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
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Replacing the unloader valve
Photo 1: Disconnect the unloader valve
Slap two open end wrenches on the compression fitting and loosen the nut on the air line. Pull it out of the fitting.
Photo 2: Remove the wiring
Disconnect the hot, neutral and ground wires from the old switch. Then squeeze the strain relief grommet with pliers and remove the cord.
Photo 3: Install the new switch
Spin on the new switch. Tighten the switch with a wrench and hold the stub pipe from the tank with a pliers.
If your air 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. Air compressor repair can be simple. The air compressor 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 air compressor unloader valve is usually built into the pressure switch assembly. Contact the air compressor repair 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). 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.
Required Tools for this Compressor Repair Project
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
Slip joint pliers
Required Materials for this Compressor Repair 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)