The compressor won’t re-start when air is used. Not an unusual air compressor problem and often relating to the malfunctioning of the unloader valve.
How does the unloader valve affect the compressor re-start? If you take a moment to read this page describing what the unloader valve is and how it works. you will be well on your way to understanding that.
We won’t get into the details of the unloader valve operation since it’s well covered on the page linked above.
Air is trapped over piston
When the unloader valve fails, however, air will be trapped over the piston or pistons on the air compressor.
That adds load to the drive train. Smaller air compressors operate with a very thin line between motor size and the power needed to drive the compressor pump. Adding even a little bit of extra load to the compressor startup process means that sometimes the motor cannot overcome that added load, and cannot re-start.
Compressor Won’t Re-start When Air Is Used
You use some air working with your air tool, the pressure in the tank drops, and eventually the pressure reaches the cut in setting, the pressure switch trips, and the compressor motor gets power through the pressure switch to start running, and pump more air into the tank. But the motor cannot start due to the extra load from the air trapped over the piston, since the unloader valve did not do its job.
When the air compressor motor cannot start it pulls more amps.
You may hear humming coming from the air compressor motor if it tries to start and cannot.
The lights in the house – at least those on the same circuit – may dim as the compressor motor tries to start.
If the motor tries to start for long enough and still cannot get going, the motor will overheat and shut down on a thermal cut out switch, if it comes equipped with one.
What to do when the compressor will not restart
Test the unloader. How?
Pull the power cord or switch off the compressor. Drain all the air from the compressor tank.
Identify your compressor unloader valve.
Power up the compressor. If the unloader valve was the cause of it not starting, then the compressor should now start.
As it runs up to the cut out pressure setting and stops, monitor the unloader valve to ensure that it is working. A visitor wrote in and said “how do you monitor the unloader valve?” While that is covered in detail here: page describing what the unloader valve is which is a link earlier on this page, to recap the info on that detailed unloader valve page, monitoring the unloader means to listen near the unloader to ensure that when the compressor reaches the cut out pressure, the unloader dumps air. That means that there should be a very short venting noise, and if a hand is held near the unloader, a short blast of venting air should be felt.
If the unloader is not venting air when the compressor reaches cut out pressure, remove and clean or replace it.
Replacing it may mean that you have to replace the pressure switch if the unloader valve is an integral part of the switch.
Other reason why the air compressor will not start when air is used
There are a number of other issues that can cause the compressor not to restart when air is used. Most of these are covered in the pages linked from the list on the troubleshooting page on this site.
If you cannot determine the reason from the existing pages, leave a comment here providing compressor make and model, and complete details of the problem.