Differential lock questions

[kirby.a.smith@lmco.com]

Certain models of Audi, such as my 1988 90q, release the differential 
lock when the speed exceeds about 15 mph.  The signal for this comes 
from the speedometer and goes to a control module under the rear seat. 
While measurements would reveal the nature of this circuit path, I 
haven't yet purchased a power converter to run my oscilloscope in the 
car at speed.  Does anyone know the answer to any of the following 
questions?

Is this signal an analog voltage proportional to speed or a pulse 
rate?

What is the speedometer source impedance?

What is the control module input impedance?

Is the control module load important to the speedometer signal, 
considering that this signal also goes to other circuits?  (Obviously 
excluding making it too low a resistance.)

In other words, it appears that opening this signal would remove the 
speed limit on a locked rear differential, and modifying it could 
change the speed limit.  Opening might require a load on the 
speedometer and/or the control module.

Obligatory Caution:  Audi added this limitation for some reason. If 
one bypasses it and operates under improper conditions, the reason 
might become painfully obvious.

***                 ...Kirby    (Kirby A. Smith)                 ***
***                      2 X 1988 90q                            ***
***                    kirby.a.smith@lmco.com                    ***
***  Opinions expressed herein are entirely those of the author. ***