Today I fixed a problem with an air conditioner circuit board. I ended up replacing the circuit board because making a repair to the circuit board is typically not in the scope of an HVAC Technicians duties nor do most of them have the tools and the time to fix a circuit board. The call from the customer was that their air conditioner blower motor was running all the time. The house was at the proper temperature and the thermostat was set to auto but the blower motor continued to run all the time. This is a good lesson for a new technician to learn the air conditioner sequence of operation as it helps one understand the sequence of how the air conditioner works and the new tech can then troubleshoot the problem easier. The sequence of operation for an air conditioner (detail) will be covered in a future article.
Air Conditioner Circuit Board Problem
First of all, knowing the sequence of operation for an air conditioner is important to know. When the thermostat contacts close for an air conditioner telling it to turn on the condenser and the blower turn on. Several things happen in this situation. The red wire going to the thermostat is a 24 volt hot wire. The yellow wire is a wire going to the air conditioner contactor. The wire runs to the air handler and then to the condenser where the contactor is located for the air conditioner. When the contacts close in the thermostat the 24 volts hot (the red wire) sends 24 volts hot to the air conditioner contactor.
That energizes the air conditioner contactor coil. The contacts close and the compressor and the condenser fan motor come on. The green wire in the thermostat is also energized by the red wire. This sends 24 volts hot to air handler to energize whatever controls the blower. In some cases a stand-alone relay controls the blower motor but in other cases this is controlled by a circuit board in the air handler. The control board also uses a relay but the relays are built into the circuit board. See the photo above. In that photo you will see a little black rectangle cube. This cube is the relay for the blower motor. That is energized by the green wire in your thermostat anytime the thermostat is set to auto and the air conditioner comes or anytime you switch thermostat to the “ON” position so the fan will run all the time.
Bad Relay in the Air Conditioner Circuit Board
In this case the relay contacts were stuck. I simply tapped the little black cube with the butt end of my screwdriver and the blower shut off. There are ways to bypass the circuit board and wire in a new relay and bypass the board altogether however it is not a good thing to re-engineer the system. There are several factors not including some liability issues that give me reason to be cautious when re-engineering the original circuit design of an air conditioner or any HVAC appliance for that matter. I simply don’t do it and I do then its to get me out of a pinch until I can get a part or something like that. So controlling the blower by adding a relay and bypassing the circuit board is only done to get me out of a pinch. When its all said and done everything is original when the job is completed.
In the situation today, I happened to have an exact circuit board on my truck. Now don’t ask me why they go bad that could be anything. Lighting strikes, power surges or any other type of power quality issue. In this case, the contacts stuck shut which can be common in relays. The electrical arc that occurs when the contacts close cause the metal of the contact points to get hot. Once the contacts are closed they cool off and sometimes stick to each other and won’t release such as the case today.