This simple method of checking the defrost system will benefit you the most after studying the chapter on “Basic Electricity,” as this test requires some understanding of schematic wiring diagrams and the heating-element ohm variation in different units. You will need a clamp-on ammeter to run this test.

1. Find the schematic wiring diagram of the unit.
2. Pinpoint the defrost heater and make a note of its ohm rating shown on the diagram.
3. Determine the supply voltage for the unit.
4. Divide the supply voltage by the heater ohm rating reflected on the diagram. This determines the approximate reading you should expect to get when using the meter.
5. Trace the wire connecting the defrost heater to the defrost timer (in fig. 72, it is the wire connected to timer terminal number 4).
6. Place the jaws of the clamp-on ammeter around the wire connected to defrost timer terminal number 4. Turn the timer shaft clockwise and stop when you hear a click. You should expect to get a reading of about 5 A (4.8 A as calculated in step number 4). If the meter registers a significantly lower reading (such as 1/2 or 11⁄2 A), you will know that the accumulation of ice on the evaporator plate must be due to a defective defrost system (the defrost heater, the defrost timer, or the defrost thermostat). Check them one by one as instructed in the chapter on “Testing Residential and Commercial Refrigeration Units” to determine the root of the problem.

CAUTION: The foregoing method has a potential shock hazard. Proceed cautiously. Be sure not to touch any wire without its insulation. If the insulation is cracked or frayed, or if for any reason it does not seem right, unplug the unit, put the meter in place, and then reconnect the unit and wait for about three minutes until the compressor starts, then proceed.

Sometimes, due to a defective light switch, the light in the fresh-food compartment will not turn off when the door is closed. The heat created by the bulb inside the unit cabinet will not let the cabinet temperature drop low enough to satisfy the thermostat. When this happens, the compressor never shuts off. Check the door switch by opening the door and pushing the light switch. If the light stays on, replace the light switch.

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