Service & Maintenance / Extra Edition

Troubleshooting Challenge: A Modular A/C Unit That’s Not Cooling

In this month’s troubleshooting situation, we’re taking you to a school, but not to the equipment room of the main building. Instead, it’s a modular classroom (similar to a double-wide manufactured home) that is brand new, and employs a heating/cooling unit that mounts on one end of the building. This system employs heating elements for the heating mode and a direct expansion system for cooling. The complaint is that there is “very little cooling” being accomplished.

Upon your arrival, you determine that the complaint would have been better stated as “blowing warm air” since only the indoor fan motor is running and the compressor and outdoor fan motor are not. Details on the thermostat settings are as follows:

1. System switch set in the cooling mode.

2. Fan switch set in the AUTO position.

3. Set point is far below room temperature.

After disconnecting the power supply and removing the access cover, you find two other factors to consider — that the compressor is very hot and that the outdoor fan motor spins freely. While allowing time for the compressor to cool down, you make a complete visual inspection, checking for any loose connections and for proper wiring within the unit (click here for a PDF of the equipment diagram), and you find everything to be properly connected.

When you turn the equipment back on, you note that the compressor starts, but the outdoor fan motor doesn’t. With the power supply again disconnected, and a check with a digital meter on the dual capacitor, you make two tests; one between C and F, and the other from C to H. The result of the first test shows a series of zeros on your meter display, and the second test shows 37.5 MFD.

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

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

Publication date: 3/4/2013

PDF - March Troubleshooting Answer

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

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