Replacing the magnetic contactor relay is one of the most common tasks when repairing air conditioners and heat pumps. Heat pumps and A/C units use these contactor relays because standard wall circuits and switches do not support the higher voltages required by compressor and condenser motors.

Replacing a contactor is a relatively simple job, and a DIYer could save a considerable amount of money. However, it is very important to recognize the dangers of installing a heat pump or AC magnetic contactor. The electrical shock hazards can be fatal, and these compressors and condensers use start/run capacitors. In other words, even with the power turned off, they can provide a nasty shock.

In order to buy the right magnetic contactor relay, simply match the part number of the relay being replaced. A dealer may also be able to provide a list of relays that are appropriately suited to that unit. However, you must be very careful when not using an exact replacement.

The two most common problems are coil strength being too low and too high. If the coil strength is too low, the unit will be unreliable and difficult to start. If the coil strength is too high, it will cause the start circuit to remain active, which will eventually burn out the starter circuit.

When mounting the relay, it is vital that you mount it exactly as the original unit was. Many relays are affected by gravity, and if you hang them incorrectly, the holding points may hold too long or not long enough. Most modern current-operated contactors can be mounted in any position, but it is still best to follow the example of the original configuration.