Yes. PID is a more sophisticated control mechanism when compared to on/off circuits/systems.

 

Nice to see systems people around here.

 

Yes, I know. I drive P-cars today in part because of adopting it for HVAC control loops in the early '80s using Andover Controls DDC controllers back when the majors were limited to on/off control.

 

I did find a partial wiring diagram. I did find a reference that said the fan speed was "infinitely variable". Couple that with the drawing and I think it's safe to say the voltage is varying across the terminals to control speed. If I could only find the full speed max voltage, I could cobble in a switch that feeds that voltage (and even cut out the controller controlled feed; but the computer might notice that)...