Air compressor not unloading

replace tank check valve first if needed unloader valve.

that's a big unit. Reach out to Atlas Copco technical department direct.

http://originair.atlascopco.com/atlascopconew/contact.aspx

http://originair.atlascopco.com/atlascopconew/industrial/compressors/atlas-copco-ga-45-ff/75ef5d89-fc23-4eca-8c0c-d5a823922470.html

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I just rebuilt my Emglo AM78 4CHV, new pressure switch, unloaded, check valve, aftercooler, and changed the #30 oil, It takes 4oz.
Runs like new after not cutting in/out correctly and purging air before reaching 125 psi. Kicked it down to 115 max psi via main spring setting. Don't need 125 for trim nailing and siding.
FYI don't use cheaper generic 125 psi pressure switch from Amazon!!
use Jenny M4-1247 (140-1074-01)

John: I had a similar problem with a brand new CH-made compressor. The problem was not with the unloader valve, but with the check valve located where the air from the pump enters the tank. The check valve was not seating correctly due to brass filings from manufacture. The defective check valve was allowing air to escape back the wrong way from the tank to the pressure switch and through the unloader valve. This backflow from the tank would start to happen when the pump stopped after reaching high pressure. The back flow would continue until the pump restarted at the low pressure point. This cycling would continue as you described. Once I cleaned the check valve, everything (including the unloader valve) started working correctly. I hope this helps. And remember, don't work on these when they are under pressure and always wear safety glasses.

This unit will shut down if unloaded for a set period of time. Usually 5 to 10 minutes. This time could be longer if some one has changed it from the factory setting. There is a timer in the control box that starts counting down when the unit unloads. If it stays unloaded long enough for the timer to count down the unit will stop until the air pressure drops to the low set point on the pressure switch. It will then start up. If the unit loads before the counter counts down the timer starts from the beginning when it unloads again. This cycle continues until you manually shut it down or you stop using air long enough for the timer to reach Zero and shut down.

If the unloader valve doesn't release the pressure from the compressor it will be hard to start. This is because there will will be pressure inside the compresser. When the tank is full and the compressor shuts off, you should hear a rush of air which is the unloader valve releasing pressure. The unloader is like a one way valve.

When a compressor reaches set pressure, there should be a release of air from discharge hose thru the unloader. This release of air unloads the compressor head pressure. The unloader is usually located within the pressure switch. Easy to check. Follow discharge hose from compressor head to tank. There should be a brass/steel checkvalve at the tank. The checkvalve allows air to enter tank, however not allow air to be released back into discharge hose. If this valve is leaking, air will be released from the unloader constantly. Follow 1/4" tubing from checkvalve to unloader and listen for leaks. Repair as needed. If air is not leaking from checkvalve then check unloader mechanism with-in pressure switch. Parts are available at www.toolpartsdirect.com Good luck

There is a line running from the pump assembly to the tank. At the tank the compressed air passes a check valve. This valve prevents the air from escaping back along the same line. After the tank reaches max psi the motor shuts off. What is undoubtedly happening or rather not happening with your compressor is the unloader valve is not functioning properly.

The unloader valve is what bleeds off the pressure that remain in the compression chamber after the motor shut off. If the unloader valve fails to bleed the excess psi off then the piston has to work against your minimum psi. or around 80psi.

This is your large load.
The unloader valve is comprised of a mechanical trigger at the switch and a small air line that runs from the switch back to the tank check valve or up at the pump manifold.

Often it is enough to clean the small mechanism and make sure all the small parts are free from corrosion or damage.
However you may need to replace it.
Hope this helps.

Are you talking about the power switch or the trigger of an air tool?
If it is the power switch then how much air are you losing?
At start up most air compressors will lose a small amount of air through the unloader valve. As the compressor builds psi this valve will close. Once the max psi is reached the motor will shut off then a few seconds later the unloader valve will open again releasing the pressure in the cylinder. If your leak does not abate as psi builds you might need to replace your switch and valve.

I have worked on many makita compressors and have run-across this problem on a few of them. Your compressor has a check valve that is supposed to hold tank pressure after the switch reaches set pressure. There is an unloader valve built into the pressure switch that releases the pressure in the discharge hose after the unit shuts off. The unloader allows the compressor to restart with-out any pressure in the discharge line. If the brass check valve is leaking pressure back into the system, air will leak out of the unloader valve. Follow the discharge line to the tank and you will see the brass check valve. To test valve for fault, run the compressor till it builds around 80 psi and unplug the unit from the wall outlet. Remove the 1/4" hose connector from the check valve and check for air leaking out of the valve. If it is leaking you can order form www.toolpartsdirect.com. or call the makita 1-800 number. The price is about the same. Good Luck on your repair.