Re: Strange Speedometer Behavior

[steveb@newkla.kla.com (Steven Buchholz)]

Happy New Year to all!

Several things regarding speedometers and flotsam and jetsam and Audis.

As was pointed out by someone else, the 200Q does not use a speedometer 
cable (I suppose that the electrical wiring could be called a cable), 
there is a transducer that is connected to the worm gear on the diff.  
I haven't looked into it to be sure, but I'll bet that there are actually 
two sensors oriented so that two out of phase digital signals are genera-
ted in quadrature.  This would allow the odometer to advance only when the 
car is moving forward.  The cars with a cable had a hall-effect sensor 
attached to the speedometer that fed speed information to the cruise 
control.  Here again, I haven't confirmed this on the wiring diagram, 
but it would make sense to replace the hall sensor with a direct connec-
tion to the same transducer that is driving the speedo.  You should 
confirm this with a wiring diagram because my reasoning will follow 
from there.  If both speedo and cruise control were affected then I 
would suspect the sending unit, perhaps the ground as Phil suggested.  
If your cruise control accepts input from the speedo, then it would be 
suspect as well.  I can't think of a problem that would not be fixed by 
powering down the system as you did.  Maybe the power for the transducer 
is on all of the time.  I haven't checked if the odometer continues 
working when the car is off.  If you know the problem will occur rela-
tively frequently you might want to hook a tach in paralle with the 
speedo to determine if the signals coming in are screwed up.  

BTW, I'll bet that your steering was not locked.  On my cars the steer-
ing wheel lock does not engage until the key is removed from the cylinder.  
I can turn the key to the off position and the steering wheel turns freely 
lock-to-lock (w/o power assist of course).  When the key is removed I can 
hear some mechaism actuating inside.  The lock does not disengage until 
the key is inserted and turned to the on position.  

Let us know when the mystery is solved ... it sounds like a good one!

Steve Buchholz