Compressor keeps running until the PRV pops?

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I have a condor md 21 compressor switch on my compressor. The compressor runs great but the compressor keeps running until the PRV pops. The pressure lowers and the compressor just keeps on pumping. For some reason it will not cut out.

I have taken apart the switch and to my surprise there is NO adjustment inside. There are two posts with springs at the top but the posts, have had green glue from the manufacturer on them so they cannot be adjusted. Can you help? Do I need a new switch? If so, how do I know which one I need? Thanks, Mike.

MDR 21 Condor Pressure switch


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Hey Mike, though the pressure switch has no easy adjustment on your compressor, and many do not, that it lacks adjustment isn’t the problem.

If the compressor runs until the PRV lets go, and that PRV cracking pressure is about where it should be, and higher than the normal cut out pressure of the pressure switch, then we believe it’s the switch that’s the problem. We suspect the points are welded shut and the switch cannot trip off, or, the diaphragm inside the switch that reacts to the changing pressure has failed, and the switch cannot trip off even when the pressure passes the normal cut out pressure.

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Ok. So you are thinking the switch itself its bad and needs to be replaced? This was my thought too but i wasnt sure if my cut in and cut out setting were right. But apparently they cannot be adjusted. So where do i go about finding a new switch?,  Mike.

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No shortage of suppliers for pressure switches on line, Mike. Use your browser to search for the same model and part number, or simply browse for compressor pressure switches. As noted on the replacing the pressure switch pages on this site, there are criteria in selecting a new… essentially the new must mirror the old in a number of ways, all covered on those pages. Please navigate to that page if you feel it will help, otherwise, Google on, and you’ll find lots of switches including an exact replacement of yours.

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Tina Matson
Tina Matson
August 25, 2021 6:23 pm

I’ve replaced my switch on my 175 psi McGraw air compressor but when it hits 150 psi it blows open the release valve and keeps running?

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Tina Matson
August 25, 2021 7:39 pm

If the compressor is set for 175 PSI cutout as norm, a new switch shouldn’t have affected the PRV… therefore, it shouldn’t be opening at 150 PSI if it wasn’t doing so before the switch change and suggests the PRV has either failed or has oil/debris in it that is allowing the piston/spring to open early.

Try rinsing the PRV in thinner, let it dry, cycle it a few times by pulling and releasind the ring, install, and test. If it works, good. If not, next step is to try a new PRV that is rated for 180 PSI or so.