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Unplug the compressor. Drain the tank down to zero. Close the drain.
Get down close to the pressure switch and have someone else plug the
compressor back in while you are closely monitoring the pressure switch.
Do you hear anything happen? Do you see any movement in the pressure
plugging it in with the tank at zero pressure, you should hear a click
as the pressure switch trips due to low pressure. It should trip and
send power to the electric motor. If this doesn't happen, you may have a
pressure switch problem.
check the pressure switch should be a little black cover with 1 screw holding it on it works the same as a water tank pressure switch contacts closed ON contacts open OFF or the pressure relief valve may be bad it is usually a couple off inches long and about the size of a quarter if it is open the compressor will not be able to build enough pressure too open the pressure switch
The right guage is the actual tank pressure. The left guage is the pressure for the air hose. If you turn the adjustment dial the air pressure guage on the left will go up or down. You should be able to go to the dewalt website and find an owners manual for this compressor. I personally own this compressor and it is a good one.
Check for leaks of course... THEN check the check valve AND the Compression Relief valve which is OFTEN cintained in teh pressure switch. This valve bleeds the compression left in the head so the thing can restart without a pressure load. This is a small pintle that closes when the pressure switch starts the compressor. The check valve is the other thing and it is often a part of either the head or the pressure line to the tank at the tank.
The fact that your compressor completes a cycle indicates that the motor and pump are probably working correctly, however when the pressure switch cycles off there should be a slight release of the air that remains in the airline. If this air is not released thru the unloader of the pressure switch the motor cannot overcome the back-pressure in the airline and just hum. There should be a 1/4 in. hose extending from the pressure switch to the checkvalve in the tank. There is a small tab and needle valve that releases the air. Also check that the tank checkvalve is not leaking air back into the airline as this would cause back pressure. Finally, failing motor start capacitor will cause motor to hum. Weak capacitor will start motor when cold and no pressure in tank, but when things heat-up and with pressure in tank, motor may not have enough power to overcome load and hum. Enjoy
Double check that the drain valve is fully closed on the bottom of the tank. Once it reached 100 PSI, did it stay pressurized or did the pressure drop? Did you run the initial break-in procedure described in the manual? Do you hear any air leaks?