What these devices do on a compressor
Every modern screw or piston compressor depends on a handful of field devices. They sit between the physical machine and the controller, and when one of them is wrong the whole package behaves wrongly, even if the airend, motor and oil are in perfect condition.
- Pressure transducers measure line (net) pressure, package pressure and sometimes the pressure inside the separator vessel. The controller uses them for load and unload set points, for minimum-pressure logic and for high-pressure trips.
- Temperature sensors, usually PT100/PT1000 resistance elements or thermistors, measure airend discharge temperature and sometimes oil, ambient or motor winding temperature. Discharge temperature is the most important protection on an oil-injected screw machine.
- Differential pressure devices watch across the intake filter, oil filter and separator element and raise a service warning as the element loads.
- Pressure switches on simpler piston compressors start and stop the motor, or load and unload the pump, without any electronic controller at all.
- Solenoid valves turn electrical commands into air movement: loading and unloading the inlet valve, venting the separator vessel on stop, operating timed or level-sensing condensate drains, and switching dryer towers.
- Level and position devices include float switches in drains and oil reservoirs and feedback switches on some inlet valves.
Because the controller trusts these readings completely, a drifting transducer or a sticking solenoid can cause problems that look like airend wear, a failing motor or a bad controller.
Why a part that fits can still be the wrong part
Two pressure transducers can share the same thread and connector and look identical, yet one reads 0–16 bar and the other 0–25 bar. Fitted to the wrong machine, the controller will read pressure incorrectly by a large margin and the compressor will load, unload or trip at the wrong points. The same applies to temperature sensors with different resistance curves and to solenoid coils that look alike but run at different voltages.
The parameters that decide compatibility are:
| Parameter | Why it matters | Where to find it |
|---|---|---|
| Measuring range | The controller scales the signal to a range; the wrong range gives a false reading | Device label, controller parameter list |
| Signal type | A 4–20 mA loop, 0–10 V, ratiometric voltage, resistance or switch contact are not interchangeable | Device label, wiring diagram |
| Supply voltage | Sensors are commonly 24 V DC; coils may be 24 V DC, 24 V AC, 110 V or 230 V AC | Coil body, controller output rating |
| Connector and pinout | Same plug style can carry different pin assignments | Old part, harness, drawing |
| Thread and sealing | BSP parallel, BSPT taper, NPT and metric threads seal differently | Old part, port on the machine |
| Wetted material | Must suit compressor oil, condensate and temperature | Manufacturer data |
| Valve function | 2/2 or 3/2, normally open or normally closed, pilot or direct acting | Valve body marking, pneumatic diagram |
| Pressure rating | Solenoid must open against the maximum differential it will see | Valve body marking, data sheet |
None of these can be safely assumed from appearance. A device with a superseded part number may also need a matching change in controller settings, which is why we look at the controller as well as the part.
How pressure transducers and temperature sensors work
Most compressor pressure transducers use a strain-gauge or piezo-resistive element bonded to a thin diaphragm. As pressure deflects the diaphragm, the element’s resistance changes, and built-in electronics turn this into a standard output. A 4–20 mA two-wire loop is common because it tolerates long cable runs and electrical noise, and because a reading of zero current clearly signals a broken wire rather than zero pressure. Many controllers treat a signal well below 4 mA as a sensor fault and stop the machine.
Temperature sensing on screw compressors is usually done with a platinum resistance element. A PT100 reads 100 ohms at 0 °C and rises in a known curve with temperature; a PT1000 follows the same curve at ten times the resistance. Thermistors (NTC or PTC) are also used, especially for motor winding protection, and have quite different curves. Fitting a PT1000 where the controller expects a PT100, or a thermistor where it expects a platinum element, gives a reading that is plausible but wrong. That is dangerous, because the high discharge temperature trip may never operate.
Electromechanical pressure switches on piston compressors use a diaphragm or bellows working against an adjustable spring. They have a cut-in and cut-out pressure and a differential between them. Their contact rating must suit the motor or contactor coil they switch, and many include an unloader port that releases head pressure so the motor can restart without load.
How solenoid valves control the compressor
A solenoid valve is a coil wound around a moving plunger. Energising the coil pulls the plunger, which opens or closes an orifice either directly or by piloting a larger diaphragm. On a compressor they typically do four jobs:
- Load and unload. The load solenoid sends control air to close or open the inlet valve so the airend switches between full output and idle running.
- Blow-down on stop. A vent solenoid releases separator vessel pressure when the machine unloads or stops, so it can restart without starting against pressure.
- Condensate drainage. Timed drains open for a set period at set intervals; level-sensing drains open only when condensate is present.
- Dryer switching. Heatless and heated desiccant dryers use solenoids to switch towers and open purge paths.
Pilot-operated valves need a minimum pressure differential to work. Fitted where that differential is not always present, such as a vent line at low pressure, they may not close fully. Direct-acting valves work from zero differential but handle smaller orifices. The normally-open or normally-closed choice is a safety decision: a blow-down valve is usually arranged so the vessel vents when power is lost, so swapping to the opposite type can leave a vessel pressurised after a stop.
Symptoms that point to a sensor or solenoid fault
| Symptom | Possible device cause | Other causes to rule out first |
|---|---|---|
| Controller shows pressure but the gauge disagrees | Drifting transducer, wrong range, scaling error | Faulty gauge, blocked sensing line |
| Sensor fault alarm, machine will not start | Open circuit, failed transducer, damaged connector | Broken cable, blown supply fuse, loose terminal |
| High discharge temperature trip on a cool machine | Failed or wrong-type temperature sensor | Genuinely hot oil: check cooler, oil level, thermostatic valve |
| No trip although the machine runs very hot | Sensor reading low, wrong curve | Controller parameter error |
| Compressor will not load | Load solenoid coil open, stuck plunger | Inlet valve fault, no control air, controller output fault |
| Machine will not unload, safety valve lifts | Solenoid stuck open or closed | Inlet valve sticking, pressure switch fault |
| Hard starting, motor overload on restart | Blow-down solenoid not venting | Minimum-pressure valve, unloader on piston machine |
| Water in the air lines | Drain solenoid or timer failed, blocked strainer | Dryer fault, drain capacity too small |
| Constant air leak from a drain or vent | Debris on the seat, worn seal | Wrong valve type for the pressure |
Device faults are real, but in our experience wiring, connectors, supply voltage and blocked sensing lines cause a large share of apparent sensor failures. Replacing a good sensor because of a loose terminal wastes money and time.
What we check before replacing a device
- Read and record the controller’s fault history and the conditions when the fault appeared.
- Compare the controller reading against a calibrated test gauge or thermometer at the same point.
- Check the supply voltage at the device connector, not only at the controller.
- Measure the signal: loop current for a 4–20 mA device, voltage for a voltage output, resistance for a PT100/PT1000 at known temperature.
- Inspect the cable, connector pins and earthing; heat, oil and vibration damage connectors on compressor packages.
- For solenoids, measure coil resistance, confirm the controller output switches, and listen or feel for plunger movement.
- Check the pneumatic side: control-air filters, small orifices and sensing lines block with oil and dirt.
- Only then identify the replacement from the old part and the machine, and confirm the controller settings suit it.
- After fitting, verify the reading against the reference and confirm the machine loads, unloads, vents and trips correctly.
How hot, dusty sites affect these devices
Compressor rooms in Telangana and Andhra Pradesh often run very hot through April to June, and many sites are dusty, especially quarries, construction projects and borewell rigs. Heat shortens coil life, hardens connector seals and pushes electronics close to their limits. Dust and oil mist get into connectors and vent ports, and condensate carries dirt into drain valves. Portable diesel machines add vibration and rough handling.
