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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 the humming air compressor is trying to tell you is that it is quite likely a power-related issue that is the problem.

This article will provide you with all the relevant information about why an air compressor humming but not starting, along with reader questions and responses.

Table of Contents

Why Your Air Compressor Hums But Won’t Start

The most common reasons for your air compressor humming but not starting include:

  • Using an extension cord
  • Faulty unloader valve
  • Dirty tank check valve
  • Pump issue
  • Capacitor problem

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

Lose The Extension Cord!

If you have had to move your air compressor to use it and as a result, 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. It may start to hum.

Extension cords are not recommended for many air compressors.
Extension cords are not recommended for many air compressors.

The first thing to do if the compressor starts to hum is to plug it directly into a wall socket. If the compressor starts without humming, your extension cord is not heavy enough gauge or is too long. Lose it.

Your compressor prefers a power supply with a 20 AMP breaker and has no other electrical appliances pulling power from the same circuit.

If where you need compressed air requires you to move the compressor and use an extension, I recommend that you instead use an air hose to extend the reach. The longer hose will diminish the flow of air to the air tool that is true, and a high-demand air tool might struggle a bit, but using an extension cord will possibly damage your 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 and destroy it in the process.

Does The Compressor 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, the motor continues to struggle to start, and begins to hum. 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 to be sure it works.

Pressure Switch Unloader Valve
Pressure Switch Unloader Valve

A quick check for unloader valve malfunction is to completely empty the air tank before trying to start the compressor. If after draining the tank to zero PSI the compressor motor starts and doesn’t hum, you likely need to get the compressor unloader valve fixed.

Is the Tank Check Valve Dirty?

Although it’s not a frequent occurrence, a partially or fully blocked tank check valve can back pressure the pump and add load to the motor, creating that humming sound.

Since I am providing tips on this page that range from the least intrusive and simplest through to more complex compressor checks, even though cleaning the tank check valve 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. If cleaning the tank check valve does not improve its operation, replace it.

Air Compressor Pump Issue?

Depending on how the compressor motor is connected to the pump, if you can remove a built or de-couple the motor, try turning the pump by hand. If there is a pulley, that makes it pretty simple. If the motor is directly connected, that makes it more difficult, for sure.

What we are trying to accomplish is to see if the pump itself has a mechanical issue that is binding it up, backloading the motor, and creating the humming sound. If the compressor pump cycles relatively easily by hand, odds are good that the pump is OK. Time to check the next item on the list.

Is it a Capacitor Problem?

Assuming that your power supply is good, and you are not using an extension cord, your tank check valve is clear, and the pump cycles relatively easily, then your humming air compressor motor problem can be caused by a dead or dying compressor motor start capacitor.

See the troubleshooting page and follow the links to pages about checking the caps and how to test capacitors and do so for the caps on your compressor, assuming the motor has them. If your start or start/run capacitor is causing the problem, you will need to replace the start cap. See the help page about finding a replacement capacitor on this site too!

If the start capacitor tests as OK, then that’s pointing to the motor itself as being the cause of the humming sound. A winding issue, a centrifugal switch… something has failed in the motor.

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. Make sure they do that.

That’s why testing the motor is the last thing on the list of diagnosing and fixing a humming air compressor. It’s complex and time-consuming, not to mention quite possibly expensive.

FAQs (Frequently Asked Questions)

Which of the following will cause a compressor motor to hum but not start?

It’s likely that if your compressor motor is humming and not starting, you’re using an extension cord that you shouldn’t be, you have a faulty unloader valve, dirty tank check valve, pump issue, or capacitor problem!

What causes electric motors to hum?

In electric motors, a slightly misaligned or bent motor shaft will lead to the production of a humming sound. A similar noise can also be created if there is a minor fault in the transmission equipment attached to the motor shaft.

Reader Questions & Responses

Air Compressor Hummin But Not Starting

Question

Plug compressor in and hit start and it hums very little and stops and I have to reset it.

Kingcraft 6 gallons 3.5 horse.

Response

Did it ever work on the circuit you have it on now?

