The clutch engage wire does indeed hook directly to the ISCM and has an
immediate and profound effect on the IAC, which is why Casey is seeing the
big idle when the TPS is open.  This idle speed regulation of the ISCM is
much slower acting (can take up 20 seconds to regulate), and could not
possibly respond in time to keep your engine from stumbling when the AC
switches on.  Keep in mind that when you increase the RPMs you are also
increasing the speed of the AC compressor, which is a big load on the
engine.

One correction on my earlier description of the workings of the ISCM and
ISV.  The voltage goes from 0 to 5 volts every 8.6 millseconds, or about
116 times a second.

Craig in Cupertino

On Sun, Jul 21, 2013 at 2:03 PM, Richard A Jones <Jones@colorado.edu> wrote:

> > The AC compressor engaging sends a signal to the ISCM and increases idle
> > speed a bit when the throttle is closed (assuming the throttle position
> > switch is actually closed).
>
> I think the ISCU just notices the drop in RPM from the AC and
> compensates for it.  The only thing connected to the ISCU in
> that way is the power steering pressure switch.  See Bentley
> 97.107.
>
> The AC is 97.140 & 141 and there is no connection back to
> the ISCU.  The only thing the AC relay tells is the radiator
> fan--to kick on low speed, at least.
>
> Richard
> '87 Syncro Westy without a waterboxer
>