Practical steps help: keep the room ventilated, keep connector boots in place, route cables away from hot pipework, clean drain strainers at each service and keep a record of when each device was replaced. Where a device fails repeatedly, the cause is usually the environment or wiring, not bad luck with the part.
Genuine, equivalent and universal devices
For many compressor makes, the sensor or solenoid fitted at the factory comes from an instrumentation supplier and carries both the compressor maker’s part number and the device maker’s own model number. That means a correctly specified equivalent can work exactly like the original. It also means a universal device chosen only on looks can fail in ways that are hard to spot.
| Option | When it suits | What to watch |
|---|---|---|
| Original part number from the compressor maker | Controller-specific devices, machines under OEM warranty, safety-critical sensors | Superseded numbers, availability |
| Same device from the instrument maker | Where the device model is clearly identified on the label | Exact range, signal, connector and calibration |
| Specified equivalent | Older or discontinued machines | Every parameter in the table above, and controller scaling |
We tell you which option we are quoting and why, so you can decide on an informed basis.
Sensor and switch types at a glance
Use this table to identify the device in your hand before you test it. Ranges are typical for industrial compressors. The nameplate or the controller’s configuration menu gives the exact figure for your machine.
| Device | Signal | Typical range on a compressor | Failure signs |
|---|---|---|---|
| Pressure transducer (current) | 4–20 mA, two-wire loop, usually 24 V DC supply | 0–10, 0–16 or 0–25 bar(g); 0–40 bar(g) on PET boosters | Reading stuck, drifting zero, 0 mA (open loop), below about 3.6 mA or above about 21 mA (failure signal or short) |
| Pressure transducer (voltage) | 0.5–4.5 V ratiometric from a 5 V supply, or 0–10 V | 0–16 bar(g) is common on small controllers | Reading follows supply voltage; noise on long cable runs |
| RTD temperature sensor | Resistance: PT100 (100 Ω at 0 °C) or PT1000 (1,000 Ω at 0 °C) | Discharge and oil temperature, about 0–150 °C | Open circuit (controller shows maximum or a sensor fault), short circuit (shows very low), offset from loose terminals |
| NTC thermistor | Resistance falls steeply as temperature rises | Small controllers and dryers; also inside some electronic drains | Wrong curve fitted (reads plausibly but wrongly), cracked bead |
| Thermocouple (type K) | Millivolt, about 41 µV per °C | Rare on compressors; some high-pressure stages and test rigs | Wrong extension cable, reversed polarity, reading tracks panel temperature |
| Motor PTC thermistor | Resistance jumps sharply at the rated trip temperature | Embedded in motor windings, read by a relay | A cool chain of three sensors normally reads below about 750 Ω. Several kilohms when cool means a damaged sensor; open circuit means broken wiring |
| Pressure switch | Volt-free contact, NO or NC, with a set differential | Load/unload on piston units; low-oil-pressure and high-pressure protection | Welded or pitted contacts, setpoint drift, diaphragm leak |
| Differential pressure switch | Contact or 4–20 mA | Separator or filter element alarm, typically around 0.7–1.0 bar differential | Alarm with a new element (blocked sensing line), no alarm on a choked element |
| Level switch | Float with reed contact, or capacitive probe | Electronic condensate drains, oil level on some packages | Float stuck with sludge, probe coated with oil emulsion |
| Dew point sensor | 4–20 mA, often scaled −80 to +20 °C | Desiccant and refrigerated dryer outlets | Slow drift wet or dry, saturation after a slug of water; needs periodic calibration |
How to test a pressure transducer with a multimeter
A 4–20 mA transducer maps its range linearly onto 16 mA. The expected current is I = 4 + 16 × P ÷ span, where P is the pressure in bar(g) and span is the transducer’s full-scale range.
- With the machine stopped and the transducer vented to atmosphere, check that the supply at the connector is within the device’s rating, usually 24 V DC. Most two-wire transducers need at least about 10–12 V at their terminals to work.
