I have an outside A/C compressor and indoor furnace fan combo in the attic. The unit was off and the 3 amp fuse was blown. I replaced the fuse and used the thermostat to turn the fan on followed by the heater then the A/C. I let the unit run the A/C. After a while the A/C turned off. I checked and the furnace was off, fuse blown and thermostat was asking for cool. I do not know if the fuse blew during operation, shutdown, or when the next request for cool happened.
I checked the wires leading to the outside unit. They show open when disconnected and 20.8 ohms when connected to the contactor. The wires leading from the thermostat to the furnace show open until fan, heat or A/C is selected and they show continuity as expected.
The volts across the red and green terminals shows 27 volts. The transformer is showing 120 in and 27 out. My first reaction is to replace the contactor on the outside unit, but that 27 volts is interesting to me because the schematic and Xformer say 24 volts.
Update
Performed the following tests. Disconnected the wires to the outside unit and ran the fan for a few hours no issues. There are 3 wires going from the furnace to the outside unit. White wire goes to 1 terminal of the contactor relay. Red and green wires were tied together and attached to the other terminal. On the furnace outside white is tied to yellow, outside green is tied to blue. Outside red was disconnected and loose on furnace. I disconnected red from green outside and put a wire nut on it. Measured the voltage on the relay of 27 volts. The resistance without wires is 16 ohms. Ran the A/C briefly without issue. Running the A/C today without 240 to see in the contactor heats up.
The furnace is a Lennox gas heater model number G40UH-36A-070-15. The outside A/C is a Lennox 13ACD-036-230-02. I went ahead and ordered a new relay because it is pretty cheap. It should arrive by this weekend.
What should I do next to troubleshoot this?
Best Answer
Set the fan to 'on' and have someone monitor the furnace blower. Go outside and hold the AC contacter down for as long as you're willing to. If the furnace blower never trips out, then it is that contacter getting hot and drawing an over current. Otherwise, happy hunting; that's my two cents.
You could also try toggling the fan to 'on' a bunch of times and see if that does it (could be a hard start after all; old motor/bearings). I guess if you want to freeze your unit and check that contacter at the same time (without anyone having to stand there) pull the power feed to the blower motor (from its clips on the control board) and call for AC. If it never trips out, it was the blower motor. If it does blow out, then it's somewhere between the control board, that contacter, or the thermostat and its wires (have fun ;).