Differential Pressure Switches Repair & Overhaul

Differential pressure switches monitor the pressure drop across a component and provide a discrete signal when that drop crosses a preset threshold. On modern aircraft they are most commonly applied to filters, where a clogging element raises the differential across it. An oil filter differential pressure switch, fuel pump or fuel filter differential pressure switch, or scavenge filter pressure switch warns the crew or maintenance system that a filter is approaching bypass, while gage and low-pressure oil switches indicate when a monitored pressure has fallen outside its acceptable band.

Internally, the switch references two pressure ports across a sensing element, typically a diaphragm or a spring-loaded piston, that displaces as the differential builds. At the calibrated setpoint, the element actuates an electrical contact set that makes or breaks the warning or annunciation circuit. The accuracy of that setpoint and the integrity of the contacts are what make the device trustworthy.

Over time, diaphragms fatigue and take a set, piston seals wear and leak, and return springs relax, all of which shift the actuation point. Contacts oxidize, pit, or weld, producing intermittent or failed switching. Repair and overhaul replace the worn sensing and sealing elements, restore the contact set, and recalibrate the actuation point so the switch trips precisely where it should.

QTC overhauls differential pressure switches under its Part 145 approval. Each unit is bench-tested on receipt, then disassembled to expose the diaphragm or piston, seals, spring, and contact set. Fatigued diaphragms, worn seals and pistons, relaxed springs, and degraded contacts are replaced, and the body and ports are cleaned and inspected for corrosion or damage.

The rebuilt switch is reassembled and recalibrated against traceable pressure reference standards. Actuation and reset (deadband) points are set and verified, contact continuity and insulation are confirmed, and the unit is leak-checked across its pressure ports to ensure there is no cross-port or external leakage. Once functional and setpoint testing confirms the switch performs to specification, it is certified and released to service with the appropriate documentation.