I am always being asked where the defrost timer is located on a refrigerator. Unfortunately the location varies depending on the manufacturer and model. Some are located in the rear near the compressor. Others are behind the louvered kick-plate in the front. Some are in the fresh-food section near the cold control. To make finding them more challenging they are usually covered except for a hole that allows them to be manually advanced. On the refrigerator that I use the timer is in the fresh-food section beside the cold-control. The only indication is the hole that permits a large flat blade screwdriver to be used to advance the timer.
Fig 1 Typical Defrost Timer
In a typical defrost circuit 120 volts is passed to the timer from the cold-control when it calls for cooling. The voltage energizes a clock circuit. If the defrost timer is in the defrost position, voltage is sent to the defrost heater circuit. If the timer is not in defrost, voltage is passed to the compressor starting-circuit and fans.
Fig 2 Typical defrost circuit (defrost cycle)
Some defrost circuits power the clock with line-voltage (bypassing the cold-control). This arrangement initiates a defrost cycle at set intervals of time regardless of how often the compressor has run. In the circuit above the clock runs only when the cold-control is made so it initiates a defrost cycle based on accumulated compressor run-time. With the defrost timer in the defrost position current goes to the defrost heater. The thermostat in the circuit (also called a termination thermostat) opens to turn off the defrost heater when the temperature in the freezer reaches its design temperature. This is usually around 50 to 55 degrees. This keeps the freezer compartment from getting too hot before the defrost timer switches back to the compressor run position. If the thermostat fails in the open position no current can flow through the defrost circuit even though the defrost timer and defrost heater are good. In this case the evaporator will frost up and eventually the freezer and fresh-food compartment temperatures will rise. The same symptoms will occur if the clock in the defrost timer fails and never switches to a defrost cycle or if the defrost heater fails.
Fig 3 Typical defrost circuit (in compressor run position)
Normally the termination thermostat will turn off the defrost heater before the defrost timer times out and switches the defrost heater out of the circuit. When the timer times out the defrost timer opens the heater circuit and closes the contacts to send power to the compressor and fans. The defrost timer stays in this position until the accumulated time to switch back to defrost cycle.