Re: 4000cs q newbie

[Luis Marques <marques@ti.com>]

>>Aaron LaPointe wrote:
> If I am not mistaken, when you lose traction to one wheel, the car
> transfers most of the power to that wheel.  This is where other awd
> vehicles stop and quattro begins.  When you lock the center diff, it
> separates the front and rear into two separate systems, creating a 
> 50-50 power split, hence then making both the front and rear wheels 
> spin.

Actually, power is split 50-50 when the differentials are _unlocked_.
When you lock the diffs, the two ends are tied toghether and power split
varies from 100-0 to 0-100 in order to keep both ends moving at the same
speed.

>>PJ wrote:
> The car make quite a racket when you drive above, say, 20mph. It 
> sounds like a whump-whump-whump, and is quite clear inside the car, 
> but not at all outside. We thought it might be a problem with the 
> 4wd, so we put the car up on blocks. The rear wheels did not turn 
> when I put the car in gear! This car is perm 4wd, correct? When I 
> locked either the center or rear differential, the wheels would turn. 
> Anyone have any ideas? Is this normal (maybe something to do with 
> LSD)? Again, I'm new to this, so any hints would be well appreciated.

I'll assume that you put the four corners on jackstands and then turned
on the engine and put the car in gear, right?  If there is more friction
on the rear drivetrain than the front, the rear wheels won't move while
the front wheels happily spin around.  This is beacuse of the center
differential that is missing in your typical 4x4 truck, but is present
in your AWD Audi.  Imagine you have a RWD car and put the rear on blocks
and put the car in gear with the engine running.  Both wheels turn at
approx. the same speed. But if you break one wheel (with something other
than your bare hand), the other will keep on turning at a higher speed
(as long as you dont have a LSD or anything).  The same thing is going
on in your quattro.  By locking the center differential, you force both
ends to turn at the same speed and that's why you see all the wheels
turning (when you lock the rear diff, the center diff. stays locked
also).

I think that there is something causing friction on the rear
drivetrain.  It could be that the rear brakes are grabbing, or you could
have a very bad center support bearing on the driveshaft (that would
explain the noise you hear).  There are no LSDs on this car!  This
driveshaft bearing is very hard to get from Audi (they prefer to sell
you a whole driveshaft), but some listers have been successfull using a
BMW bearing.  Check the archives.  I doubt there is anything broken
internally, as the drivetrains on these cars are _very_ durable,
although it would be a good idea to change all the fluids on the car.

Good luck and congratulations on your new Audi! :)

Luis Marques
'87 4kcsq