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

January 4, 2010
Figure 1. (Click on the image for an enlarged view.)

Yes, it’s January, but it’s always nice to look ahead to a busy summer season. Hence, a problem of ‘not cooling’ instead of ‘not heating.’

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 resistance-type 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 is set in the cooling mode.

2. Fan switch is set in the AUTO position.

3. Set point is far below room temperature.

After disconnecting the power supply and removing the appropriate access cover, you find two other factors to consider … that the compressor is very hot to the touch, 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 (see the diagram in Figure 1), and you find everything properly connected.

When you turn the equipment disconnect switch 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, the other from C to H. The result of your 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 in repairing this unit?

Compare your answer with ours by clicking here.

Publication date: 01/04/2010