The Reversing Valve: Operation, Wiring, and Common Failures
Objective. The student will describe how a four-way reversing valve shifts refrigerant flow and demonstrate how to test the valve solenoid coil and pilot tube operation using a multimeter and manifold gauge set.
How the Four-Way Reversing Valve Works
A heat pump moves heat in two directions — into the building for heating and out of the building for cooling — using the same refrigerant circuit. The four-way reversing valve makes that direction change possible. It is a slide-type valve with four pipe connections: the discharge line from the compressor (high-pressure vapor), the suction line back to the compressor (low-pressure vapor), and two lines that connect to the outdoor coil and the indoor coil. By shifting a sliding spool inside the valve body, the refrigerant path is rerouted so that which coil acts as condenser and which acts as evaporator is swapped.
The valve body contains a main slide (sometimes called the shuttle or piston) and a small pilot valve controlled by an electromagnet — the solenoid coil. When the solenoid coil is de-energized, spring pressure holds the pilot valve in one position, directing high-pressure gas to one end of the main slide and low pressure to the other. The pressure difference pushes the slide to one end of the body. When the solenoid coil is energized, it pulls the pilot valve to the opposite position, reversing which end of the main slide sees high pressure, and the slide shifts to the other end.
Most residential heat pumps energize the solenoid in cooling mode and de-energize it in heating mode, but this varies by manufacturer — always check the unit's wiring diagram before diagnosing.
The Pilot Tubes
Three small capillary tubes (pilot tubes) connect the pilot valve section to the ends of the main slide cylinder and to the discharge port. These tubes carry the pressure signals that move the main slide. A blocked or kinked pilot tube prevents the main slide from shifting even if the solenoid coil works perfectly. This is one of the most common causes of a valve that appears to be stuck.
Wiring the Solenoid Coil
The solenoid coil receives its signal from the thermostat control board — typically through the O terminal (some manufacturers use B). When the thermostat calls for a mode change, it energizes or de-energizes the O/B terminal, which in turn energizes or de-energizes the coil. The coil is a simple electromagnet: a coil of wire wound around a hollow bobbin that slides over the valve stem. Coils are field-replaceable without reclaiming the refrigerant charge — only the coil is changed, not the valve body.
Common coil voltages are 24 V AC (most residential controls) and 240 V AC (some commercial units). Always confirm supply voltage before touching wiring.
Safety Before You Start
- Lockout/tagout (LOTO): Apply lockout/tagout procedures to the unit's disconnect before opening any electrical panels or handling wiring. Verify absence of voltage with a non-contact voltage tester after locking out.
- PPE: Wear safety glasses at all times around refrigerant lines. Wear insulated gloves when working near energized terminals.
- Refrigerant hazard: Reversing valves operate under system pressure. Never loosen any brazed or flare fitting while the system is charged. When manifold gauges are connected, open valves slowly and watch for frost or oil indicating liquid carry-over.
- Refrigerant handling: Only EPA 608 certified technicians may purchase and handle regulated refrigerants.
Testing the Solenoid Coil with a Multimeter
A failed solenoid coil is one of the three most common reversing valve problems. Use a digital multimeter set to the ohms (Ω) function to test the coil.
1. De-energize the unit and apply lockout/tagout.
2. Disconnect the coil leads from the control circuit.
3. Measure resistance across the two coil terminals.
4. Compare to specification. Manufacturer service bulletins list the acceptable resistance range for each coil model. A reading of OL (open loop / infinite resistance) means the coil winding is broken — replace the coil. A reading of zero or near-zero ohms indicates a shorted coil — replace the coil. A reading within the published range means the coil is likely serviceable.
You can also perform a voltage check at the coil terminals with the unit energized in the correct mode to confirm the control board is delivering the signal — but only do this if LOTO is not practical at that step, using full PPE and keeping clear of live terminals.
Using Manifold Gauges to Confirm Valve Shifting
Once the electrical side checks out, confirm mechanically that the slide is moving:
1. Connect manifold gauges to the system's service ports.
2. Start the unit in one mode (e.g., cooling) and note suction and discharge pressures.
3. Command a mode change at the thermostat and observe the gauges. Within a few seconds, suction and discharge pressures should redistribute — the former high-side coil drops in pressure and the former low-side coil rises.
4. If pressures do not redistribute, the slide has not shifted. Check pilot tubes for blockage and listen for the faint "click" or "whoosh" of the slide moving.
Three Common Failure Modes
| Failure | Likely Cause | Diagnostic Clue | Typical Repair |
|---|---|---|---|
| Stuck slide | Contamination, worn slide, blocked pilot tube | Pressures do not equalize between modes; coil resistance OK | Clear pilot tube or replace valve |
| Leaking slide | Worn slide seal | High superheat on suction; unit short-cycles; pressures equalize slowly | Replace valve |
| Failed solenoid coil | Open or shorted winding | OL or 0 Ω on multimeter; no audible click | Replace coil only |
A leaking slide allows high-pressure discharge gas to bypass into the suction side continuously; this raises suction pressure and lowers system capacity. It is confirmed by suction pressure being higher than expected for the operating conditions and by comparing actual superheat to expected superheat.
Key Points to Remember
- The solenoid moves the pilot valve, not the main slide directly — pressure differentials do the heavy lifting.
- Coil resistance outside the manufacturer's range = replace the coil.
- Pilot tube blockage can mimic a stuck slide even with a good coil.
- Always check the wiring diagram for your specific unit to confirm which mode energizes the coil.
Key terms9
Worked examples4
- Identify the measured value: 48 Ω.
- Identify the manufacturer's specification range: 40–60 Ω.
- Compare: 40 Ω ≤ 48 Ω ≤ 60 Ω — the reading falls within the acceptable range.
- OL means the circuit is open — current cannot flow through the coil winding.
- An open winding means the coil cannot create a magnetic field, so the pilot valve will not move regardless of the control signal.
- Confirm by re-measuring in case of a loose probe connection; if OL persists, the coil has failed.
- Measured value: 1 Ω.
- Specification range: 40–60 Ω.
- 1 Ω is far below the minimum of 40 Ω, indicating a shorted coil winding where turns of wire are contacting each other, drastically reducing resistance.
- A shorted coil may draw excessive current, blow a fuse, or fail to produce sufficient magnetic force.
- The coil resistance is within spec and the correct voltage is present — the electrical side is working.
- The slide has not shifted despite the electrical signal being correct.
- Possible mechanical causes: blocked pilot tube, stuck or contaminated main slide.
- Inspect pilot tubes for kinks or blockage; tap the valve body gently with a rubber mallet (unit running) to free a stuck slide — if the slide then moves, the valve is worn and should be replaced.
Materials: Cut-away reversing valve display board, Ranco or Sporlan reversing valve service bulletin, digital multimeter, manifold gauge set, manufacturer wiring diagram, troubleshooting worksheet
Check: Instructor observation checklist: student correctly measures coil resistance and interprets result; correctly traces the valve position on a wiring diagram; pass mark 80% of checklist items.
Practice check8
Answer each question, then check it. Nothing is recorded; this is your own practice.
