Nicole

When I drove down to Southern California last week, my car was vibrating strongly from about 67 mph to 96 mph, but even at higher speeds the vibration never went away completely. It seemed to be stronger on deceleration (stepping off the gas).

At Greg Brown's shop, he re-balanced all wheels and pointed out that the left rear wheel is a bit out of round.

He also pointed out that both my outer rear axle boots had started to split in the crease that's towards the inside of the axle. No grease had been lost, apparently, as everything around the axle was dry.

After re-balancing, the wheels, the ride was smoother. The vibrations didn't really start until I was in the upper 70ies, then peaked in the low to mid 80ies, and were still stronger on deceleration. Note that this is now quite consistent with Bill's original description of his vibration issue.

A fellow 928 owner, who rode with me on the freeway, pointed out a whining noise from the right (passenger) side, which he thought was a rear wheel bearing. This is consistent with a concern by Marvin Weitz from The RennShop in Santa Clara, who had previously mentioned a noise on that side that should be investigated.

So, I don't remember, if Bill's car got rear bearings lately, or if it makes any whining noises. But now I'm wondering, if bad bearings could in fact cause a such vibration, and if this would not be obvious from turning the wheels while the transmission is in neutral.

Bill Ball

I replaced the rear wheel bearings not that long ago.

Nicole: We can take a look at your wheels bearings. John Fagerland's right rear bearing was dry as toast when we examined it.

http://www.youtube.com/watch?v=GtDEvlzDhXU

If your wheel is out of round, that could be the residual vibration. I have a recollection that I noticed that as well when we were checking the runout of your rotors.

Chuck Schreiber

Bill,
Did you ever cure this?

White Lightnin'

Any updates? Fixed?

Bill Ball

No, not fixed. I don't think it's the play in my diff, as the play has not changed in 10 years. I replaced all shocks and springs. Still there, although somewhat less noticeable perhaps because the new "custom" suspension is minus some of the stock rubber. Still haven't done the test of running the car on jackstands. Needs to be over 80 MPH. May try that when Jeff is up here, as I need a helper to do that test.

Lizard928

I know that you probably know better, but for those that do not.

It is not recommended to run the tires up to 80mph off the ground......

Find an empty rim, or leave off the tires....

Speedtoys

Never heard of a failure doin that...and could be related to the vibration.

borland

Running the test on a lift may give different results, as the suspension will be unloaded. Consider using an inspection camera mounted under the car to record video while driving the car at speed.

Here's a very inexpensive USB camera for about $30 that can be connected to a laptop PC for recording and viewing...

http://cgi.ebay.com/5M-USB-Waterproo...item3f09040880

Landseer

" guys, bet you can't guess why there are 3 tire marks that lead to the back of my garage "

Speedtoys

Thinin about that..


I think mythbusters kinda tested this..the HP delivered to maintain a speed is so low..than when they did the Knight Ridder drive up the ramps into the semi at highway speeds, the momentum in the drivetrain was all but instantly used up when the rear tires hit the ramps.

With even less, -far- less being consumed to drive gear to 80 with no chassis weight or drag..its likely..worst case, to do more than stall the car in place.

dr bob

Key to the test is to have the suspension at normal height, so wheels/tires are off and the car is supported on the lower suspension with blocks. Front wheels are blocked and I like to put a safety strap or two down to the rails. A little drag on the rear brakes will be a good idea too.

Bill, my vote is still on CV joints. Been there, done that, with same symptoms, got a really nice Saab T-shirt to show for it. It was the inners, so the car would shake at speed (63+) and almost any load. Changing the suspension changed amplitude slightly as the chassis/gearbox movement was dampened differently, but the primary cause remained. Have your assistant put a hand on the bottom of the rer gear case, and you'll find a vibration at wheel speed. Amplitude increases until you find a nice resonant spot for the trans mounting.

White Lightnin'

Sounds familiar Dr. Bob (post #4)...

Bill Ball

COULD THIS BE IT??

You may remember that I said the vibration seemed to start after I replaced the tranny mounts. I had been aware of a gap that needs to be maintained around the tranny mount and I made sure there was a gap there....or so I thought.

Here is the WSM section regarding this.



Pretty clear, eh? Well, when I replaced the mounts I read it too quickly, not noticing exactly where the arrow pointed. I looked up and saw a gap between the mount and the side of the crossmember tab and thought, great there's a gap. Today I was reinstalling my tranny after a leak repair and partial rebuild. For some reason I decided to check the WSM and I saw my error. I went back and looked at the tranny and found no gap on one side between the tranny mount stop and the tranny case. The picture below shows the correct gap with the red arrow and the incorrect gap location by the red arrow with the black X through it. You can have a gap at the second location, but it's not where you need a gap. It took some prying the create the gap where needed. So, If you loosen the tranny mounts from the crossmember for some reason, this gap could disappear.



I'm not quite done with reinstalling the tranny, so I haven't had a chance to test this out, but I am hopeful I may have found the problem.

AO

Bill, as much as I hope this cures your problems, I cannot imaging that this would be the culprit. Fingers and toes crossed in any event.

Bill Ball

Looks like I won't get to test my hypothesis - the tranny is bad. Thank you rebuilder.