db500

Hi guys,

Bit of a newbie here with some daft questions. Hope you dont mind? I have a 90 S4 auto that seems to vibrate at speeds over 85mph.. Im pretty sure its not the wheel balancing as the vibration is sort of throughout the car rather than just the steering wheel. Never encountered this before.There are no strange noises otherwise and the car is happy to continue pulling...

also a warning message has come up saying 'differential inoperable' (or soemthing to that effect) yet when the rear loses traction the PSD light continues to operate-this has only started recently in the icy weather conditions we have been experiencing.

Any ideas appreciated!

Bill Ball

Welcome db500:

Rear wheel out of balance would do this. I've had this happen.

Somebody with PSD experience will chime in about that.

Sharkbody

I'd get a thorough "road force" analysis to properly position the tires on the rims and root out any wheel damage. In my experience 80-85 mph is the magic zone for balance and true issues. G

Nicole

Yep - 80-85 mph is when the wheels seem to turn at the resonance frequency of the springs, and can make the whole car shake or vibrate.

From what I've learned, this can be caused by one or multiple of the following:

1. Wheels out of balance
2. Tires out of round (e.g. flat spots from sitting more than a week or two)
3. Rims out of round
4. Half shafts damaged or badly worn (rather unlikely)

If they offer road force balancing, or balancing on the car in your area, that might help - providing the wheel/tire combo is not out of round.

But make sure you drive the car for a while before doing this - if you have flat spots from letting the car sit, those might disappear after half an hour of driving.

db500

Thanks for the replies guys. Im pretty sure that the wheels themselves are ok as the car has not been stored for sometime. I had new rear tyres fitted recently so perhaps they didnt balance them/properly.

Does anyone know about the PSD warning that comes on (even though it still appears to function)?
Somebody suggested that I disconnect the battery for a while and let it clear itself. havent tried that yet.

AO

It sounds like your PSD is inopperable, or at least sending a warning message to the main computer. It may just be low on fluid, but should probably be bled to make sure.

I think the reason the light still comes on is that the car still records/detects the slip of one wheel and is trying to activate the PSD. It might work... it might not.

You may try contacting John Speake. Hoe's on your side of the pond and on this list. He's developed a diagnostic tool which may be handy for you to find out what the error code is. Not sure if he offers free help or not.

In the meantime I suggest you look at this proceedure on how to bleed your PSD. I'm sure it needs it.

http://928oc.org/journal/psd1.pdf

RKD in OKC

This can cause some weird vibrations...

db500

Now why didnt I think of that!!!!

Mrmerlin

check the steering rack bushings, you will need a helper to turn the steering wheel about an inch each way. while you look at the rack
Plus also check all of the other ideas posted.
Also what are the condition of the motor mounts. a quick check is to see if the pan is flush with the bottom opf the front cross member, if it lower than this then is a good chance the motor mounts need to be replaced, also check the rear trans mounts

db500

Thanks for that, to be honest I didnt even know there was fluid involved in that part of the car! I will check...is it something that needs doing on a ramp?

killav

I also vote for tire. I had this exact same problem and road force balancing the old tires did not help much. New rear tires and road force balancing completely alleviated the problem. I was ready to get rid of my car it was so bad.

The other possiblilities are still valid, but don't overlook the tires.

Bill Ball

I've had more than one occasion where a reputedly good tire shop left one rear wheel out of balance by an ounce causing a vibration starting at 85-90 MPH. I would have this easy thing checked first before you go onto more complicated stuff. The fact that you recently had tires put on the rear is all the smoking gun I would need.

Sharkbody

In terms of the wheel well issue. It's ok to have that "on the brain", just not on the tire. G

RKD in OKC

On my 2001 Boxster S I added some of those nifty Porsche crest valve stem caps. Got a vibration from 80mph up. Took em off, no vibration. Found the wheel/tire needed balanced with them as they weigh about 1/2 ounce.

Bill Ball

1/2 ounce valve stem caps! A bunch of us stopped using a local "race" shop for tire installation after the owner claimed 1 ounce out of balance was within spec! Uhuh. The spec is 1/10th ounce.