Cannot start up my compressor when there is air in the tank.

by Fred Holmes
(UK – Hartlepool)

Hi There,

I have a cheap 45 litre tank compressor.
When i use the compressor at the start of the day , it will not pump and buzzes for a few seconds , and blows the thermal trip , i then reset it , and same again, it will always do this when there is air in the reservoir.
If I bleed the air out all is well, and the pump fires up , charges up the reservoir , and then cuts off at 110psi. when pressure drops off , it kicks in again , and all is well, I only have a problem at the beginning of the day ?
Do you think it is the pressure switch leaching back into the pump, but very slowly… ?


Hi Fred. When compressors have a hard time starting when there is pressure in the tank, I’d look to the unloader valve first.

Here’s info on how Unloader Valves work. Please check it out and let us know if this was the problem. K?

Not all unloader valves are the same. This one is off a Condor MD-11 compressor switch.

Compressor motor starts or runs slowly

by johnnyh
(loudun, france)

i have a single phase motor, direct drive compressor that often, though not always, starts slowly or runs slowly for several seconds. eventually it picks up and runs fine but i have gone so far as to take the heads off the cylinders to check it is not a stuck valve or something causing the slow start. it used to run fine but was left unused for several years and now does this. i have wired the motor (plus it’s start capacitor) directly without the pressure switch and it behaves the same so it’s not the switch or, i think (due to head removal) any part of the pneumatics.
could it be the motor itself or it’s capacitor?


Hello Johnny, it could be the motor, but I’m actually inclined to think that you might have extra load on startup, and that extra load may be caused by the compressor not unloading when it reaches cut out pressure and shuts off.

If you aren’t sure what Unloading is, please read the page on this site, and make sure that you are getting unload each time the compressor shuts off.

Compressor motor stalls when re-loading


Our compressor fills to cut-off pressure, motor goes to idle then when ready for re-load motor immediately stalls.
Any idea’s. Thanks!


Sounds like your unloader valve isn’t working properly

The link is information about the unloader. Could you have a look at the page, and ten tell us, Is yours working properly? If not, that’s the problem, and if it is, we need to know that too.

Air Compressor Stalls on startup

by KL
(Bay Area,CA)

My air compressor Central Pneumatic 2hp 8 gallon is having problem when it try to starts automatically.

The motor runs fine when its first started, and it builds pressure to its max cut off of 115psi. The problem occured when it tried to start automatically when the pressure goes low. When it attempts to start, the motor is having a hard time, it create this noisy popping sound like its going to die, and it will eventually die. It wont turn on if I turn on the switch. I found out that it will turn on again if I press the reset button on the side.

I was wondering if it has do with my garage’s electrical supply, if I have enough electricity to power the compressor.


KL, it might be the power supply, but I suspect from the symptoms it’s more than likely it’s your unloader valve that’s not working. The link is information about the unloader valve. Please check to make sure yours is working properly, then add a comment if you still need help.

Do you have a question about a Compressor struggling to restart? Please add it below. Upload photos of the compressor and problem areas on it too, please!