In a previous post, I mentioned that control boards are replaced and there is really nothing wrong with them. In recent months, there has been an increased number of defrost boards from split system heat pumps returned for warranty. In-house testing of some returned boards has found that a majority of the boards were not defective.
The following information will help you better understand the operation of the board, and give some insight to other possible sources that may lead a technician to condemn the board when it is actually not defective.
These defrost boards have an internal five-minute compressor time delay that will be initiated anytime the room thermostat satisfies or the high pressure switch opens. In regards to the time delay, one thing that has been seen is that if there is a problem with the compressor contactor coil (such as being shorted or pulling in rough, or sticking occasionally) the board will have a tendency to go through the five-minute time delay, attempt to energize the compressor contactor, but will immediately return to another five-minute time delay and continuing repeating the time delay.
Before diagnosing the board as defective, use the following troubleshooting procedures .
- Ensure that the indoor unit transformer is tapped for the correct line voltage on the job. Don’t assume it is 240 volt. If the primary voltage is 208 volts and you leave the transformer set for 240 volt primary — you are starting off with low voltage to the control and this, in itself, will cause problems, especially with the contactor pulling in.
- Remove the wire attached to the “M” terminal on the board (see below).
- Give unit a call for cooling (measure for 24 v between Y and C).
- With a screwdriver, short between the two test pins on the board until you hear the board click. (see below)
- Place voltmeter leads on the “M” terminal and “C” and you should read 24 volts.
- If you read 24 volts after completing the above steps, the board is good and you should check your compressor contactor coil and wires.
- If you do not read 24 volts at the above steps, ensure that you have continuity through the high pressure switch.
- If there is continuity through the high pressure switch, the board is likely defective.
- Make sure the defrost curve jumper is not left at the P position (see picture above)
This applies to both heating and cooling since the “Y” call on a heat pump is the COMPRESSOR CALL. Heating and cooling is determined by the reversing valve. Hopefully this will help you correctly diagnose a problem and not condemn a control board that may still be good.
Thanks to Alan Dukes Technical Service Manager Virginia Air Distributors, Inc. Richmond/Fredericksburg for the information in this post.