Capacitor Starter Motor Troubleshooting


Capacitor start motors are used for high torque applications, such as in air conditioners and air compressors. Located inside and at the rear of the electric motor is a centrifugal switch. This switch remains closed until the motor comes up to full speed. When the switch opens, it disconnects the capacitor from the motor windings. If a capacitor motor fails to start, the best place to begin checking is the capacitor itself.


  • Remove all electrical power from the motor and its circuits. Use a screwdriver to pull the protective metal cover from the motor that houses the capacitor. It will sit in top of the motor. The cover will be a long cylindrical shape. Use a pair of pliers to remove the two connectors that tie the capacitor to the centrifugal switch. Stand the capacitor on end so it is standing upright. Use a volt ohmmeter and switch the meter to the volts position. Place the red lead into the volt connector and the black lead into the common connection point. Touch the leads, one each, to the capacitor connectors. Allow the capacitor to dissipate any stored voltage. Switch the volt ohmmeter to ohms and place the read lead into the ohm connector. If the meter has a capacitor-checking position, select this option. Touch both leads to the capacitor connectors. The capacitor check option meter will read either "good" or "bad." The ohmmeter method will read either "open" or "infinite ohms." If the meter reads any type of resistance, then the capacitor is bad and should be replaced. The same goes for the capacitor-checking option of good or bad. If the capacitor is good, move to the next section.

Centrifugal Switch

  • Remove the rear cover from the motor. The power must still be off and the capacitor disconnected. Pull the rear cover directly rearwards. Inspect the interior bearing for any signs of wear. If the bearing has a burnt smell, replace the bearing. The switch should be sitting just on the inside of the rear cover with two wires coming from it and into the motor windings. A set of copper discs called contacts will open and close from the mechanical action of the centrifugal movement mounted on the motor shaft. Inspect the contacts to be sure that they open and close by pressing on the brass actuation bar. It should not take much pressure to perform this action. If the contacts fail to open, then the contacts maybe burned together. Replace the contacts. Check for any signs of pitting or black carbon marks on the contacts. Clean the contacts with some emery cloth, and polish them until a bright copper color is revealed.

Mechanical Centrifugal Movement

  • This movement is a pair of balanced weights that move rearward into the electrical switch. As the motor comes up to speed, the weight pushes against the electrical switch and opens the contacts. Two springs force the movement weights closed or back to the motor. If the springs are broken or stretched, replace the springs. If one of the weights is missing, the movement may have caused other damage to the windings. Inspect the windings for any signs of damage. Check all wire connections to the switch and motor windings. Test the motor windings for continuity with the ohmmeter. There should be a continuous path for all the winding connections. Check the manufacturer's specifications for individual motor winding schematics.


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