[Richard]

I haven't figured out the answer to Chester's issue, but I enlarged the control schematic to try to noodle it out.

Using colored markers, I discovered that indeed on the LOW setting Compressor 2 is prevented from starting even if a signal is sent to start it. Relay 8 is activated when the fan is on LOW. It took a magnifying glass to discern Relay 8 is normally closed. Relay 8 interrupts the signal from the thermostat to power Comp 2. When the relay is activated the trigger to close the power relays to Comp 2 is interrupted.

In studying the control schematic, I can see that problems in the logic would occur IF the neutral for the outdoor fan is wired around the circuit board. Relay 2 switches the common when going from LOW to HIGH. I am really keen to examine the wiring now from the board to the motors, all four of them. All of the commons should originate from the board. If they don't I would fix that before doing any more diagnostics.

My head hurts.

I'll draw more careful sketches in color tomorrow and post them.

This has been a GREAT learning exercise for me.