Failure to start – air compressor.

Home Air Compressor
(Last Updated On: August 3, 2020)

I have a problem with my compressor that sometimes fail to start when the pressure drops.

Usually when the motor is running and pressure builds up in the tank, once it reaches pressure the motor switches off with a shhh sound.

It now sometimes seems that this particular actions doesn’t take place, i.e the compressor reaches pressure, and the motor stops, but the pressure doesn’t get released. What happens then, is that the next time the motor starts, it tries to start against compression, which then draws to may amps and makes the circuit breaker trip. This happens about 3-4 times per day. This is what i have been told by a technician, who replaced the pressure release switch. This initially improved the problem in so far that it only happened about once a week or so, but of late it is happening more and more, to my frustration of course. If i then turn the motor/flywheel by hand a few times it helps and the motor then starts again turning the head.

Your advice on possible causes, other than the pressure release switch, will be appreciated. In other words, where can i start investigating to try and recify this niggle.

Bill answers…

Hello J Swanepoel:

Nice to hear from S.A.

Your description of the operation of an unloader valve is spot on.

And what you have described in your diagnosis is indicating to me that you clearly have an unloader valve problem.

Most of the modern pressure switches come complete with the unloader valve built in.  It’s the pressure switch, reaching the high pressure cut-off and the flipping of the contact to the motor that physically toggles the unloader valve to dump the air over the cylinder head.

If the pressure switch is shutting off the motor at high pressure, and you aren’t hearing that characteristic “pshhhhhht” as the unloader dumps that air, then you still have air trapped.

And if you have air trapped over the piston, and it’s not exiting through the unloader valve when the compressor shuts off, then you have an unloader valve problem.

Since the unloader valve is integral with the pressure switch, you need to change the pressure switch.

Tell your mechanic to replace the pressure switch, and maybe this time, to use a new one??? 🙂



Hello, I am Bill, the Compressed-Air-Man. I have years of experience in industrial and residential compressed air applications, air compressors and general pneumatics. I created this site to help professionals, students, and DIYers understand and properly implement and maintain compress3ed air systems.


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