On this page
14 answers covering pressure switches, no-start faults, industrial controllers, refrigerant readings and compressor sizing.
Pressure switches
Cut-in, cut-out, differential, wiring and replacement — the questions technicians search most.
air tank pressure switch
What does the air tank pressure switch do?
The air tank pressure switch is the mechanical control that starts the motor at cut-in pressure and stops it at cut-out pressure. It senses receiver pressure through a port on the tank and opens or closes line-voltage contacts feeding the motor directly — on most shop compressors there is no relay or contactor between them.
Nearly every tank switch also carries an unloader valve. When the contacts open, the unloader bleeds the air trapped between the pump and the check valve so the motor restarts against no load. A short hiss after shutdown is normal; a continuous hiss while running means the check valve is leaking back.
Look up your switch by part number for its factory cut-in and cut-out, its differential range, and whether it is a two-pole or four-pole body before you order a replacement.
Pressure switch librarycompressor switch
Which switch on an air compressor controls the motor?
On a typical shop compressor, the pressure switch is the motor control. One body holds the sensing diaphragm, the line-voltage contacts, the auto/off lever and the unloader valve, which is why a single failed part can cause a no-start, a no-shutoff or a constant air leak.
Around it you will find a thermal overload on the motor, a manual disconnect, and on larger three-phase machines a magnetic starter that the pressure switch pilots rather than feeding the motor directly. Knowing which arrangement you have decides whether the switch carries full motor current or only a control signal.
If the machine runs when you jumper across the switch contacts, the switch or its setting is the fault. If it still does nothing, move upstream to the overload, starter coil and supply.
Identify your switchhow to adjust the pressure switch on an air compressor
How do you adjust the pressure switch on an air compressor?
Bleed the tank to zero and lock out the supply before you remove the cover — the terminals inside are at line voltage. With the cover off you will see one large adjustment (range, which sets cut-out) and on most switches a second smaller one (differential, which sets how far pressure falls before the motor restarts).
Turn the large nut clockwise to raise cut-out and counter-clockwise to lower it. Move it a quarter turn at a time, refit the cover, restore power, and let the compressor run a full cycle while you watch the tank gauge. Note both the pressure where it stopped and where it restarted before making a second change.
Never set cut-out above the tank's or the pump's rating, and keep the safety relief valve setting above cut-out. Too small a differential short-cycles the motor; too large starves tools at the bottom of the band.
Cut-in / cut-out calculatoradjusting air compressor pressure switch
What cut-in and cut-out settings should I use?
Start from the equipment, not the gauge. Cut-out must stay below the lowest rating among the receiver, pump and piping, and below the safety relief valve. From there, the differential decides how often the motor starts: a 20 to 40 PSI gap suits most shop machines.
Work backwards from the tool. A gun that needs 90 PSI at its inlet loses pressure through the regulator, hose and quick-couplers, so cut-in should sit well above 90 — 115 PSI cut-in with 145 PSI cut-out is a common, safe combination.
Short cycling wears contacts, unloaders and motor windings faster than anything else on the machine. If the motor starts more than a few times per minute under steady demand, widen the differential or fix the leaks first.
Square D adjustment guidereplacing air compressor pressure switch
How do you replace an air compressor pressure switch?
A replacement has to match on four points: the pressure range (cut-in and cut-out), the electrical rating and pole count for your supply, the port thread and size, and whether the switch needs an unloader port for your pump. Substituting a four-pole switch on a single-phase machine, or an unloaded switch on a pump that needs one, causes hard starts and tripped overloads.
Depressurize and isolate power. Photograph the terminals before removing anything, then move one wire at a time to the same position on the new switch. Use a sealant rated for compressed air on the port threads — PTFE tape shreds and can end up inside the switch.
Before it goes back into service, run a full cycle and confirm the actual cut-in and cut-out on the tank gauge, plus a clean unloader blowdown at shutdown.
Find a matching switchwiring pressure switch air compressor
How is an air compressor pressure switch wired?
The supply lands on the line side of the switch, the motor on the load side. The switch is simply a pressure-operated contact in series with the motor, which is why a burnt contact shows up as a machine that hums, trips or refuses to start.
On single-phase 240 V installations both legs should be broken by a two-pole switch, not just one. On 120 V, the switched conductor must be the ungrounded (hot) leg. The ground bonds to the switch enclosure and continues to the motor.
Three-phase machines rarely run motor current through the switch. There, the switch is a pilot device on the starter coil circuit, and the starter contacts and overload carry the load. Check which arrangement you have before assuming a rating.
Wiring diagramssingle phase air compressor pressure switch wiring
How do you wire a single-phase compressor pressure switch?
For 240 V single-phase, both ungrounded conductors must be switched. L1 and L2 land on the line terminals of a two-pole switch and the two motor leads land on the load terminals opposite them; the equipment grounds bond to the switch enclosure and continue to the motor frame.
For 120 V, only the hot is switched. The neutral runs through to the motor without interruption and the ground bonds to the case. Switching the neutral instead of the hot leaves the motor energised when the compressor is 'off'.
