Air Compressor Pressure Switch — Wiring, Adjustment & Replacement

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

Kill power at the disconnect. Lock out.
Bleed the tank to zero PSI.
Remove the switch cover. You'll see a large range spring and a small differential spring.
Restore power, restart the compressor. Note the actual cut-out pressure on the gauge.
Bleed the tank slowly through an outlet. Note the actual cut-in pressure.
Adjust the range screw clockwise to raise both settings, counterclockwise to lower.
Adjust the differential screw clockwise to widen the gap between cut-in and cut-out.
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.

Brands & families

Square D / Pumptrol (Schneider)

The North American default. Class 9013 Type F (light duty, fixed differential) and Type G (higher HP, adjustable differential). Vertical visible contacts, drift-proof settings, 1/4" NPSF standard port.

Type FHG = compressor. Type FSG/FYG/FRG = water pump — do not substitute one for the other on a compressor.

9013FHG12J5295/125 psi shop standard, fixed 30 psi differential 9013FHG2J2780/100 psi low-pressure variant, 20 psi differential 9013FHG42J55110/150 psi, 40 psi differential 9013FHG42J59135/175 psi high-pressure recip switch 9013GHG seriesUp to 300 psi, adjustable differential, higher HP 9013FSG2J2430/50 psi water pump switch — commonly misapplied to compressors

Furnas / Siemens / Hubbell (Class 69)

The OEM switch on Emglo, Jenny, Champion, Curtis and thousands of shop recips. Class 69MB and 69JF are the compressor families; many are now obsolete from Siemens and are supplied as Lefoo or Hubbell equivalents.

69MB7LY and 69JF7LY are the two part numbers you'll see most on 95/125 psi shop compressors.

69MB7LY (and 69MB109xx OEM variants)Classic Emglo / Jenny OEM switch, 95/125 with lever and unloader 69JF7LYFour-port Furnas build with adjustable differential

Condor (Condor-Werke, Germany)

European industrial standard, common on Atlas Copco, Chicago Pneumatic, ABAC, Fini and imported recips. MDR series has both a switching-point screw and a separate hysteresis screw, plus a built-in motor disconnect.

MDR 3 versions carry three-phase motor current directly and can include a thermal overload — a real differentiator vs. Square D.

MDR 2 / MDR 11German industrial switch with separate switching-point and hysteresis screws MDR 3Three-phase switch that carries motor current, optional thermal overload

Lefoo

High-volume Chinese manufacturer supplying most aftermarket replacements and a large share of OEM switches. LF10 is a drop-in form/fit match for Furnas 69MB and Square D 9013FHG in the popular ranges.

UL listed and CSA certified — check the listing mark, not just the shape, when buying online.

LF10 (LF10-1H, LF10-L1, LF10-4H)The aftermarket workhorse — 20 A, one or four port, up to 200 psi

Industrial pilot-duty controls

On rotary screws and any motor above about 7.5 HP the pressure control never carries motor current — it pilots a contactor coil or feeds a PLC/controller input. Allen-Bradley 836T, Danfoss KP/RT and Telemecanique XML/XMP live here.

Pilot-duty switches are rated in VA, not horsepower. Never wire one in series with motor leads.

Pilot-duty pressure controlsContactor-piloting switches for large recips, screws and plant air

Every pressure switch design, decoded

Fixed differential (Square D FHG, Lefoo LF10)

One range adjustment. The gap between cut-in and cut-out is set by the mechanism and cannot be changed.

Use when: Standard shop recips where the factory 20–40 psi gap already suits the tank and duty.

Field note: If it short-cycles, the fix is receiver volume, a check valve or a leak repair — not the adjustment screw.

Adjustable differential (Square D Type G, Condor MDR, Furnas 69JF)

Two independent screws: range (both setpoints) and differential/hysteresis (the gap only).

Use when: When you need to hit a starts-per-hour target on a fixed tank, or match an existing header pressure band.

Field note: Always set range first, then differential, then re-verify range — on most designs they interact.

Standard action (opens on rising pressure)

Contacts open as tank pressure rises to cut-out. This is every compressor switch.

Use when: All compressor motor control.

Field note: Continuity across the terminals with an empty tank is normal and expected.

Reverse action (opens on falling pressure)

Contacts open when pressure falls — used for low-pressure protection and pump-down interlocks (e.g. Square D FRG).

Use when: Loss-of-pressure alarms, dry-run protection, header interlocks.

Field note: Fitting a reverse-acting switch as a compressor control will run the pump until something bursts or trips.

With unloader (two-way relief valve)

A small valve in the switch vents the line between the pump head and the check valve at cut-out.

Use when: Every reciprocating compressor without a separate centrifugal or head unloader.

Field note: A short hiss at shutdown is correct. A continuous hiss while running means the unloader seat failed.

Without unloader

No head-pressure vent. The compressor must be able to start against whatever pressure remains.

Use when: Rotary screws, pilot-duty applications, and recips with a separate unloader valve.

Field note: Fitting a no-unloader switch to a recip causes hard starts, tripped overloads and burned start capacitors.

Low-pressure cut-off (Square D M4 option)

Latches the switch OFF when tank pressure falls about 10 psi below cut-in; requires a manual restart at the lever.

Use when: Protection against a broken line or a run-away leak emptying the system unattended.

Field note: Only available on standard-action switches. A customer reporting 'it just stopped and won't restart' often has an M4 that latched.

Four-port manifold

The switch body carries tank inlet, gauge, ASME relief valve and unloader line in one casting.

Use when: Shop recips where the switch is the entire control head, and on replacements where the old manifold leaks.

Field note: Seal with air-rated thread sealant. Tape shreds migrate into the unloader and hold it open.

Power duty vs. pilot duty

Power-duty switches carry motor current directly. Pilot-duty switches only make and break a contactor coil circuit.

Use when: Power duty up to roughly 5–7.5 HP single-phase; pilot duty for everything larger and all three-phase without an integrated starter.

Field note: Match the switch amp rating to motor FLA using the motor amperage calculator, then add contactor and overload for anything above.

Electronic / transducer control

A pressure transducer feeds the compressor controller, which drives the starter or VFD. No mechanical contacts in the pressure path.

Use when: Rotary screws, VFD machines and any unit with a sequencer or load/unload logic.

Field note: Set points live in the controller menu. A mechanical switch on these machines is usually a high-pressure safety backup only.

Enclosure rating (NEMA 1 / 3R / 4)

NEMA 1 is indoor general purpose, 3R sheds falling rain, 4 is wash-down and hose-directed water.

Use when: Anything outdoors, in a wash bay, or in an agricultural building needs at least 3R.

Field note: Water in a NEMA 1 switch cracks the diaphragm and corrodes the contacts — it is the leading cause of premature failure.

Related diagnostics & tools

Calculator: cut-in / cut-out / starts per hour Diagnose: short cycling Diagnose: won't start Diagnose: trips breaker / overload Receiver tank sizing (fewer cycles)Motor amperage calculator Blog: pressure switch adjustment walkthrough Electrical reference — contactors, overloads, wiring Master 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.