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. 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.
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.