There are many myths and wives tales about equipment I hope to dispel. Some of the problems can be solved by the home owner some need to be corrected by a skilled service technician with the proper tools.
Some of the problems are minor and may cause increased energy consumption and others will cause destruction to your equipment if not corrected. Scott Meenen N3SJH
No power: Check for power at the disconnect and contactor on most residential units you should have 240 volts ac between the terminals and 120 volts to "ground" or "earth". Some disconnects have fuses in them. If you have circuit breakers I see little reason for fuses other than to create a service call. I usually replace them with copper tubing.
Most contactors will allow you to push them in manually (DANGER HIGH VOLTAGE). If the fan runs and the compressor does not then you have an electrical problem with the other component. There are cases where a grounded compressor or bad fan motor will cause the fan motor to run slow even though the contactor is not pulled in. A bad run capacitor will cause the compressor not to run and the same for the fan motor. Some units have a dual run capacitor that takes care of both and if it fails (it has an internal fuse) the motors will just hum (drawing high current) but not run. The compressor will have an internal overload that will disconnect the compressor after a few seconds of not starting. It will remake as soon as it cools, this condition can be mistaken as a bad compressor. Some compressor will require a "hard start kit" to solve starting problems and if it doesn't have one I recommend one as a preventative measure.
Use a clamp on ammeter shown above to verify which parts are drawing current. If there is no current flow through the capacitors then that is a clue they may be bad.
DO NOT let your system run if the either fan motor (inside or out) is not working you will destroy your compressor, the same goes if your unit is icing up or short of charge you will burn up your compressor motor.
No control voltage: Most residential units use 24 volts ac to pull in the contactor. The power for this usually comes from a step down transformer in the furnace or indoor unit through the thermostat. Some outdoor equipment will have its own transformer, but this is rare for residential.
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