The termination thermostat is clipped to the evaporator. When the evaporator temperature rises to about 50°F, the contacts within the thermostat open, causing the defrost heater to become de-energized. When the evaporator temperature drops to about 20°F, the contacts within the thermostat close and the defrost heater becomes energized when the unit is in the defrost mode.
Buy a small container of CO2 (a fire extinguisher) before starting this test.
1. Disconnect power to the unit.
2. Remove the evaporator panel.
3. Disconnect the two wires from the termination thermostat and remove it from the unit.
4. Set the ohmmeter on the RX1 scale and zero it. Then connect the two meter probes to the two thermostat wires with alligator clips.
5. Spray the thermostat with CO2 for about five seconds. Be sure the CO2 does not touch the ohmmeter. The ohmmeter should register a zero reading. Otherwise, replace the termination thermostat.
6. Hold the termination switch in your hand for about one and a half minutes or long enough to warm it. With the alligator clips holding the probes to the thermostat wires, the meter should register an open-circuit reading. Otherwise, replace the thermostat (see fig. 62).