Humming Air Compressor

All of a sudden you have a humming air compressor. What's with that?

If your air compressor hums instead of the motor firing up, it's telling you something. The message is that it is quite likely a power related issue that is the problem.

Here are some steps for you to try and resolve a humming air compressor motor issue, from the least complex through to the more complex diagnosis and fix.

Added An Extension Cord?

If you have had to move your air compressor and in order to use it you needed to get power to it via an extension cord, the extension cord is quite possibly the problem. If you are using a 14 gauge x 100' extension cord, for example, you are almost certain to be starving your compressor motor, and it cannot get enough juice to get going.

The first thing to do, then, is to plug the compressor directly into a wall socket, and preferably one that is supplied by a 20 AMP breaker, and has no other electrical applicances pulling power from the same circuit.

If where you need compressed air is far from the compressor, do not move the compressor. Add air hose to extend the reach. The longer hose will diminish the flow of air to the air tool, but using an extension cord will possibly damage the compressor motor. Better under-supply the air to the tool (since that will not hurt the air tool) than to under-supply the electricity to the compressor motor.

Did this resolve the problem with the humming air compressor? If not, please read on.

Does The Unloader Valve Work?

When an electric air compressor motor cannot start against a too-great load, the motor may make a humming sound.

If you unloader valve does not work, then the added load of air trapped over the piston may be enough to prevent the motor from starting. See the pages on this website about what the unloader valve is and what it does if you are not sure, and then check the unloader valve on your compressor.

pressure switch unloader valve

A good indication that it is the unloader valve causing the humming air compressor motor is if you completely empty the air tank before trying to start the compressor, and after doing so, the compressor motor starts and doesn't hum. If that's the case with your compressor, you need to get the unloader valve fixed.

Air compressor still humming when it's trying to start?

Clean The Tank Check Valve

Although it's not a frequent occurrence, a blocked tank check valve can be a factor in creating conditions on the compressor that result in a humming motor. Since we are providing tips from the least intrusive and simplest through to more complex checks, even though this does require some work, it's an easier check than those that follow.

Unplug the air compressor, open the tank drain and the PRV valve, and drain the tank completely.

Remove the line from the pump head where it reaches the tank, and typically you will find the tank check valve there. It's a one way valve. Blow through it both ways. Air should flow easily in one direction, and not at all in the other. Wipe your lips! :-)

If the check valve isn't working properly, quite often it only needs a good cleaning. Rinse in a clean solvent a few times, let it dry thoroughly, and reassemble.

Air Compressor Still Humming

Assuming that your power supply is good, and you are not using an extension cord, you want to do this check first, as a humming air compressor motor is frequently caused by a dead or dying compressor motor start capacitor.

See the troubleshooting page and follow the link to testing capacitors to find out if yours is causing the problem. If so, you will need to replace the start cap.

If the start capacitor is good, you are probably looking at a pump failure of some kind that's backloading the motor so it cannot start, or the compressor motor itself is failing.

Tearing down the pump means pre-ordering a valve plate and gasket kit. Otherwise, once you have the pump disassembled and you find a problem, you are stuck until you can get parts. Sometimes kits are not available and you may have to cobble up valve plates and make your own gaskets. Information on how to do that is found on this website. Use the search box to find details.

Testing an electric motor means removing it from the compressor and taking it to a shop. In my opinion it's not enough for them just to add power and watch the motor spool up, there needs to be load on the motor shaft to replicate the environment on the compressor, where the shaft is attached to a crank and there is load. That's why testing the motor is the last thing on the list of diagnosing and fixing a humming air compressor. Good luck with yours.