Heat Pump Reversing Valve Electrical Diagnostics
The valve requires a minimum pressure differential (typically 50-100 PSI) across the suction and discharge lines to physically move the internal slide. If the system is severely undercharged or the internal pilot port is clogged by debris, the solenoid coil might hum and vibrate perfectly, but the valve will never actually shift.
The Pilot Port Logic
A 4-way valve does not shift because of the magnetic pull of the coil alone—the coil only moves a tiny pilot needle. That needle redirects high-pressure refrigerant to one side of a large internal piston. If that tiny pilot port is clogged by brazing slag or moisture, the piston remains trapped, even if the coil is energized.
| Symptom | Electrical Check | Mechanical Check |
|---|---|---|
| Valve fails to shift | Check for 24V at coil | Check for 50+ PSI differential |
| Humming/Vibration | Check for magnetism | Solenoid is good; port is clogged. |
| Coil gets hot but no shift | Normal operation | Verify compressor is pumping. |
Diagnostic Gateway Challenge
You replace a reversing valve coil because the original was open. The new coil has continuity, and you hear a distinct “click” when you energize it, but the discharge pressures do not change. What is the fault?