Air Compressor won't start unless empty

[ps2cho]

Hey guys

I have a 5hp 27 gallon Coleman/Sanborn Powermate air compressor and it just spins then dies if I try to turn it on with air in the compressor.

It's never done this before just started today. If I let all the air out it'll start fine, but anything more than 30psi and it spins for about a second and doesn't catch and stops. I turn it off before it pops the breaker.

Plugged directly into the wall and still no luck.

What could be the problem?

 

[dnschmidt]

Most likely the unloader. This is the thing that hisses once the compressor has reached it's maximum pressure and shuts off. If you don't here this hiss (which is the air bleeding from a tube that's pressurized) the motor will not start as it believes it would have to pump against full head pressure.

 

[Milton Shaw]

It should not be trying to run at the 300psi you told about. Did you mean 30 psi. 300 is the pressure the tank blows up at if it is a really good tank most won'g to 200 psi before they explode.

 

[larry_g]

Unloader valve, check valve , or motor capacitor would be my first checks. Pump the thing up to cutoff, unplug, and then loosen the discharge pipe from the pump head. It should have no air pressure. If it puffs out a fair amount and then quits your unloader valve is not working. If a constant flow then the check valve is leaking. If no air then check the motor cap/ start circuit.

lg
no neat sigline

 

[454ragtop]

I had a portable compressor with this problem once, it was a check valve called a Load Genie, screwed into the tank right where the discharge tube from the comp. head goes in. Got a new one from Grainger, all was well.

 

[kunkernator]

I agree with Larry_g, most likely an unloader valve or check valve. The motor wont beable to create enough momentum to turn the pump if there is pressure on the line. I recently replaced my unloader valve for a similar issue, cheap, easy, $15 repair. There is also a check valve on the tank that would allow air back to the compressor head if it is stuck open, resulting in the same failure.

 

[Fixin'Stuff]

I have an old Sanborn also. Many years ago the check valve somehow managed to get cocked sideways in its bore. I released all the pressure from the tank and then removed the check valve. I cleaned it up, put it back together and it's been working ever since. Cost = $0

 

[sld961]

As was mentioned, I would check these I this order: unloader valve, check valve, start capacitor, start windings/switch inside the motor.

 

[ps2cho]

It should not be trying to run at the 300psi you told about. Did you mean 30 psi. 300 is the pressure the tank blows up at if it is a really good tank most won'g to 200 psi before they explode.

Fixed it yes 30psi.

Thanks for suggestions I'll look into them.

 

[taumac]

If a portable unit his unloader valve might be part of his pressure switch.

OP does your compressor build pressure up to it cuts off? If does what happens with it builds pressure and drops down below say 90 to 100 psi? Does it try to kick on and build pressure?

 

[Jmonnty]

Can you post a model number on the compressor?

 

[ps2cho]

Bumping thread as I now am getting around to needing this fixed as I have to do some valve stem seals and need constant air to keep the valves in place.

I disconnected the silver pipe that goes to the top of the cyclinder head and got zero air out of it at all. Does that mean the check valve is good and it is likely this unloader valve?

To the question 2 posts up -- yes once its empty, I start it and it does fill all the way to 120psi then cuts off.

If its the unloader valve, I cant find much on this compressor...will any generic one work off eBay?
EDIT: Will this unloader fit? It "looks" identical to the one on mine...Is this "style" pretty much the same across the board? http://www.ebay.com/itm/CW001000AV-...867334?hash=item3cbe3f7446:g:M8sAAOxypthRxNZ7


Photos:

 

[ps2cho]

wanted to update unloader valve fixed it. $13 on ebay. Thanks guys. It now makes a significant hiss when I turn it off. I know for a fact it was not quite as pronounced before, more of a whimpering hiss than the strong rush of air it does now.

 

[bsaint]

Glad you got it!

Thankfully my compressor doesn't need them. It holds open the inlet valves until it gets to speed.