Service & Maintenance / Extra Edition

Troubleshooting Challenge: A Condensing Unit Problem

May 10, 2010
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Figure 1. (Click on the image for an enlarged view.)

This time around our problem centers on a split-system air conditioner. The condensing unit is 240-volt, single-phase, and the indoor air handler (which our customer is able to tell us is “blowing warm air”) consists of a standard gas furnace and an A-coil housed above the furnace plenum.

Since our unit is a split system, our diagram (see Figure 1) only shows segments of two distinct circuits because the problem is not with the indoor air handler. Both the condenser fan motor and compressor are PSC type motors, and the unit employs a high-pressure switch, low-pressure switch, and compressor safety thermostat for protection.

When you arrive at the customer’s home at 8 a.m. on a day that promises to be a warm one, you confirm that the wall thermostat is set in the cooling mode, and you also note that the fan switch is set in the AUTO position, and that the set point of the thermostat is below room temperature.

Moving outside to the condensing unit, you find it quiet, with no indication of attempts to start. Based on that observation, you use your meter to make specific voltage checks. Your results are as follows:

1. At L1 and L2: 240 volts

2. At T1 and T2: 0 volts

3. At test points 1 and 2: 0 volts

4. At test points 3 and 4: 0 volts

5. At test points 5 and 6: 24 volts

6. At test points 7 and 8: 0 volts

Your troubleshooting question: Based on the above tests, what direction does your troubleshooting take in order to solve this problem and get the unit back on-line?

Compare your answer with ours by clicking here.

For additional articles on troubleshooting, visit The NEWS’ HVACR In-Depth page.

Publication date: 05/10/2010

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