Compressor struggling to restart?

Published Categorized as Air Compressor Won't Restart 15 Comments on Compressor struggling to restart?

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

by Fred Holmes
(UK – Hartlepool)

Hi There,

I have a cheap 45-liter tank compressor.
When I use the compressor at the start of the day, it will not pump and buzz for a few seconds and blows the thermal reset button, 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 the 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, a 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 its 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 its capacitor?

Hello Johnny, it could be the motor, but I’m actually inclined to think that you might have an 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, the motor goes to idle then when ready for re-load motor immediately stalls.
Any ideas. 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 then 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 gallons is having a problem when it tries to start automatically.

The motor runs fine when it 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 creates this noisy popping sound like it’s going to die, and it will eventually die. It won’t 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 to 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.

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Hello, I have an A.C.P. (Jacksonville, Florida) air compressor that’s 30 years old with an 80 gallon tank, 2 of Challenge Air (FS Curtis) E-23 pumps, 2 of Baldor 1HP 3-Phase 240 volt motors, 2 of Furnas 69M B6Y Pressure (cutoff) Controls, 2 of Furnas 14BP32AA71 Non-reversing motor starters, and a Furnas 47AB10BG Alternator (Alternating Relay). When either of the pumps is working separately (other pump belts removed) both work fine. But when both are working together they work fine from 0 psi (to 100 psi cut off) but when starting up from 65 psi they both start and stop (turn… Read more »

Thanks for the equipment detail. I’ve not looked any of it up, yet.
I presume this has been working. If not please clarify.

For starters, verify your power is still good.
That is not just voltage on each leg, but motor direction.

I take it that normally both pumps run below some set point, and that they alternate at another higher set point with modest demand, correct?

When running one at a time they both work just fine. The wiring is (I think) the same as it has been for 30 years. Both pumps work to raise pressure to 100 psi where both cut off. As the pressure bleeds down 1 kicks back on at 60 psi and the other at 65 psi, but the alternator (I think) causes them to take turns. I don’t understand the proper function of the alternator very well. Coming on and cutting off is entirely controlled by the 2 Furnas 69M B6Y Pressure Controls (pressure cutoffs). Thank you.

OK. I’m not sure I’m clear on this, though: “When either of the pumps is working separately (other pump belts removed) both work fine. But when both are working together they work fine from 0 psi (to 100 psi cut off) but when starting up from 65 psi they both start and stop (turn a fraction of a rotation, stop, hesitate, try again) and the motor starters clack like a machine gun” Do they both try start and stop simultaneously, or one and then the other like a “pom-pom” gun? And this starts happening at 60, not 65psi? Have you checked… Read more »

Last edited 2 years ago by Doug in

I’ll have to look again more closely to be clear whether they’re simultaneous or alternating when they start up.
As far as checking the power, the voltage to both motor starters for 240 volts. What else should I check regarding power? Thank you.

You indicated the motors are 3 phase, so you want to verify that all three phases are at about the same voltage, either to neutral or across the phases.

Hi Doug, Recently I measured between the 3 phases and all voltages were the same, and the correct value (I wrote it down but can’t find it right away, I’ll post it when I find it). So this part looks OK. Thank you.


I have a 60 Gallon Kobalt Compressor that is struggling to start. Its makes a humming noise but it does not turn over to make the belt turn until I try a few times or we add some oil but thats also not working. What do you suggest.


I have a Champion R-15 pump. In colder weather the motor starts fine but the pump is very slow and the unloaded spits for a minute. Then she catches up and runs fine the rest of the day. I was thinking the unloaded, but it’s only when she starts up in colder weather.


You know the unloader is in that bump on the front, right? There isn’t supposed very much oil up there. I suspect there IS too much and/or it’s just too thick.
If you want, maybe just changing the oil would help, or you can try removing the unloader and cleaning the gunk out of it. Be careful – things can go flying and hard to find.


No need to shout… and if you look at this page, I suspect if you scroll down to the section about compressors that won’t start, you’ll find the answer. Hint, it’s likely not the pressure switch.

My model 67501 is about 10 years old. It will not start even after draining tank.. Does not run unless re-set button is pushed then it runs for 2 or 3 seconds and shuts down. I replaced cold start valve and no change. Plugged directly into wall outlet, no extension cord.

Hey Mike… thanks for including what and how you’ve checked. I suspect you’ve already drilled down to the problem as being electrical, and it’s either the motor start capacitor, the reset switch itself, or the motor as being the problem I think. Maybe start by checking the capacitors. Here’s how: