This is usually a burned out start relay on the compressor. The function of the start relay is to momentarily energize the start winding in the compressor motor and then cut power to the start winding as soon as the motor is running. In the good ol’ days, these relays were mechanical: heavy copper wire wound around a plunger that would open and close in response to current… and they almost never failed.
Most of the start relays on modern compressors are solid state and use a special material whose resistance increases with temperature. So, as current flows through the relay and the relay heats up, its resistance increases to the point that the start winding is isolated from the rest of the circuit, accomplishing the same thing as the old relays. A common failure of these relays is that the solid state material “cooks” and breaks up, staying open and thus never allowing the start winding in the compressor motor to energize. The end result is that the compressor tries to start, usually you’ll hear a humming noise, and then, after a few seconds, the compressor’s overload protector takes the compressor offline with a loud CLICK!
Easy way to tell if the start relay is bad is by simply removing it from the compressor’s control pod and shaking it. If you hear any rattling, it’s fried.
The most common start relays are shown below. Match ’em up to yours by sight. If you don’t see yours, you can use the 3-in-1 start kit or look it up here using your model number.
The other, more ominous possibility is that the compressor itself is FUBAR. You can check this with your ohm meter. Set it on the lowest setting and then measure the resistance between each of the three pegs sticking out from the compressor in the control pod– the same ones that the start relay and overload connect to. Should read something in the low ohms. If the reading between any two pegs is infinite resistance, then one of the windings in the compressor motor is burned out. You should also measure from one of the compressor pegs to ground with your ohm meter on the highest setting. If you read anything, the compressor has developed a path to ground and is bad.
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