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How to Fix an Air Compressor

A faulty air compressor unloader valve can cause the compressor to stall out, but replacement valves are readily available and the fix is easy and inexpensive.

By the DIY experts of The Family Handyman Magazine

How to Fix an Air Compressor

A faulty air compressor unloader valve can cause the compressor to stall out, but replacement valves are readily available and the fix is easy and inexpensive.

Replacing the unloader valve

If your compressor starts up fine on the first start of the day, but stalls and makes a loud humming sound on subsequent tries, you've got a bum “unloader” valve. The unloader valve is what makes the “psssssst” sound when the motor shuts off after reaching operating pressure. It's actually venting the residual pressure from the compressor head so the motor doesn't have to work as hard on the next startup.

The unloader valve is usually built into the pressure switch assembly. Contact the compressor manufacturer to buy a new assembly. Or, remove the cover from the switch (unplug the compressor first) and locate the pressure switch part number. Then do an Internet search for that part. I found a complete four-port pressure switch assembly replacement for this compressor for $20 (at amazon.com). Here's how to replace it.

Start by unplugging the compressor. Then open the drain valve at the bottom of the tank to depressurize it. Next, remove the 1/4-in. line going to the unloader valve (Photo 1). Then use a slip-joint pliers or small pipe wrench to remove the quick-connect fitting, pressure regulator and gauge from the pressure switch.

Disconnect the wires from the pressure switch (Photo 2). Then remove the old switch. Coat the pipe threads with pipe dope and install the new switch (Photo 3). Finally, apply pipe dope to all the plumbing parts and reassemble.

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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

    • 4-in-1 screwdriver
    • Adjustable wrench
    • Slip joint pliers
    • Wrench set

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.

    • Unloader valve (usually part of the pressure switch assembly)
    • Pipe dope

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