> The lowest of the speeds is actually from the AC running when the compressor
> is engaged. There is very little difference when the radiator switch turns
> on the fan as compared to the AC running the fan.
>
> Either the AC or the radiator switch can call for the high speed. If the
> radiator calls for the high speed the AC compressor gets disabled. There is
> also a high pressure cut out for the compressor if pressures still get too
> high.

Dennis:

Thanks for the information.  It made me look at 97.141 closer and
see that the compressor clutch relay doesn't turn on the high
speed fan--if the fan goes to high speed, that relay shuts off
the compressor.  I was reading the relay symbol backwards.

But it seems clear that if the high pressure switch closes, all
it does is feed power to the 2nd stage relay and run the fan
at the middle speed.  It doesn't turn off the compressor.
Only the fan on high speed does that.  Of course, if the
pressure is TOO high, it will come to that!

The fan on low speed is fed by either the AC being ON or
the temp sensor kicking in.  They feed the same input to
the resistor then on to the fan.

So I understand now:  I observe low and high speed and never
the middle.  Or if middle speed comes on, I don't notice it
over the road noises.

I have R134a in my system, driven by a Subaru compressor, so
I guess it was filled using pressures.  Isn't that the only
way?  Anyway, it was recharged recently--one of the schrader
valves was leaking.  We just drove 2711 miles from Boulder to
northern Idaho and back.  Air temps were in the 90s--up to
98 blasting through Twin Falls and again around Cisco.  We
were cool enough, unless directly baked by the sun.  I have
no complaints about my AC.

Richard