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 compresser 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... ?
Regards...Fred

fred@bayview.be






Bill says...

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.

Cheers,

Bill

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Oct 03, 2016
Changed unloader valve
by: Bill

Hello Lan... you've changed the pressure switch and the integral unloader valve, correct?

Have you monitored the compressor operation to confirm that the unloader valve actually dumps air when the compressor stops at the cut out pressure setting?

As to this "Could it be the check valve between the unloader valve and the cylinder head?", I don't know the make or model of your compressor. But, certainly, if there is a check valve in the line to the unloader valve that would be suspect.

Normally the check valve is in the tank to keep air in there when the unloader opens to vent air over the piston and the line from the pump head, through the tank unloader valve fitting, is open to the unloader all the time.

Oct 01, 2016
Changed unloader valve, still stalling.
by: Lan

Hello,

I have this issue as well, where the motor will stall if air is in the tank. I changed the pressure switch/unloader valve. Same problem as before. Could it be the check valve between the unloader valve and the cylinder head? Thanks!

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motor starts or runs slowly

by johnnyh
(loudun, france)

hi,
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?

_______________________________
Bill says...

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.

Cheers,

Bill

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May 27, 2010
motor starts or runs slowly
by: johnnyh

hi bill and thanks for the reply, but with the heads removed surely there is no pneumatic load on the machine?

__________________
Sorry, I missed that you were running it with no heads, and it was still slow on start up.

An intermittent problem is a witch to sort out, isn't it.

Good luck.

Bill

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Motor stalls when re-loading

Hi,

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






Bill says...

Sounds like your unloader valve isn't.

Is it?

Cheers,

Bill

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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.

Thanks.

___________________
Bill says...

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.

Cheers,

Bill

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Aug 31, 2018
Won't start under load. Is it the check valve?
by: Mike McCabe

Hi Ive been battling my air compressor for about 6 months.

Its a 5hp 60 gallon bohemouth from the 70’s.

The problem is the motor wont start up under load.

I’ve replaced the check valve twice now (both times unloader was leaking) and I've torn the motor down and cleaned the centrifugal clutch,and replaced starting capacitor; only thing I haven’t done is replaced bearings in motor; also tore down pump and cleaned valves.

My question is am I possibly buying the wrong strength of check valve? I figure there has to be ones with stronger springs to withstand back pressure. if so could you please tell me where I could find the sizing information.

Also its a 240v motor and instead of running a service to my shop I bought a transformer. the motor is rated at 15 amps but i checked the amperage at startup under load and it was hitting 35! I know it should peak around 25 amps at startup any ideas will the excess draw could be coming from?

thanks

Aug 12, 2016
Is it the unloader
by: Anonymous

Yes but does the hiss mean that the unloader valve is definitely working?

Aug 12, 2016
Humming compressor
by: Bill

Andrew, this is covered in the troubleshooting pages.

Aug 11, 2016
Compressor just humms and trips breaker
by: Andrew Haywood

Hi Bill, I just bought a Coleman 40 gallon power mate portable compressor. I got it cheap because sometimes it doesn't want to start. I brought it home, took the cover off to turn the pulleys and they moved freely. So I plugged it in and turned it on and it started pumping no problem! It shut off and I heard that hiss of the unloaded valve for a second. Then after a few minutes I flipped the switch from on to off and then back on and this time the motor received power but was not turning and dimmed my lights until it flipped the breaker. Well, I reset the breaker and tried again and the same thing lights dimmed and power to motor but no pumping. So I turned it back off and turned the compressor wheel to see if it was free and it moved easily and then I tried to turn it on again and this time after I turned the compressor pulley it fired up and ran normally!
It is wired for 110 and I do hear my unloader valve hissing for a second when the compressor shuts off.

I am new to air compressors and need guidance please and thank you!
Andrew Haywood

Jun 13, 2010
How do I fixed the unloader valve.
by: KL

Where is the unloader valve, and how do I fixed it? My compressor is 2hp 8 gallon Harbor Freight Central Pneumatic.

Thanks,

____________________
The unloader valve is usually adjacent to the pressure switch, and it's a lever protruding out of the pressure switch that toggles the unloader valve open and closed as the pressure switch reacts to changing system pressure.

I've updated, with a photo, the unloader valve page.

As to fixing it, maybe you could tell me what it's doing or not doing?

Cheers,

Bill

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