In this month’s troubleshooting problem, you have been called by a colleague who is relatively new to HVACR to assist in repairing a split system in a manufactured home located in a rural area. The original diagnosis regarding this unit that is sitting dead was that the PCB 3-amp fuse (click here for a PDF of the wiring diagram) was blown, and when it was replaced, the new fuse also failed.
When you arrive and survey the situation, you note the gas furnace that serves as the indoor air handler when the system is operating in the cooling mode, and the condensing unit that sits on a concrete pad near the building. Since the new fuse that was replaced failed when the system was started, you take the following steps to evaluate the control wiring system: