# Trouble starting when air in tank



## ken (Jul 12, 2020)

I have a small Cambpell Hausfeld oilless compressor that suddenly developed a starting up issue. when I first turn it on it starts up fine and fills the tank to the set capacity. my problem occurs when it needs to restart to replace the air I used. at this time the motor just hums and will not engage the pump. If I let some air out of the tank it will start up like it should and build up to the set capacity of air and turn off. Any assistance would be greatly appreciated.


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## motormonkey (Jul 10, 2016)

It sounds like the unloader is not working, and the compressor is trying to start under a full head of pressure. There is a check valve in the air line between the air tank and the compressor output to prevent air in the tank from flowing back into the compressor output line. The unloader is between the check valve and the compressor, and when the compressor cuts off, the unloader opens and releases the air on the output line of the compressor. When the compressor restarts, the unloader recloses, but this gives the compressor time to get up to speed before it builds a full head of pressure on its output line. The compressor doesn't have to start under a full load. You should hear air momentarily hissing out of the unloader when the compressor kicks off.


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## iowagold (Jan 22, 2020)

the check valve could be stuck open with trash or rust as well.

what is the model of the compressor?


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## stevon (Mar 5, 2019)

I concur, sounds like a check valve or start capacitor/switch issue

Stephen


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