Symptoms
- No brief hiss or puff of air right when the motor shuts off
- Motor struggles or hums at every startup
- Motor trips the breaker specifically at startup
- Compressor is louder or strains audibly when starting
Likely causes
- CommonStuck or worn unloader valve mechanismThis small valve, part of or near the pressure switch assembly, can gum up or wear out.
- CommonSmall tube connecting the unloader to the pump head is blocked or disconnectedA kinked, cracked, or popped-off tube prevents the valve from doing its job.
- PossibleDebris in the unloader valve seatDirt can prevent proper opening and closing.
- Less commonPressure switch assembly failureThe unloader function may be integrated into a switch that has failed as a whole.
What to do
- With the compressor unplugged and pressure released, locate the pressure switch and the small tube running from it to the pump head (this operates the unloader).
- Check that the tube is intact, properly connected at both ends, and not kinked or cracked.
- Reconnect or replace the tube if it's damaged or disconnected.
- Plug in and let the tank build to cutoff pressure, then listen closely right at shutoff for a brief hiss from the unloader; if there's none, the valve itself may be stuck.
- If accessible, clean around the valve mechanism per the manual, since some are a simple pin-and-spring design.
- If cleaning doesn't restore the puff of air at shutoff, replace the pressure switch assembly, since the unloader is usually integrated into it.
- Have a technician handle this if the switch and unloader are combined in a way that's not straightforward to service yourself.
Tools and parts
- Small wrench
- Flashlight
Pressure switch with unloaderSearch: “air compressor pressure switch unloader valve assembly”
When to call a pro
If the unloader isn't releasing pressure and the tube is fine, have a technician replace the pressure switch assembly.
Safety
Use caution
Release all pressure before inspecting or working near the pressure switch and unloader tube.