How to Troubleshoot a Definite Purpose Contactor

Determine if your unit runs constantly by turning the thermostat completely down. Switch it to a different mode in the case of an air conditioning condenser that will not shut off. A bad contactor will also cause a no start condition in a unit when the power switch is turned on.

Shut off the circuit breaker that feeds current to your unit, or turn off both the air handler and condenser breakers, if separate and labeled as such. Shut off the secondary power supply switch at the thermostat, or any other utility switch for the AC unit. Trace the power cables to their source, which will be a control box mounted on your air conditioning unit. Wait an hour or so for the capacitors to discharge; capacitors can hold residual electricity and deliver a shock.

Use a screwdriver to remove the access panel to the control box on the unit. It might require small sockets or a screwdriver. Identify the contactor as the small plastic box that has wires connected to it, and contact points that are made of silver coated steel. The contacts might also be bronze. The coil is situated inside the lower part of the contactor box, and the electrical wire connectors are on the ends of the contactor box.

Use a narrow head paint brush to clean away any spiderwebs, nesting materials, dried grass, dirt and dust around the contactor box area. Look for any burnt, brown or black wiring at the screw connection points, or slightly larger incoming power wires. Remove the spade connectors one at a time and clean the spades with a cotton swab and alcohol. Use a screwdriver to tighten the electrical eyelet screws, if they appear loose. Inspect any eyelet connection that is blackened and clean it with alcohol and a swab.

Locate the contactor mechanism, which sits in the middle of the contactor box, and rides on a small track way. A small spring keeps the contactor points in the open position. The contact points are similar in appearance to automotive distributor points. Push your finger down on the contactor. If the contactor is frozen, due to high voltage arc welding, it must be replaced.

Push the contactor down and pull up, several times. If it moves in both directions, leave it open and inspect the contact points. The points must be clean and making a good electrical connection. If they are worn down, melted or discolored you can temporarily clean them with a small nail file or Emory board. However, this will be a temporary fix.

Set the dial on a volt-ohm meter to the resistance setting. Attach one lead to one coil side contact pole and the other lead to the other coil pole on the other side. Read the resistance. A normal reading will show 10 to 20 ohms. If the reading is significantly higher or lower, it indicates a defective coil. Replace the contactor.