Shumy Fu

I have a 90 S4 AUTO, just as I took it for a drive just now. When I let go the brakes and as I turn the steering wheel, I could felt vibrate from the back.
But as I accelerate, the vibration disappear.
Could it be the differential?
The last time I have the oil changed was in 2020.

FredR

First thing to check would be the gearbox mounts. With age the rubber collapses into the valley of the mount and at some point there will be metal to metal contact. Thus under dynamic loading conditions contact is made as/when components of the system move slightly.

Mrmerlin

does this car have a limited slip diff?
if so the clutches are probably causing this ,
it usually happens on tight turns at slow speeds while accelerating.
Its a normal thing with the LTD slip diff,
it could indicate that its time to rebuild the clutch pack

Kiln_Red

MY90 has PSD as a standard option.

Petza914

Could also be a worn wheel bearing. Does it do it the same when you turn to the left or to the right or is it worse in one direction?

Shumy Fu

It happens when I turn left and right.

Mrmerlin

It happens when I turn left and right.


when starting off from a stop ? probably clutches in the diff.
or
while at speed loading the suspension left to right and back? probably a wheel bearing


NOTE check the lug nut torque,
and put a 32MM socket on the axle nuts torque to 320 Ft lbs. if the wheel bearings are suspect.
Also lift the rear wheel off the ground and see if you have play in 12/6 and 3/9

NOTE check PSD tank to see if the fluid level has been draining out,
possibly into the differential and ruining your PSD unit inside.

Shumy Fu

I have just drained the fluid of the differential, the fluid looks fine.
But it seems the differential is causing the vibration.
There's gaps between the diff. and whatever it is attached to it ( I don't know if it's suppose to have gaps, but one side seems has a bigger gap).
And also when I jacked the car up and turn the rear tires, there is also gap, and it seems one side is bigger than the other side.
On my diff. it says A2816 3L 01402, they say it's a option diff.
What parts should I start looking at?

FredR

Please post some photos of what you are seeing.

Mrmerlin

what are the gaps your talking about IDK what you mean,
as Fred asked Pictures please.
Even though you drained the trans,
Make sure to check the PSD tank level