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 herefor 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:

• Remove the wall thermostat and disconnect the control wiring harness.

• Disconnect the thermostat wiring harness from the PCB field connection terminals shown in the wiring diagram.

• Disconnect the condensing unit control wiring harness connected to the contactor coil and the PCB field connection terminals.

With all of the control wiring harnesses disconnected and making sure that the wire ends are not touching each other, you use an ohmmeter to check between the red and yellow wires of the condensing unit control wiring harness, and you get a continuity reading.

Your troubleshooting question: What is the next step you need to take to get this unit back on-line?

Compare your answer with ours by clicking on the PDF link below.

Publication date: 7/9/2012

PDF - July Troubleshooting Answer

For information on Jim Johnson’s HVACR technician training DVDs, go to