Understanding the Defrost Control Board - Part 4 Testing

If you suspect there is a problem with the defrost control board, thermostat, or defrost timing here are the procedure for testing. This assumes that the system is properly charged.

Checking the Defrost Control Board

• Turn off all power to the outdoor unit.
• On the control board, locate the two terminals where the defrost thermostat is connected.
• Disconnect the defrost thermostat.
• And connect a jumper across the defrost thermostat terminals on the control board. (Make sure the jumpers are connected properly, touching only the defrost thermostat terminals).
• The jumper simulates a cold coil and a defrost condition.
• At this time, locate the two test terminals on the defrost control board. Depending on the manufacturer it may say "Test", "Speed Up", or on a Trane system "Test" and "Test.Common". Double check!
• Place another jumper between the two test terminals.
• This will speed up the defrost timing cycle. 90 minutes = 21 seconds, 60 minutes = 14 seconds, and 30 minutes = 7 seconds.
• Turn the electrical power back on.
• Place the thermostat into heating.
• The unit should cycle in and out of defrost with the accelerated time limits as listed above.
• It is not recommended to leave the unit in the test mode for a prolonged period of time.

If the unit cycles in and out of defrost, check the defrost thermostat.

Checking the Defrost Thermostat The defrost thermostat is what determines when a defrost is needed and when the system should go out of defrost. Most defrost thermostats close at 28F +- 5F.

• Turn of the power to the unit.
• Disconnect and remove the sensor from the unit.
• Fill a small glass or container with ice and just enough water to cover the ice.
• Place a thermometer in the water and add salt until the temperature reads 20-25 degrees (I know...  Who carries salt with them?).
• Fill another small container with water that is in the 70-75 degree range.
• Connect your ohmmeter between the two sensor leads.
• Place the the defrost sensor in the cold water solution.
• In about 60-90 seconds, the sensor should close and read continuity.
• Place the sensor in the warm water container.
• After another 60-90 seconds, the sensor should open and read infinite.

If the sensor fails to open or close, replace the sensor.

Final Notes

If you still are having a defrost problem, check the all wiring and connections for corrosion, opens, or shorts.

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by Ron Walker