How to diagnose an outdoor condensing unit

When it comes to diagnosing an outdoor condensing unit, the process is pretty easy. In a standard condensing unit, there are only a few parts in the unit. There is a contactor, a fan motor, a compressor and a capacitor. These parts all work in tandem. The way a condenser works is the thermostat sends a signal to the contactor. When this happens, the contactor becomes magnetized and pulls down the bridge completing the power circuit from one side of the contactor to the other. On one end of the contactor, there are 2 wires which are the live 220 volt power lines coming from the house. At the other end of the contactor, there are the wires from the compressor and the fan motor. So once the power bridge has been engaged, the compressor and fan motor are now supplied with power and should begin to run. The capacitor which is connected to the compressor and fan motor is simply a start assistant to help them both get running. The compressor’s purpose is to pump refrigerant into the house to provide the cool air, and the purpose of the fan motor is to suck air through the coil and keep the refrigerant and unit cool. Now that you know the functions, you can begin the process of diagnosing the unit.
At this point, you will need to run down a check list such as the one below…
1. Is the contactor engaging when the thermostat is turned on and calling for cooling?
    a) If yes, then proceed to step 2.
    b) If not, you will need to test the line voltage from the thermostat wire to see that you are getting 24 volts. If there is 24 volts, the contactor is bad. If not, the thermostat wire could have a break in the line somewhere or the thermostat itself could be bad.
2. When the contactor engages, what happens? Does the motor run and not the compressor? Or vice-versa? Or nothing at all?
    a) If one or the other runs but not both, the problem could be either a bad motor or compressor or a bad capacitor. If the motor is not running, try spinning the blade with a stick to see if it gets going. If it does, it is the capacitor. If it doesn’t, it most likely means the motor is bad and will need to be replaced along with a new capacitor. If the motor is running but not the compressor, unless you have the tools to test the capacitor, you will want to replace the capacitor. If after replacing the capacitor the compressor still does not run, your compressor is bad and will need to be replaced.
    b) If nothing at all begins to run, odds are it is just the capacitor. It is highly unlikely that the compressor and fan motor would fail at the same time. Either way, the capacitor is going to be the least expensive part to try first.
3. Are all the parts functioning but you are still not getting any cool air?
If this is the case, you are likely to be low on freon which should only be happening if there is a leak somewhere in the system. Only thing left to do now is call a tech.
So there you have it. Diagnosing the condensing unit in 3 simple steps. For more helpful tips, keep checking back here for our latest blog entry. For any parts needs, please visit or call toll free to 866-432-8551.
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