Problems with Cruise Control & Diff Lock (long)

Houston Texas
Thu, 19 Apr 2001 12:25:45 -0500 (CDT)

Cruise Control Doesn't work? 
Rear Diff Locks don't disengage above 15mph? 
Do you have an aftermarket radio installed???? (1989-91 Audi 100/200)

I ran across a strange problem recently on a 1991 Audi 200TQ 20V. The
cruise control didn't work, and the rear differential lock didn't
disengage above 15mph as it should. The Bentley wiring diagram shows the
speed signal comes out of the instrument cluster and is routed to the
cruise control (control unit) and is also routed to the Rear
Differential Lock control unit that is located under the rear seat. This
signal comes from the instrument cluster from pin T26/2 (Black Connector
on right side of the back of the instrument cluster) using a White/Blue
stripe wire.
Given all the problems the instrument clusters have on these cars with
poor solder joint connections, I made the assumption that there must be
a connection problem where the speedo head connects to the circuit board
with the two sets of connection pins. Two of the pins are for the +12V
and ground, the other two have the signal coming in from the
transmission hall effect speed sensor, the other pin is for the output
signal that goes out to the cruise control/diff locks.
Well, after taking the instrument cluster out of the car, and repairing
the usual suspect solder joints, there still wasn't any signal coming
from the speedometer head unit for the cruise control/diff lock. I went
so far as to run the car in gear with all 4 wheels off the ground, with
the instrument cluster torn apart, while probing the back of the
speedometer head unit with an oscilloscope trying to determine why there
was no speed signal out to the cruise control/diff lock control units.
At one point, I had removed the head unit from the instrument cluster
circuit board and had it connected using jumper wires for the +12V,
ground and the speed signal input from the transmission. At this point I
noticed the speed signal for the cruise control/diff locks was magically
there when the head unit was removed from the circuit board which
connected it to the wiring that goes out to the cruise control and diff
lock control units. 
I finally realized that there was something in the circuit that was
loading down this speed signal to prevent it from being measured with
the oscilloscope. There could be a short in the wiring that goes to the
cruise control and diff lock control units or the problem could be in
either the cruise control or the diff lock control units. Normally this
speed signal used by the cruise control and diff lock control units is a
0-12V square shaped waveform which is low (near ground) about 10% of the
time, and high (+12V) 90% of the time. The frequency of this signal
naturally depends on the vehicle speed.
I took another look at the Bentley wiring diagram and noticed that there
was an additional current track (wire) that was routed somewhere else
besides the wiring that went to the cruise control and diff lock control
units. This vehicle speed signal is actually sent to the radio! I assume
the OEM radio uses this vehicle speed signal to adjust the volume as the
vehicle speed increases. Well, come to find out that this car had an
aftermarket Radio/CD player installed, and yes you guessed it, they had
incorrectly connected this White/Blue stripe wire which carries the
vehicle speed signal to the aftermarket radio, which was loading down
the signal. Cutting the White/blue stripe wire from the aftermarket
radio allowed the correct operation of the cruise control and rear diff