Air Compressor Pressure Switches

How they work, how to adjust, how to replace.

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Air compressor pressure switches

The pressure switch is the brain of every reciprocating air compressor. It decides when the motor starts, when it stops, and whether the unloader vents head pressure at shutdown. Get it wrong and you cook motors, burn contacts, or short-cycle the pump to death.

The three numbers that matter

Cut-in pressure
The tank pressure at which the switch closes and the motor starts. Typically 20 PSI below cut-out.
Cut-out pressure
The tank pressure at which the switch opens and the motor stops. Never set above the tank's stamped MAWP.
Differential
The gap between cut-in and cut-out. Wider = fewer starts per hour = longer motor life. Minimum 20 PSI on shop compressors.

Adjusting a Square D / Furnas Pumptrol

  1. Kill power at the disconnect. Lock out.
  2. Bleed the tank to zero PSI.
  3. Remove the switch cover. You'll see a large range spring and a small differential spring.
  4. Restore power, restart the compressor. Note the actual cut-out pressure on the gauge.
  5. Bleed the tank slowly through an outlet. Note the actual cut-in pressure.
  6. Adjust the range screw clockwise to raise both settings, counterclockwise to lower.
  7. Adjust the differential screw clockwise to widen the gap between cut-in and cut-out.
  8. Re-test through a full cycle. Recover cover.
Safety

Never set cut-out above the tank's Maximum Allowable Working Pressure (stamped on the tank data plate). Never disable the safety relief valve. The relief valve must pop at or below the tank's MAWP.

Picking the right pressure switch

Pressure range
95–125 PSI (portable/light shop) · 140–175 PSI (industrial recip) · 100–150 PSI (single-stage 5–10 HP)
Amp rating
Match to motor FLA. Standard shop switches: 15A/240V or 25A/240V. Above 7.5 HP or any 3-phase — pilot a contactor. Use the motor amperage calculator to size.
Port count
1-port (basic), 2-port, 4-port with gauge and safety relief tapping. Match your existing plumbing to avoid re-piping.
Unloader configuration
With unloader (reciprocating) or without (rotary screw pilot). Never mismatch — a recip without unloader relief will stall on restart.
On/off lever
Auto/Off lever is standard on shop switches. Industrial switches usually omit it — the contactor handles start/stop.

When to replace, not adjust

  • Visible arcing or pitted contacts
  • Motor chatters at cut-in (weak spring)
  • Continuous hiss from unloader port while running (bad unloader valve)
  • Cracked diaphragm — water in the switch
  • Won't hold adjustment
  • Sticking closed — motor won't stop at cut-out

Wiring notes

Single-phase, direct-drive (≤5 HP): pressure switch carries motor current directly. L1/L2 in, T1/T2 out to the motor. Bond ground to the switch box.

Single-phase >5 HP or any 3-phase: switch pilots the contactor coil. Motor current flows through the contactor, not the switch. Overload relay sizes to the motor FLA — see the electrical reference.

Voltage drop matters: long feeder runs cause motor to trip overload under load. Use the voltage drop calculator to size wire.

Related diagnostics & tools

Calculator: cut-in / cut-out / starts per hourDiagnose: short cyclingDiagnose: won't startDiagnose: trips breaker / overloadReceiver tank sizing (fewer cycles)Motor amperage calculatorBlog: pressure switch adjustment walkthroughElectrical reference — contactors, overloads, wiringMaster air compressor troubleshooting guide

Frequently asked

What does an air compressor pressure switch do?
It starts the motor at the cut-in pressure and stops it at the cut-out pressure. On reciprocating compressors it also vents the unloader line so the motor can restart against no load.
How do I adjust the cut-in and cut-out pressure?
The large range spring raises or lowers both settings together. The small differential spring changes the gap between cut-in and cut-out. Always kill power and bleed tank pressure before opening the switch cover.
Why is my air compressor short cycling?
Almost always a narrow pressure differential, a bad check valve letting tank pressure back-bleed into the head, or a leak on the discharge side. Set the differential to at least 20 PSI on shop compressors.
What size pressure switch do I need?
Match the amp rating to the motor's full load amps (FLA), match the working pressure range (95–125, 140–175, etc.), match the port count (1-, 2-, or 4-port), and match the unloader configuration (with or without).
Can I use a pressure switch on a 3-phase compressor?
Yes, but the switch pilots the contactor coil — it does not carry motor current. The contactor carries the load. See the electrical reference for typical 3-phase control wiring.
Why is my pressure switch leaking air continuously?
The unloader valve inside the switch has failed. Head pressure is bleeding through the unloader port whenever the motor is running. Replace the switch — the unloader is not serviceable separately on most models.