If it idles fine with the ac on, check into the tune up for the freeway speed misfire?
It will not be the diode if it runs okay at idle with the air on
The field coil (ac clutch hub connector) is just an electromagnet
Plug another one in if you want the processor to see the ac on
The problem IMO is the 10hp draw the compressor takes to spin it
That is a load, and the plug wires / spark plugs could be breaking down
Open the hood at night and watch for sparks near the plug wires is one way to test the wireset
Spraying some water on the wires from a squirt bottle is another
A 3-5 ohm resistor will simulate a field coil being plugged in
It will not be the diode if it runs okay at idle with the air on
The field coil (ac clutch hub connector) is just an electromagnet
Plug another one in if you want the processor to see the ac on
The problem IMO is the 10hp draw the compressor takes to spin it
That is a load, and the plug wires / spark plugs could be breaking down
Open the hood at night and watch for sparks near the plug wires is one way to test the wireset
Spraying some water on the wires from a squirt bottle is another
A 3-5 ohm resistor will simulate a field coil being plugged in