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Read the manual Steve! (Speedos & Auto Locks)
Well, I went to the Bentleys last night and found I made some incorrect
assumptions and spread some misinformation. Let me try to rectify them ...
The speed sensing transducer connected to the transmission on newer 5000/
100/200s has only 3 terminals, with only a single output signal. There
must be circuitry in there to put out pulses only when the car is moving
forward. That's why the thing costs so much.
The cruise control still receives its input from the gauge cluster. I
don't have a schematic for the thing so I can't tell if there is a direct
connection from the speedo transducer output to this signal. After last
night I wouldn't be surprised to find a little servomotor in the speedo
that locks itself to the incoming pulse rate, simulating the rotation
of the non-existent cable! Then they could use the hall effect device
to drive the cruise control (and I thought Rube Goldberg (sp?) was dead!).
The bi-pressure pump on the ur-Q is also a 3-terminal device. Depending
on which of two sources is providing power the pump will try to either
suck or blow. This means that it doesn't matter what the position of
the passenger's door lock is. Bentley does have two very nice trouble-
shooting flow charts (one electrical and one pneumatic), but in this
case I think it's pretty apparent that the bi-pressure pump has a problem.
Sorry about any misinformation ... I guess I'll have to buy another
set of manuals so that I can reference them here in my office!