Diagnosing a Compressor Problem
First, we will check to ensure that the compressor does not have an electrical problem. You will need a
basic multimeter to measure resistance. Shut off the power to the unit and verify with the meter that no
power is present. Next, you need to remove the terminal cover. Check to make sure that the wires and the
terminals are not damaged. The typical unit will have three terminals labeled C, R, and S. You should
check for continuity (less than 30 ohms resistance) between C & R, C & S, and R & S.
If the resistance is significantly higher, then check to see if the compressor is hot. If the unit is
hot, the internal overload is open. You will need to wait until it cools off to continue troubleshooting.
If the resistance reading is high and the unit is cool to the touch, the compressor motor is bad. Next,
you should take resistance readings from each of the terminals to the body of the compressor. A low
resistance reading, means the motor is grounded and the compressor will need replaced.
You should also check the capacitors. If your tests do not
indicate a bad compressor motor but the unit trips the breaker or fuse, the problem may be mechanical. In
some cases, as the unit gets older it may require a boost to get started. Special capacitors called hard
start capacitors are made for these situations. They are a combination of a capacitor and a built in
relay. They give the compressor extra starting torque and the relay disconnects the capacitor after the
unit starts. They are very inexpensive and can often extend the life of the compressor for several years.
If you install a hard start capacitor and the unit will still not start, you will need to proceed with air
conditioner compressor replacement. In other cases, the air conditioner compressor may run but not provide
cooling. This can happen when a failure allows liquid refrigerant instead of vapor at the compressor
inlet. The result is usually broken valves in the compressor. In that case, the air conditioner compressor
or the whole condensing unit will require replacement.