Service & Maintenance / Extra Edition

Troubleshooting Challenge: A Split System That’s Still Sitting Dead

In this month’s troubleshooting problem we have a homeowner who has attempted to repair his own equipment, and he has replaced two components. He installed a new thermostat and found that the problem was not solved, then followed that part replacement with a new transformer, which also didn’t solve the problem. The original symptom of the split system sitting dead still exists.

When you arrive, you find this seven-year-old, 3-ton system, which employs a gas furnace and outdoor condensing unit, is in good condition, and (see the equipment wiring diagram) you confirm that 120 vac is applied to the air handler equipment connections at L1 and L2.

After confirming that the wiring for the thermostat and transformer is correct, you connect a voltmeter at L1 and 1 at the Interlock Switch, and your reading is 120 vac. When you depress the switch manually, you maintain the 120 volt reading.

Your troubleshooting question: What is the next step you need to take in repairing this equipment?

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

Publication date: 5/6/2013

PDF - May Troubleshooting Answer

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

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