[s-cars] Transmission Vibratons? (long)

[Eric Renneisen]

Patient is a '93 S4 with 170K miles on the clock.  I'll try and condense
the history of this problem a bit, but it's still gonna be long:

Last year the car was all stock, including original suspension bits.  At
highway speeds I'd get a nasty tank-slapper (violent oscillations in the
steering) after hitting a pothole or other such suspension upset.  It
was easy to supress, by briefly stabbing the brakes or even a quick
flick of the steering.  I chalked it up to the rubber stuff and shocks
that had long ago worn out.

Then, my transmission died.  Long story short, it refused to allow
selection of 1st or 2nd.  No matter, I already had a line on a 6-speed
from a Euro S6.  I dropped that in and was a happy camper, until I
noticed that the tank slappers had become more common.  I could induce
them on a smooth road by simply running the car up to 80 mph.  Also, it
wasn't so easy to quell them.  The only solution was to drop the speed
back down below 80.  I just thought that the suspension degradation was
getting progessively worse.

So, I ordered up all the parts: Bilsteins, H&Rs, control arms, inner and
outer swaybar bushings, outer tie rod ends, upper shock mounts and
bearings, motor mounts, front wheel bearings and maybe some other stuff
I've forgotten.  I also replaced the inboard rear diff mounts because
one was torn.

After all that, the car is nice and tight as you'd expect.  No more tank
slappers!  However, I still feel vibrations when I hit 80 mph.  I also
feel them now at 40 mph.  I suspect they were there before, but the
mushy ride was absorbing them.

I've gone over everything to track down the source.  I ruled out the
wheels by driving around with the urq R8s--that was an intersting
look--and the problem was still there.  The propsahft carrier looks
good, but I was going to take it to a driveshaft shop to have it
balanced and checked out.  While I had the propshaft off, and the car on
the lift, I decided to spin up the driveline for the heck of it.  It
still vibrated at an indicated 40 and 80 mph!

So, I've isolated the problem to the front of the car, right?  I pulled
off the halfshafts and ran the tranny up again.  It felt smooth.  OK,
halfshaft problem, but all the joints feel OK.  The fears of a bum
tranny crept into my head.  To test this, I put the propshaft back on
and spun it up without the front halfshafts.  Big fat vibes again.

I pulled the propshaft and put the halfshafts back on.  This time I ran
it with just the hubs, no wheels or brakes.  I think I felt some faint
vibes, but it's tough to tell.  Put just the rotors on, and the vibes
were still faint, but a bit stronger.  Then I put on the wheels (a third
set, just to be sure) and the vibes were there in force.

So, all signs point to the gearbox.  With nothing connected, I guess the
motor and tranny mounts isolate it pretty good.  It seems to need either
the propshaft or the halfshafts to transmit the vibes into the body.  I
thought maybe it was the flywheel or new clutch, but it's always at 40
and 80, no matter if the box is in 4th, 5th, or 6th.  That rules out
everything from the input shaft forward.

I have an S2 6-speed I'm going to throw in, but before I go to the
trouble I thought I'd bounce the situation off you guys.  Has anyone
ever heard of a tranny having this kind of harmonic vibration?  Is there
anything I've overlooked?

TIA,

Eric R.
'86 urq
'93 S4