by Joe
(Virginia)

Hi, and thanks for your site – full of valuable information!

I have an elderly Sears compressor that more than 50% of the time can not overcome the residual head pressure when the pressure switch signals for more air. If I am there, I can pull the ring on the control valve to drop the pressure so the motor can overcome the pressure, if not, it blows the breaker.

Having reviewed your trouble-shooting tips I am stumped but have a few questions. Looking at the attached picture, I do not see a check valve between the head and the tank. I also do not see any other line from the head that would be an unloader valve. There is a plugged port on top of the head. Also, the pressure control valve does not have a second connection – just power in and out and the male adapter to the manifold/tank.

Is this unit designed to operate based on the toqure of the motor? Should it have a check valve, or does it and I just can’t find it? Should I install a Square-D control valve that includes an unloader valve and plumb it to the head? If I do that, I bet I’ll need a check valve, or a new comtressor…

Thanks!

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Nice photo Joe. Thanks. What I can’t see is where the manifold is connected to.

In order for your compressor head to unload after the tank reaches cut out pressure, and if it doesn’t unload then you will get the symptoms you describe, there MUST be a way for the air trapped over the piston to exhaust to atmosphere when the compressor stops. Have you taken the line off from the compressor head to the tank to be sure that there is no check valve, in particular, where the line enters the tank?

On your pressure switch, was there no “toggle” or “lever” (not the on off or run lever) that interfaced with an air line?

Since these eyes of mine don’t work so good anymore, can you tell me what the “hook” looking device downstream from the empty pressure switch port is?

Maybe older compressors worked on some sort of torque situation as you suggest, but it’s news to me.

If everything on the compressor works well and the cost to retrofit it with a new pressure switch, unloader valve and check valve at the tank makes economic sense, that makes sense to me.

Have a look at the tank size and the motor HP and see what a new one of the same specs would cost first though.

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My old Sears compressor
by: Anonymous

Hi Bill, thanks for getting back to me.

In the mean time, I found and tested the check valve. It is at the base of the fill tube in the tank.

I also bought a new pressure switch with an unloader valve that I plumbed to the port in the head ($20 including shipping). I am sad to report that though the reduced pressure helps the motor on startup, it quickly gets bogged down and stops. Only way I can get it to work is to back the low pressure cut in down to ~50 psi. Me thinks the motor may be getting tired.

The hook you asked about is just the wire to the motor. The manifold connects to the tank by a tube between the pressure switch and the safety valve.

Thanks Bill!

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Joe, the photo link was broken due to system change. Please provide additional photo(s) as a comment and they will be added to the page. Thanks.