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.
Slap two open end wrenches on the compression fitting and loosen the nut on the air line. Pull it out of the fitting.
Disconnect the hot, neutral and ground wires from the old switch. Then squeeze the strain relief grommet with pliers and remove the cord.
Spin on the new switch. Tighten the switch with a wrench and hold the stub pipe from the tank with a pliers.
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
switch assembly replacement
for this compressor for $20
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.
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.