AC Compressor won't turn OFF

[eze2bme]

I have a 1979 Hatteras 43 DC; 2-16,000 BTU Cruisair units with three knob controls; 10 years ago pump wiring changed so that pump cuts ON-OFF when compressor cycles due to thermostat demand. This past weekend on Sunday morning I awoke and found the temperature in the salon at 64 degrees (Columbia, SC). When I turned the thermostat to a warmer setting nothing happened – compressor kept running. I turned OFF the system and everything turned OFF. I waited approximately 10 minutes, turned the system back ON, and everything was running but still without responding to the thermostat. When I turned the controls to FAN, the compressor and pump remained running. I have noticed lately that the thermostat knob seems to be "loose" … it sort of wobbles. Any thoughts?

 

[Genesis]

It may be the switch but it may also be the triac in the power box; they do fail shorted on occasion (not often, but it does happen.)

The older-style triggers are reed relays and also are prone to failure, but those usually fail open. The cause is a horrifyingly-bad design that I reamed CruiseAir over years ago. The newer trigger boards are a simple relay and shouldn't have that problem.

I'd start by checking the rotary switch to see if it properly opens and closes. One word of warning - there's high voltage all over that system so DO NOT work on it with the breakers closed unless you use extreme care - line-voltage surprises are not fun!

 

[eze2bme]

Turns out it was a bad thermostat. Thanks to DL Cameron - who sent me one of his three knob controls when he converted to digital several years ago - I have the system running!