Furnace Debugging

A few days ago, the blower motor on my Carrier Weathermaker 9200 furnace stopped working. Luckily, this model of furnace provides quite a bit of debugging information printed on the inside of the door. It was very straightforward to test the blower motor and confirm that it was not the source of the problem. After a bit of poking around with a multimeter, the only remaining possibility was the blower motor relay.

Resoldered Relay Trace on Control PCB

This furnace is equipped with a HK42FZ008 control board. This board holds the blower motor relay along with the control circuitry. There is a clamshell style case that snaps in place around the board; the relay is not accessible without removing this case. Once removed, the source of the issue was obvious. It appeared as if the solder around one of the legs of the relay burned up. The damage was enough to burn the PCB copper around the leg so that there was no electrical connection between the board and the relay. After testing the relay to make sure it still function properly, the solution was to expose enough copper of the trace leading up to the relay so that a large amount of solder could be added. Hopefully, the solder will be enough to handle the large motor currents that likely caused the issue in the first place.

The blower motor has been running fine ever since the repair. It is possible that an issue with the motor resulted in an unusually large amount of current draw causing the relay connection to fail. However, there is a 3 amp fuse on the motor supply line that was still intact. To my knowledge, the control board is original to the furnace which is about 20 years old. It may simply be a case of failure over time.