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Shawn Pettersen

Technical Service Manager • North Georgia

Defrost Board Troubleshooting

I want to go over the process of troubleshooting a heat pump defrost board. This includes checking for common issues. Start by examining the board for visible damage or burnt components. Now we need to use a multimeter to test continuity, voltage, and resistance at various points on the board.

The first place we need to check is to see if we are getting power to the board. Here we are checking for the 24 volts at the contactor.

  1. Now we check the board for 24 volts. The T1 terminal on the defrost board powers the defrost timing function. This terminal must be energized before any defrost function will occur.
  2. If the unit does not go into defrost after performing the speed up, check for 24 volts on the T1 terminal. This terminal powers the defrost circuit, and must be energized before any defrost function can occur. The T1 should be energized once the Y terminal is energized and the pressure switches are closed. Ensure the T1 wire is connected at the contactor, and that 24 volts is present on the T1 spade terminal.

Check to see if the defrost switch is open or closed. Disconnect defrost thermostat leads from control board, and connect to ohmmeter. Thermostat leads are pink, insulated wires connected to DFT and R terminals on control board. Resistance reading may be zero (indicating closed defrost thermostat), or infinity (OL for open thermostat) depending on outdoor temperature.

Now we want to check for 24 volts on the DFT terminal. Checking that the defrost board is supplying power to the defrost Switch.

Meter leads on the DFT position 1 and the C terminal.

Schematic: The circles represent where meter leads were making contact for testing.

Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion. 

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