 to 60 it's vibrating pretty bad, and from 60 on up it gets a bit better, but it's definitely still there. The steering wheel never vibrates, it's all coming from the rear, and it vibrates the same regardless of engine speed or gear.

These are the parts I've replaced so far:
>two bent wheels (lessened the vibes somewhat)
>new rear tires (helped the ride, but not the vibes)
>transmission mount (helped some)
>swapped nivomats and springs for standard shocks (didn't help, but one nivomat was leaking, so they needed replacing anyway)
>all three u-joints, driveshaft center support, and bearing (this made take-off much smoother, but it actually made the higher speed vibes worse. I think the soft worn out rubber of the old center support was absorbing some of the vibrations at high speeds. With the new firmer support, those vibes are being transferred to the chassis now.)

While under the car, I checked the following: trans tailshaft shows no fluid leakage and there is no lateral play. differential flange shows no leakage and no lateral play. Rear end locating bushings (torque arms?) are intact, with very little movement.

My theories (guesses) right now are either a bent driveshaft, or maybe motor mounts. The motor runs smooth as silk, so I don't know if I should be suspecting the mounts or not.

Any thoughts about my guesses, or anything else I may be missing, would be greatly appreciated. Sorry for the long post, but this thing is really getting me flusterated.

 

Dan: you didn't say on the simplest -- balanced tires and wheels (all four). Assuming this is done, you've done pretty much everything else.

When center bearing rubber and transmission mount -- are both bad at the same time, out-of-balance driveshaft is usually to blame. Besides it's always good to balance it ($ 60) when center bearing and u-joints are replaced.

Another thing -- I would check for hub play. Lift the wheels, keep the wheels on -- grab wheel at 12 and 6 o'clock and 3 and 9 o'clock and move them inward-outward. You shouldn't have any play.

 

OK, I think I've found the problem. I did a quick hub play check when I did the driveshaft r&r, but there was no play. Thanks to your suggestions, I decided to go back and do a more thorough check, and I found that if I pulled outward on the entire wheel, it would move axially about 1mm or so. Once it was pulled out, then I could feel movement when rocking back-and-forth at 12 and 6 o'clock.

I'm not very familiar with live axle rear ends, so is there any way to adjust the preload on the bearing, or is this just a regular worn out bearing? I plan on replacing it anyway, but I don't want the new one wearing out too because it hasn't been adjusted correctly.

 

You don't have a live axle. It's independent.

Is the axle being pulled out when you pull out the wheel? Do you see any rip in the rubber or oil leak?

You could try to swap both sides and see what happens. There is a procedure on Brickboard on this.

 

No, it's definitely a live axle. Looks just like my old '70 Cadillac. Only 92-94 wagons had them, I think.

 

Sorry, got confused with the other car. Your job just got easier.

 

Ha ha, that's good to know. So, you think it's nothing more than a worn out bearing?

 

That bearing sounds suspect. They are $ 60 or $ 70 each. However, like I said earlier the driveshaft should be balanced after replacing everything.

Are all 4 wheels balanced?

 

On bearings -- they should be the same with 740 and 940, so maybe people here or on Turbobricks sell them on For Sale Forums.

 

All right, I finally got it figured out.

First, I changed rear wheel bearings. That didn't help at all. The inner seal was leaking on the passenger side, and the grease was washed out of the bearing, so that needed to be replaced anyway.

So, the next step was getting the driveshaft balanced. I took it to the machine shop, and the guy said the front section of the shaft was bent. He said he might be able to balance it out, but I might be better off with a new one. So, I went down to the local U-Pull-It junkyard and pulled a complete shaft off of a 940 sedan (amazingly, they use the exact same shaft) for $35. I just put it on this morning, and it was smooth like butter, baby.

All hail!!! The monster is dead!!!

Thanks for all the help, Henry.