Thanks for the post Kevin.

What is the model number of the compressor?

Most likely the Start Capacitor is bad. It's probably drained or the terminals are burnt, which can happen if the unit ran too hot or if there was a surge of voltage at one point.

I would replace the Capacitor(s) (if more than one) and this should take care of the problem.

Does it not start and hum when air is in the tank or when the tank is empty? Or does it not matter?

Also, do you hear the short burst of air from the pressure switch unloader valve after the unit reaches max pressure and shuts off? If the Unloader Valve isn't bleeding the excess air from the pump head (and supply line), it can cause this to happen.

- MTR

The Start Cap is typically the one with the lower microfarad rating. This will be denoted on the capacitor as "uF", along with a voltage.

E.g., "90uF, 250 VAC". This should certainly be marked somewhere on the capcitor.

Unfortunately, the manufacturer doesn't offer the capacitors any longer. I would contact them directly at 888-895-4549 as they may be able to direct you to a source to find the capacitor. Or, you can try Grainger or another electrical supply outlet.

But, it certainly does sound like you have have a defective start cap.

Also (just to be safe), make sure the unit isn't plugged into an extension cord.

- MTR