Air compressor switch shutoff problem

I just purchased a switch to repair my old sears 1.5hp compressor.
The original problem I had was there was a leak in the diaphram as
well as once the topend shutoff was reached (at 150psi) it would leak
air rapidly. If I interrupted the power before it reached shutoff it
would only leak from the diaphram and not have the rapid leak. I
requested a new part from sears and installed it. This solved one
problem, the diaphram no longer leaks, but I have kept the problem
where once it reaches shutoff (at 150psi) it leaks until the motor
fires again. If I interrupt the power while it is leaking it will
eventually stop leaking around 135 psi. Do I have a 2nd bad switch?
Have I not connected things correctly?
A friend of mine gave me this air compressor for free. I know that
generally things are worth what you pay for them but that did not
deter me. I have a small hot-dog air compressor and this is a pretty
substantial move up in compressors. The switch is new from Square-D
although I purchased it for double its cost from sears (hindsight gets
me every time). I noticed that the power cord from the wall is not
grounded and that the new switch has a small screw for that ground.
In other reading I have learned that the rapid leak from the switch
after shutoff is expected. This leak releives pressure but should
stop immediately. Generally most of the operation of the switch is a
mystery to me, if anyone cares to explain how the switch operates at a
level not requiring electrical engineering training I would enjoy the
Thanks for your thoughts!
Reply to
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Are you sure it's not the pressure relief valve bleeding the air ? Should be one on the block the pressure switch screws into , and they *do* go bad . You might also consider lowering the high limit on the new switch , that PR valve is probably set at around 140 psi . They function with an overcenter spring that is preloaded by the spring under the adjustment screw . The air pressure turns it off until the pressure drops enough for the spring to push it back overcenter to make the circuit again . The tighter the screw/spring , the more pressure required to shut it off . I hope that makes sense ...
Reply to
Terry Coombs
Many air pressure switches have a relieving valve in them. This is to bleed the air pressure out of the compressor, allowing an easier start. This requires a check valve between the compressor and tank, and the switch is connected to this line. When the switch shuts the compressor off, there should be a short burst of air released, and then it stops. It sounds like this is the correct type of switch for this compressor, but your check valve is not closing off above 135 PSI.
Reply to
Jon Elson
I think I would check and clean the check valve going down into the tank from the compressor head - it may be allowing the air pressure to go back into the "head line" and over to your cutout switch where it would bleed off the pressure. Take the check valve out of the tank and clean it thoroughly and make sure it is sealing - if not, replace it. Ken.
Reply to
Ken Sterling
Is the "check valve" possibly referred to as a "pressure plate"? I am looking at a diagram of my compressor and there is a large tube running from the head into the manifold which I am sure is the main supply of air to the tank. In line in this large tube is a "pressure plate" with a spring on top of it with a retaining ring and seat on either side.
What you are describing seems to be the problem I am having.
on another note, another gentleman suggested that I lower the overall pressure which I would like to try, but the square d switch has the adjusting bolt welded so I cannot adjust it. I may just purchase another switch if all else fails as I have no interest in running at 150 psi.
Thanks again for your thought!
Reply to
The check valve only allows the compressed air to flow one direction - into the tank. It can be located (probably) just about anywhere, but is usually a fitting going down into the tank - onto which the main piping from the cylinder head threads. If you have a manifold (which I personally have no experience), it is possible that the check valve is located at the input of the manifold. Take it apart - check the seats and seals and springs and o-rings or whatever. Make sure that it moves freely (under spring pressure) and that it would only allow air flow one direction (into the manifold/tank) and then reassemble and test. Ken.
Reply to
Ken Sterling

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