Question about Dewalt 2 Hp 4 GAL Air Compressor

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Compressor motor keeps running and the tank relief valve opens and pressure is released. why will the motor not stop when pressure reaches 150psi

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Hi,
The pressure switch has gone bad....there is a switch that is to stop it and then restart it on pressure...either the switch is bad or the tube is plugged...going to the switch.

heatman101

Posted on Sep 14, 2010

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1 Answer

Leaking around Pressure Switch. Ordered & replaced switch but still leaking


Some pressure switches have a valve that reliefs the compressor head pressure when the motor stops and closes the head pressure relief valve when the motor runs.
There is a check valve that keeps air pressure from coming back to the compressor head and the relief valve.
I think the check valve is leaking tank pressure to the relief valve and compressor head.
The check valve is where the pressure switch mounts on to and it mounts threaded on to the compressor tank.
Try replacing the check valve

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The over-pressure relief valve may be defective if the valve is opening before the desired tank pressure is reached. If not, then the shut-off pressure adjustment (if there is one) may be set wrong. You'll need to find out from the manufacturer exactly what static tank pressure the compressor is designed to provide and then determine if it is working correctly. If so, then the relief valve might be defective. DO NOT attempt to defeat the relief valve under any circumstances - it can result in compressor damage or a dangerous tank failure.

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That is the compressor's holding capacity 150psi. Therefore, the cut-out pressure has been reached. The safety valve opens to relieve the excessive psi. Compressor should come on at 120 psi.

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Check your Safety Valve. It is on the back of the tank sometimes on one of the pressure switch ports or on the air manifold. It has a ring on it to pull to relieve pressure from the tank. The spring in these can get overheated so they don't hold air unless the tank gets over 165 psi. When the motor is running hold the center pin in to see if you have the same results. If the valve holds air until you let it go, replace it.

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I have a C2006 Type 4 Porter Cable pancake compressor which will build pressure all the way up to 150psi then cut off as it should but there will be a loud hissing and pressure drop until the cut on...


first you have a reciprocating pump and the hissing you ear probably comes from the unloading valve on the pressure switch(it will look like a bike tire valve)if there is air coming out the valve when the compressor is off it probably means you have a bad check valve.This valve is use to unload the pressure inside the pump for an easy start .a motor always drive more amps on start up if you cant unload this pressure you will trip the overload(you probably have a reset button on your motor)check the specs on your pressure relief valve ,its a small pump maybe its just adjusted to high.in this case your pressure relief valve will open .If the pressure is fine and there is air coming out the relief valve then you have to replace it!to adjust your pressure turn the screw on your pressure switch clockwise to increase and counterclockwise to decrease .usually you have a 30 psi between cut-in and cut-off called the pressure differential.It can be adjusted on some pressure switch.be sure you put some teflon tape on the treads if you replace something to avoid air leaks.hopes this help!

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1 Answer

My compressor will start and run well but when it turns off it will turn but not run stalls and kicks the breaker


It sounds like the small valve that releases the pressure out of the main pipe that runs between the compressor and tank is not working once the tank is full and the motor automatically cuts off. You know this valve is working by the sound of a short spurt if air being released when the motor cuts off. Releasing this pressure means that the motor is able to restart easily because there is no load. If the valve is not releasing this air, the compressor is trying to start under full load which the motor is not capable of doing, hence the humming and popping of breakers due to a current overload as the motor tries to start. Motor burnout is possible if it keeps recurring.
This small release valve is attached to the Pressure control switch that switches the motor On & Off and usually has a small pipe going to it or is incorporated inside the Pressure Control Switch.

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Likely the check valve is bad or has contamination on the seat. The result is the pressure will bleed back out the compression relief valve which is often as part of the pressure switch.

If the safety valve is the one that is bleeding, then the pressure is set too high at the pressure switch.

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That is what the relief valve is set at, you need to
either increase the pressure on the relief valve or
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1 Answer

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Is the air "leaking" from the pressure relief valve? If so it is either faulty or it is doing what is supposed to do. The pressure relief valve is there in case the pressure switch fails.The tank could blow up if the pressure switch fails, so the pressure relief valve is there as a secondary measure to insure the tank does not receive too much air pressure. Assuming your gauge is operating properly, watch the gauge to see if it reaches the maximum pressure (check your manual to find the maximum PSI) or if it goes over the maximum pressure. If it goes over the maximum pressure then replace the pressure switch. If it the motor stops at the maximum pressure, replace the pressure relief valve.

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Once compressor reaches 125 lbs. pressure, the relief valve opens and releases pressure. the motor then tries to engage but cannot come to full speed because of back pressure I am trying to find and...


Generally, the safety/relief is set to 140 to 160psi. The pressure setting is stamped on the side of the valve. Replace the valve if it will not hold stamped pressure(cost about $6 at Lowes Lumber).The pressure switch is usually set at the factory to 120 - 130 psi. The unloader part of the pressure switch will not release backpressure unless it reaches set cut-off. The 1/4 inch line attached to the pressure switch connects to air discharge tube at the intank checkvalve (brass valve). The safety valve should very rarely release pressure, only when pressure switch malfunctions and allows pressure to reach up to 160psi. The tank will hold over 300psi so dont worry about the 160psi safety. I am not sure about your exact model number or brand, however most small compressors work the same and share similar parts. For manuals to other similar compessors to to www.dewalt.com. Here you will find manuals to dewalt and porter cable compressors. I prefer to use generic replacement parts from www.grainger.com or from Lowes Lumber. These parts will cost less than factory replacement. If you need to replace the pressure switch, use SquareD brand from grainger. Watch fitting sizes and pressure range. Good Luck on your repair.

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