Intermittent vibes...

[billy bee]

My wife's 1991 900S convertible gets a pretty good shake going at highway speeds...but not all the time. Sometimes it is smooth as silk up to 70. Sometimes it's like a paint shaker at 60. No apparent rhyme or reason to it. Strictly road speed related. RPMs are not a factor.

I have some thoughts that may or may not be consistently true. First is that is is ambient temp related. Seems to shake more in hotter weather. Not som much when it cools down.

Second, that it may be axles that can be made in or out of phase depending on turning radii or something like that.

Any suggestions? Front mount just replaced. Was a problem prior to that. Other mounts look newer.

Thanks in advance,
bb

[DeLorean]

Very likely the inner-drivers are packing it up. if your tires are balanced but there is a shimmy under power, that's almost always the issue. Question, when it's shaking, if you pop the clutch in (or put an auto in neutral) does the shaking stop? If it does, that's the for sure inner driver test.

[Crazyswede]

My wife's 1991 900S convertible gets a pretty good shake going at highway speeds...but not all the time. Sometimes it is smooth as silk up to 70. Sometimes it's like a paint shaker at 60. No apparent rhyme or reason to it. Strictly road speed related. RPMs are not a factor.

I have some thoughts that may or may not be consistently true. First is that is is ambient temp related. Seems to shake more in hotter weather. Not som much when it cools down.

Second, that it may be axles that can be made in or out of phase depending on turning radii or something like that.

Any suggestions? Front mount just replaced. Was a problem prior to that. Other mounts look newer.

Thanks in advance,
bb

I would start with your tires...are they new, when were they installed and balanced? have you tried rotating them front to rear? if the tires are old or warn or have not been balanced in a while this would be a good place to start.

if the vibration is primarily under acceleration or worse under acceleration than your problem is most likely worn inner drivers.

[billy bee]

Makes sense to me that it's the inner shaft joints. I've seen a replacement procedure for them. Seem to recall you have to replac one at a time lest something in the trans comes apart. I'll get to it soon. Wifey loves driving the car. Shame she cannot hit the highway comfortably.

Thanks,
Bill

[gmreider]

Don't know if anyone else said this, But odds are the passenger side inner driver is the one that is bad. And yes do one side at a time. If you pull out both drivers at once, the diff as a tendency to move around and make things a bit harder to reassemble.

[Crazyswede]

You could pull one driver off and remove the driver from its aluminum housing and then mount the housing back on the transmission as this is what supports the bearing on the diff. Then you can pull the other side, swap drivers side to side and reassemble...this is assuming that someone has not already swapped them from side to side.

[DrewP]

You can also jack up one front corner at a time, and rock the road wheel forwards and backwards and yank around on the inner end of the halfshaft - if it's worn you'll feel relative motion between the halfshaft and the driver cup.

[99Super]

There is a good chance it's the passenger side inner tripod bearing coming apart. Pull back the boot and have a look.

[billy bee]

There is a good chance it's the passenger side inner tripod bearing coming apart. Pull back the boot and have a look.

I an hear an occasional clicking from the pass side.

I wonder how likely it is that I'll also need the cup in addition to the universal joint? The factory manual does not suggest taking the cup out of the trans to replace the joint. That seems to save a lot of work...unless, of course, the cup also needs replacement. Anybody have photos of worn cups?

Bill

[billy bee]

You could pull one driver off and remove the driver from its aluminum housing and then mount the housing back on the transmission as this is what supports the bearing on the diff. Then you can pull the other side, swap drivers side to side and reassemble...this is assuming that someone has not already swapped them from side to side.

I've been thinking about this possibility. Will the driver cup wear unidirectionally? If it is worn from clockwise rotation on the passenger side, would it still serve OK as a driver's side cup due to the change in rotation?

Thx,
Bill

[Jordan]

Yes it wears unidirectionally, flipping the drivers is an old trick. To check for wear, remove the axle and run your finger along the contact surface of the driver. If you feel significant cupping then it may need replacement. If you feel the opposing side, you'll see if that is worn too and if the swapping the cups will work for you.

[99Super]

Swapping cups works, but be aware you will now have vibration on decel.