Air Conditioner Compressor, A Troubleshooting Guide
The air conditioner compressor is the heart of your cooling system.
It is the
most commonly misdiagnosed component in the system. At least one third of the
time that the unit is
said to have a "bad compressor", the compressor is in fact not the problem. The
typical air conditioning service call would cost over $100. We will show you how
to correctly troubleshoot your unit and avoid unnecessary replacement. This
procedure will help you to check out the typical single speed units.
Types Of Air Conditioner Compressors
There are basically four types of compressors that are used in air conditioning systems. A
rotary compressor is commonly used in window units and some central ac units.
They have been around for many years and are inexpensive and durable. A
reciprocating compressor is the most common in lower efficiency split air
conditioners. They are relatively inexpensive but not as durable. A scroll
compressor is the most common type in higher efficiency equipment. They are very
durable and efficient but are typically more expensive. The latest innovation is
variable speed compressors which are commonly referred to as inverter drives.
These are in the top of the line high efficiency units but they are normally the
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
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