HVAC For Beginners

Air Conditioner Compressor

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.

AC Compressor

**HOT TIP** If the unit is no longer under warranty and the estimated cost of replacing the compressor is over half the price of a new unit, it would be a good idea to replace the system.

**NOTE** Before replacing the air conditioner compressor you should identify and correct the cause of the failure. An acid test should be performed in the case of an electrical failure.

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.

Rotary Compressor

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.

Scroll Compressor

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 most expensive.

Inverter Compressor

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.


Sometimes the compressor terminals and wires become damaged from operating with a loose connection. When the terminals are damaged, a kit is used to attach new wires to the terminals.

Terminal Kit

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 to be 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.

Hard Start Capacitor

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.

Compressor Replacement