Speedaire 3Z421 compressor keeps running

by John Young
(San Pedro, California)

The compressor pressure relief valve (PRV) pops, makes loud noise (scares the _$#%* out of you) does not hold pressure and compressor starts cycle again? is this a matter of simply repacing the PRV? or is the pressure control valve not working? Is there a Schematic of this compressor?

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May 10, 2017
Which pressure switch
by: Bill

John, I haven't been able to find a photo of the pressure switch for a 3Z421 on line, so I cannot recommend one.

Please read this page: https://www.about-air-compressors.com/pressure-switch.html

It will help you understand how they work, and to define which switch will help.

If you Google compressor pressure switches, you will find many. Get one that has the same pressure range, voltage, and mounting configuration as the old one.

May 09, 2017
Pressure switch
by: John Young

THanks Bill! I think the I will try to replace the pressure switch it makes the most sense. Can you direct me to the correct part number?


May 08, 2017
Compressor keeps running
by: Bill

John, are you saying that the pressure in the tank rises past the normal cut out pressure setting, and the compressor runs until the PRV lets go?

That being the case, this is typically caused by the pressure switch not tripping off at the correct pressure, and replacing the pressure switch should fix it.

When you say it does not hold pressure, if the PRV lets go, then the air will bleed down until the PRV piston is reset by pushing in on the end of it.

If this is not what you meant, please add a comment.

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