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Old 07-02-2011, 04:00 PM
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Compressor hard start with ait in tank

I've got a small 110 Craftsman air compressor, 2HP 12 Gallon that was recently given to me. It starts and runs great when the tank is empty, however, when it kicks on again after the pressure drops, it the motor won't turn the compressor and usually kicks the breaker. Does this sound like a motor issue, or perhaps something in the compressor?

Thanks
John

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Old 07-02-2011, 04:24 PM
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Sounds like your unloader valve isn't working. There should be a check valve at the tank where the line from the pump attaches. In the line, there should be a tube attached which usually runs to the pressure switch. When the switch cuts the power, an finger on the switch hits the unloader valve, dumping the pressure in the pump and line. The check valve prevents air from the tank escaping. The unloader valve assures there is no pressure on the pump when it starts again, making it easier to start.

Listen to when the switch cuts off the power, you should hear a shhhhhh as the pressure is released. If not, you may need to adjust the valve or the finger so the pressure is released.
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Old 07-02-2011, 06:16 PM
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Exactly what was already said, it almost certainly is a leaking back-flow valve in the tank and/or an unloader valve. However I think some of the little direct drive units only have the back-flow valve in the tank and it may or may not have an unloader in the pressure switch, most likely it will so if you don't hear that momentary "hiss" when the motor shuts off you will need to check for that unloader.
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Old 07-03-2011, 06:28 PM
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Nope, now woosh of air when it turns off, just a slow steady leaking from this valve;



It looks like it pushes down on a switch that turns the compressor on and off. I disconnected the main line from the compressor to the tank and there was no other valve. It just feed straight into the tank. Should I start by replacing the valve pictured above?
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Old 07-03-2011, 07:29 PM
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That valve is from the pressure switch right? It is supposed to be opened by the switch when the switch opens the electrical contacts and it is supposed to stay open until the motor is switched back on again. If it is leaking continuously then that means the back-flow valve, where-ever it is located, is leaking. That back-flow valve is necessary to keep air from flowing back to the compressor pump and switch when the motor shuts off and that switch you have in the pic is then supposed to drain off any pressure between the pump and the tank, if the back-flow valve works properly the valve pictured will "hiss" for only a second or so (unless air is leaking back from the tank as yours apparently is). If there is no valve located in the tank where the supply line attaches then check to see if it is located at the pump where that same line is attached, it has to be somewhere.
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Old 07-03-2011, 08:08 PM
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There should be a little check valve like everyone said... I would get a lay out of your compressor to see where it is,, Mine was right in the top of the tank.... It was bad, And mine was doing the same thing,,, I put a new one, and it worked fine.... It stops the air from backing up to the pump head... The one you have there is the one by the on and off switch..(This was on my old compressor..)
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Old 07-03-2011, 08:23 PM
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continous run

I have a gasoline and elect continous run construction compressors. Construction sites usually don't have enough capacity or the air compressors can overlaod a generator when they start so the elect or gas engines run continously, then have an unloader that lets the thing run but not pump air when the pressure is high enough. I have had to take the pieces apart and clean them every couple of years from running in dirty sites. If you can find the unloader and use some carb cleaner then lube with air comp oil it should be ok.
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Old 07-03-2011, 08:38 PM
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Quote:
Originally Posted by timothale
use some carb cleaner then lube with air comp oil it should be ok.

That's what I have often found to work, most times the valve just needs cleaning or sometimes it has a chip of some type of material stuck in it.
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