Unloader + pressure switch service
Diagnose and replace pressure switches and pilot unloaders on tank-mounted recips.
This guide is general field reference for trained, qualified service technicians. It is not a substitute for the original equipment manufacturer's service manual, applicable codes (NEC, OSHA, ASME, local), or the judgment of a licensed professional. Procedures, torque values, pressures, refrigerants, and safety requirements vary by make, model, jurisdiction, and revision. Always verify against current OEM documentation and follow your employer's safety program. Working on compressed-air, electrical, pressure-vessel, and refrigerant systems can cause serious injury or death. By using this guide you accept all risk; AirCompDX and its authors disclaim all warranties and are not liable for any damage, injury, loss, or code violation arising from its use.
LOTO and bleed tank to 0 psi. Pressure-switch contacts may carry 240 V — verify zero voltage before touching.
- Adjustable wrench
- Phillips/Slotted drivers
- Multimeter
- Thread sealant rated for compressed air
- Pressure switch (matched to cut-in / cut-out and HP)
- Pilot/unloader valve if leaking
- Check valve (often replace at same time)
- 1Diagnose
Confirm symptom: pump won't restart loaded = unloader stuck closed (or check valve passing). Pump won't cut out = switch failed closed or differential too wide.
- 2LOTO and bleed
Open the disconnect, LOTO, bleed tank to 0 psi at the drain.
- 3Pull switch
Document wiring with a photo. Disconnect leads, unscrew the switch from its tee. Thread sealant on male threads only, never on the first 1.5 turns.
- 4Set cut-in / cut-out
Install matching switch, set main range to cut-out (typ. 175 psi), then set differential to give the desired cut-in (typ. 25 psi differential).
- 5Pilot unloader check
Energize the pump empty, observe the unloader bleeding off head pressure within 5 seconds of cut-out. If not, replace the unloader.
- 6Cycle test
Run a full load/unload cycle. Verify cut-in, cut-out, and that the unloader vents fully before the next start.