1. Disconnect the power and wires connected to the timer terminals.
2. Check the diagram inside the timer door for normally open (NO) and normally closed (NC) contacts such as the ones in figures 53a, 54a, and 54b.
3. Remove the wires connected to the terminals.
4. Set ohmmeter scale on X1 and zero it. (No adjustment is necessary if you are using a digital meter.)
5. Turn the dial clockwise until it reaches the zone where the distance between the clicks is longer. (The unit is now in the cooling cycle.) Touch the probes to the normally closed contacts (numbers 1 and 2). The meter should register continuity. Otherwise, replace the timer.
6. Turn the timer dial clockwise to the area in which the distance between the clicks (start/stop time tabs) is shorter. This is the defrost cycle.
7. Touch the ohmmeter probes to contacts 3 and 4; the meter should register a continuity reading. If not, replace the timer.
8. Touch the two terminals connected to the timer motor (numbers 5 and 6). The meter should register an ohm reading. If not, replace the timer.
9. Turn the dial slowly clockwise. If you feel any snag, replace the timer.
10. If you notice any charring or burn marks on the terminals, replace the timer. Also, if the timer fails to change cycles, check the terminals connected to the timer motor (numbers 5 and 6) with a test light to make sure proper voltages reaches the timer. In cases where the timer motor operates on low voltage (40, 24, or 12 VAC), if the test light doesn’t glow, check the transformer.
A good way to find out if the timer motor fails to change cycles is by visual observation. Watch the timer dial while it is connected to the power for a short time. If the timer dial turns, there is nothing wrong with the timer motor.
Some commercial-type timers are equipped with a sensing bulb, which is attached to the evaporator coil. When enough ice accumulates on the evaporator coil, the sensing bulb transmits the changes in temperature to the timer bellows and causes the timer to take the unit into the defrost cycle by stopping the compressor motor, the evaporator fan(s), and energizing the evaporator heater or the hot gas solenoid. Then, when sensing a rise in evaporator temperature, power is restored and the unit restarts.
The defrost period on these timers can be regulated by turning an adjusting knob inside the unit cabinet to lengthen or shorten the defrost period. Some large commercial units with multiple evaporators use more than one timer.