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Differential lock questions
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
Is this signal an analog voltage proportional to speed or a pulse
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 ***
*** email@example.com ***
*** Opinions expressed herein are entirely those of the author. ***