The system uses two pickups and compares the signals.   If the signals stay in
sync, the sustem knows the engine to wheel connection is not broken - ie that
the clutch is not disengaged.   If the signals get out of sync, the system knows
that the clutch has been disengaged and that the engine is beginning to
over-rev.    It then disengages the speed control device.

Wendell

"Michael A. Radtke" wrote:

> Eric,
>
> Thanks for the detailed HOWTO.
>
> I installed a similar unit a while back and it seems to work fine on my '84.
>
> 1) Why did you use the magnetic pickup?  I thought that was only
>    necessary with an automatic transmission?  My installation senses
>    engine speed through the coil wire.
>
> 2) A disengage down hill is a safety feature.  The idea is that the
>    car shouldn't continue to speed along, or increase speed over some
>    length of time with that the cruise thinks is a closed throttle.  I
>    haven't experienced it with my current cruise control, but I did
>    experience it with the previous one on long mountain descents.
>
> 3) Just a week ago, I made a wiring change in my installation to
>    reflect behavior that I was used to in all my previous cruise
>    control installations.  The AudiVox unit acts like most factory
>    installations in that there is no provision to disengage the cruise
>    without losing its setting except by using the brakes.  All of my
>    aftermarket units, until the AudioVox, retained the last setting
>    when switched off and then back on again.  Only using the set key,
>    or turning off the ignition unset the unit.
>
>    I measured the current draw of the unit and found it to be less
>    than 100 milliamps when disengaged, so I reasoned that I could
>    leave the unit active all the time.  So I made the following wiring
>    modifications.
>
>    a) I connected the power to the unit from the control, and
>       connected directly to the fused power source.  This leaves the
>       unit powered on any time the ignition is on.
>
>    b) I disconnected the wire from the unit that senses brake light
>       battery voltage and connected it to the switched power source
>       from the control unit.  This is the place the cruise unit
>       power lead was originally connected.  This change supplies the
>       cruise with brake light reference voltage only when the cruise
>       switch and the ignition switch is on.
>
>    c) I added a diode in the wire from the brake lights to the cruise
>       unit.  I don't know whether this was necessary, but with the
>       ignition or cruise switch off, stepping on the brake pedal
>       causes this lead to go to 12 volts.  With my other
>       modifications, the brake light reference voltage is zero and bad
>       things could happen to the cruise.  The diode blocks reverse
>       current flow.
>
>       I determined that the current flow was out of the control unit,
>       so the arrow on the diode should point toward the brake lights
>       and away from the cruise unit.
>
>    With the above modification the cruise acts the same as before, but
>    will return to its last speed setting if the unit is on and resume
>    is pressed.  It does lose its setting on ignition off.
>
> Thanks again for sharing.
>
> Mike      '84 GL  '58 Isetta (no cruise)