Sound like too many appliances drawing power from the circuit before it gets to the compressor, or the start capacitor is failing or has failed, or the motor itself might be flawed.

Before you check any of these further, unplug the unit, open the drain valve to drain the tank to zero, and plug it back in. What happens then?


Air Compressor Hums But Won’t Start

Question

When some air is used the air compressor hums instead of pumping more pressure?

Response

Carlton; your compressor hums? Do you recognize the tune

I think I do.

It’s the “my motor can’t start as there isn’t enough amperage in the circuit, or the motor start capacitor isn’t working” song.

Those are the things I would check, as I suspect the humming is coming from your compressor motor that can’t start when the pressure switch trips cut in at the low-pressure level.


Air Compressor Just Hums Won’t Start

Question

Hi, I wonder if you can help, please. I have a Rondy RDY100CA50 compressor. I was using it okay and then it stopped as normal when the tank was full of air.

Rondy RDY100CA50 compressor
Rondy RDY100CA50 compressor

The trouble is it didn’t re-start and just made a humming sound.

I drained what air was left in the tank and opened the condensation valve- only a very small amount of water came out. I checked the oil and that was okay. I then checked all the electrical connections and they were okay. I was using a power extension lead so I unplugged it and plugged it straight into the wall socket but still no joy.

Have you any ideas please and do you know how I can check to see if the capacitor is okay.
Thanks a lot

Response

Hi Tony.

Dumping the air, and having the problem still persist likely rule out what my first guess was the unloader valve. By draining the tank you effectively unloaded the air compressor, and if the compressor won’t start, then your unloader valve is probably OK.

Plugging the compressor directly into the wall socket was a good move. Are you sure that there is nothing else new drawing power from the same circuit?

I tend to think it’s your capacitor, too. I will be posting a page soon on how to check the capacitor. Not being an electrician myself, I’m relying on someone to provide me the info, and that hasn’t come yet.

If anyone else knows how Tony can check his capacitor, please post.


318v IMC Belair Air Compressor Hums and Then Trips Breaker

Question

IMC belair standup compressor, 318v IMC.

Belaire 318V air compressor
Belaire 318V air compressor

Belaire hums on startup, got it wired straight to the fuse box, then it pops breaker?

Response

Visit the troubleshooting guide!


Ingersoll Rand Air Compressor Hums But Won’t Start

Ingersoll rand 60 gallons 5 hp 220 single-phase runs but after a fill cycle, the motor hums and then won’t come back on for 30 mins or so.

I recently had a heavy jack run across the extension cord I made to go to the garage and it tripped the circuit breaker in the house. so I went out and bought new 10-2 solid copper wire the same as I had before only this time I bought the stuff with the bendable metal covering.

New Ingersoll Rand 5 HP x 60 gallon air compressor
New Ingersoll Rand 5 HP x 60-gallon air compressor

I also changed the plug end to the bigger 3 prong as I just got a welder for Christmas so since I had to change the wire because of the miss-hap with the wire being ripped apart so I change the receptacle to the 3 prong that would accommodate my welder as well.

now the compressor will run thru a cycle to fill the tank with no problem but when the next cycle gets ready to start the motor will hum and then the safety switch kicks,

now me being some what a diy I checked all the wires and im getting 110 at each prong from the house and 220 touching both wires on my multimeter, so I did some looking inside the motor and realized it had no ground wire from the motor to the pressure switch so I did that thinking that would help but its still doing same thing.

I use the compressor to work on cars so using the air ratchet uses more air and the compressor goes on and off frequently.

I’m really no pro at this 220 motor but it has worked fine for over 2 years and now its giving me the flux its on a 30 amp circuit breaker but it never gave me this problem before

please help as I’m really unable to go back to wrenching by hand if you need other info please let me know and I will provide also the side of the motor does get hot but not so hot that it will burn you if you touch it.

Response

James, please see the troubleshooting page on this site about how to check the compressor motor capacitors, and check those on your compressor.

From what you’ve written I think this is a good first step, and failing caps may be the issue.

Please add a comment here letting us know if the caps were weak or failed will you?


If you have any questions about your air compressor humming, please leave a comment below, with a photo if applicable, so that someone can help you!