[V8] servotronic connection on 90 V8?

Buchholz, Steven Steven.Buchholz at kla-tencor.com
Thu Oct 14 15:06:57 EDT 2004

... very sure ... if you go back to the old digitalized analog speedos
from the '80s you will find a Hall sensor mounted to the back of the
speedo assembly counting the rotations of the internals ...

Trying to send DC voltages around a car to convey information is a very
difficult thing to do consistently ... remember the recent grounding
discussion?  Pulse rate or pulse width modulation is fairy easy to
generate and decode and eliminates the reference error situation.  

By some chance were you using an analog VM?  You might very well notice
the reading on an analog VM increase and decrease as the number of
pulses per unit of time changes ... 

... as to the function of the modulator in the rack ... the Audi
documentation clearly states that it is current controlled.  For some
reason just recently I came to the realization that the modulator is
very similar in function to the differential pressure regulator used in
the CIS-E systems to modulate the mixture based on the OXS output ... no
confirmation, but it would make a lot of sense ... 

Steve B
San Jose, CA (USA)
> Are you guys sure about this?
> I have had a volt meter on this signal while driving. It is a 
> variable voltage
> (0 to 12 volts) - proportional to speed. It is not pulsed.
> Are you thinking of the speed sender? That is a pulsed signal 
> (the sender is a
> Hall Switch), but the output from the speedometer that feeds 
> all these various
> control units is most definetly a voltage proportional to speed.
> The speedometer is what does the conversion from the pulse 
> train to voltage. In
> fact, I am 98.76% sure the speedometer is nothing more than a 
> voltmeter
> calibrated in MPH (and the odometer is driven by a small DC 
> motor, driven by the
> same proportional voltage).
> There are other ways to make an electronic analog speedo/odo, 
> but I am pretty
> certain this is how they did it.
> For another useless point of reference, the Tachometer works 
> the exact same way.
> It receives a pulse train from the ECU, converts it to a 
> voltage proportional to
> RPM and sends it to a voltmeter (the tach) calibrated in RPM.
> I have no idea what the actual control signal from the 
> servotronic box to the
> rack is - but I would have bet money on it being a square 
> wave that varied the
> duty cycle to a control solenoid. That would be seem to be a 
> cheap and effective
> way to get the job done, but they could have used a linear 
> actuator too.

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