If your switch has an extra pair of terminals for a pilot light, an unloader solenoid or a low-oil switch, confirm from the printed diagram inside the cover which pair is line and which is load before landing anything.
Single-phase wiring referencepressure switch for air compressors
How do I choose the right pressure switch for an air compressor?
Start with the pressure band: the switch's adjustable range must comfortably contain your intended cut-out, not just reach it. Then confirm the electrical rating covers your motor's horsepower at your voltage, and that the pole count matches the supply.
Mechanically, check the port thread and size, the mounting style (direct tank port, manifold or bracket), and whether you need an unloader. Manifold switches with a built-in relief valve and gauge port replace several fittings at once but only fit certain pumps.
Common families — Square D, Condor, Furnas and Lefoo — are not interchangeable port-for-port, so compare the specification rather than the look of the body.
Compare switch specsCompressor won't start
No-start and no-run faults, from tripped overloads to failed unloader valves.
air compressor will not turn on
Why will my air compressor not turn on?
Start at the supply. Confirm voltage at the disconnect and at the switch line terminals, check the breaker, and on extension-cord installs check for voltage drop under load — a long, undersized cord is a frequent cause of no-start and nuisance tripping.
Then check the obvious control causes: tank pressure already above cut-in, the auto/off lever off, or a tripped thermal overload that needs a manual reset after cooling. Jumpering the switch contacts briefly separates a failed switch from everything downstream.
A motor that hums but will not turn is a different fault: a failed start capacitor or centrifugal switch, a stuck unloader leaving head pressure on the pump, or a mechanically seized pump. Do not repeatedly retry a humming motor — it will cook the windings.
Won't-start diagnosticControllers and wiring diagrams
Industrial controller pinouts, fault codes and service passwords.
delcos 3100 wiring diagram
Where can I find CompAir Delcos wiring and terminal information?
CompAir Delcos controllers use a fixed set of inputs for the pressure transducer, discharge and airend temperature sensors, motor overload and emergency stop, with relay outputs for the star-delta or line contactor, the loading solenoid and the alarm.
The controller page collects the terminal assignments, sensor types, alarm and fault codes, and the parameter levels that require a service password, so you can trace a fault without the paper manual on site.
Machine options change wiring: an integrated dryer, remote start/stop, sequencing or a soft starter reassign terminals. Use the page to orient yourself, then verify against the diagram in the machine's own enclosure before landing a wire.
CompAir Delcos controlleringersoll rand r series
What should I know about servicing Ingersoll Rand R-Series compressors?
Ingersoll Rand R-Series machines span small fixed-speed screws through larger variable-speed units, and are controlled by the Xe series of controllers. The two shutdowns you will meet most are high airend discharge temperature and high discharge pressure.
High temperature usually traces to a dirty cooler, low or degraded fluid, a failed thermostatic valve or restricted ambient airflow rather than the airend itself. High pressure points at the inlet valve, the pressure transducer or the load solenoid.
The model page lists the flow and pressure range, controller fitted, separator and filter service intervals, and the fault codes with what each one is actually measuring.
Ingersoll Rand R-SeriesRefrigerant checks
Pressure-temperature readings, superheat and subcooling on compressor-side work.
refrigerant pt chart
How do you use a refrigerant pressure-temperature chart?
A P/T chart converts gauge pressure into the saturation temperature for one specific refrigerant. Read the low side to get evaporator saturation temperature and the high side to get condensing temperature — those two numbers drive superheat and subcooling.
Blends such as R-407C and R-410A have temperature glide, so the chart gives two values at each pressure: the bubble point for saturated liquid and the dew point for saturated vapour. Use dew for superheat at the suction line and bubble for subcooling at the liquid line.
Never read one refrigerant's pressures on another's column. Confirm the charge type on the nameplate before you interpret anything.
Refrigerant P/T charthow to calculate superheat
How do you calculate superheat?
Superheat is the temperature of the suction vapour above its saturation point. Take suction pressure at the service port, convert it to saturation temperature on the correct refrigerant column, then subtract that from the line temperature measured at the same location with a well-insulated clamp probe.
Low superheat means liquid is reaching the compressor — the main cause of slugging, diluted oil and failed valves. High superheat means the evaporator is starved: undercharge, a restriction, a failed metering device or low load.
Read superheat with subcooling, not alone. The two together separate a charge problem from a metering or airflow problem.
Superheat calculatorSizing and flow
CFM measurement and compressor sizing for real tool demand.
compressor cfm calculator
How do you calculate a compressor's actual CFM?
Nameplate CFM describes a new machine at a rated pressure. To find out what yours delivers now, run a pump-up test: close the discharge so nothing is consuming air, time the receiver's rise from one pressure to another, and apply CFM = V × (P2 − P1) ÷ (14.7 × T), where V is tank gallons divided by 7.48 and T is minutes.
Compare the result against a rough expectation of 3 to 4 CFM per horsepower for a two-stage piston and 4 to 5 for a rotary screw at 100 PSIG. A machine well below that has worn rings, leaking valves, a slipping belt or a restricted inlet filter.
To size rather than measure, total the CFM of tools running at the same time, apply duty factors, and add 25 to 50 percent headroom before choosing a machine.
CFM calculator