When you arrive, you confirm that the equipment, which is designed to have a box temperature of 0 to 5° F, is maintaining a temperature near 35°F. You also note that the compressor, condenser fan motor, and evaporator fan motor are all operating.
As your first step in evaluating this situation, you check the condenser coil and find it to be clean. When you check the condition of the indoor coil, you find that it is clogged with frost. When you check the wiring diagram (see Figure One - top), you note that this equipment is designed to defrost on a time schedule, and when the timer advances to the defrost mode, a normally closed switch wired in series with the compressor opens, and a normally open switch wired in series with the defrost heater closes.
Following the manufacturer’s recommended procedure, you manually initiate a defrost mode and find that:
- An ammeter check of the defrost heater circuit shows the proper current draw.
- A voltage check at the DFTM connections shows the proper voltage.
- The evaporator fan motor does not shut down at the moment the defrost cycle is initiated.
Your troubleshooting question: What is the next step you need to take in servicing this equipment?
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