- Break the loop at one terminal and put the meter in series on its mA range. Do not put a meter on its current range across the supply terminals, as this shorts the supply.
- Vented, the meter should read 4.00 mA. A good transducer is within about ±0.5% of span, which is ±0.08 mA.
- Run the machine to a steady pressure. Compare the current with a calibrated test gauge fitted at the same point.
Worked example: Scaling and a range mismatch
- A 0–16 bar(g) transducer at 7.0 bar(g) should give 4 + 16 × 7 ÷ 16 = 11.0 mA. Each 1 mA is worth 1 bar on this range.
- If the meter reads 11.6 mA while the test gauge shows 7.0 bar(g), the transducer indicates (11.6 − 4) × 16 ÷ 16 = 7.6 bar(g). It is reading 0.6 bar high, and the machine will unload 0.6 bar early.
- Now suppose a 0–25 bar(g) transducer has been fitted but the controller is still set to 0–16. At 7.0 bar(g) it sends 4 + 16 × 7 ÷ 25 = 8.48 mA. The controller converts this as (8.48 − 4) × 16 ÷ 16 = 4.48 bar(g). The machine never sees its unload pressure, and only the safety valve stops the pressure rising.
Some controllers read the loop as a voltage across a 250 Ω resistor. Here 4, 11 and 20 mA appear as 1.00, 2.75 and 5.00 V, so you can test without breaking the loop. A ratiometric 0.5–4.5 V transducer on a 0–16 bar range should read 0.5 + 4 × 7 ÷ 16 = 2.25 V at 7 bar(g).
How to test a temperature sensor
Disconnect the sensor from the controller and measure resistance across its leads. Compare the reading with a separate thermometer on the same pipe or the oil. For a platinum RTD, the standard curve gives these values:
| Temperature | PT100 | PT1000 |
|---|---|---|
| 0 °C | 100.00 Ω | 1,000.0 Ω |
| 25 °C | 109.73 Ω | 1,097.3 Ω |
| 40 °C | 115.54 Ω | 1,155.4 Ω |
| 85 °C | 132.80 Ω | 1,328.0 Ω |
| 100 °C | 138.51 Ω | 1,385.1 Ω |
| 110 °C | 142.29 Ω | 1,422.9 Ω |
| 120 °C | 146.07 Ω | 1,460.7 Ω |
A PT100 changes by only about 0.38 Ω per °C at compressor temperatures. On a two-wire connection, 1 Ω of extra resistance from a corroded lug or loose terminal adds about 2.6 °C to the reading. This is why a hot compressor room and loose terminals can give an early high-temperature trip. Any reading on the meter’s open-circuit (OL) range means a broken element or wire. A reading close to 0 Ω means a short. Between either lead and the sensor body, the meter should show open circuit.
An NTC thermistor has no single standard curve, so get the curve from the controller documentation. As an illustration, a 10 kΩ (at 25 °C) NTC with a B value of 3,950 K reads about 5.3 kΩ at 40 °C, 1.09 kΩ at 85 °C and 0.70 kΩ at 100 °C. A PT1000 fitted in place of that NTC reads 1,328 Ω at 85 °C. The controller would show about 79 °C, a plausible but wrong figure. A type K thermocouple gives about 4.1 mV at 100 °C relative to 0 °C. Measure with the hot junction in place and correct for the panel (cold-junction) temperature.
Solenoid valve types and coil voltage checks
| Type | Where it is used | What to confirm when replacing |
|---|---|---|
| 2/2 direct-acting, normally closed | Condensate drains, small pilot circuits | Orifice size, maximum pressure differential, coil voltage |
| 2/2 normally open | Blowdown on many screw compressors, so the machine vents on power loss | Normal state, which must match the original |
| 3/2 pilot (loading) solenoid | Sends control air to open or close the inlet valve | Port functions (supply, outlet, exhaust), as mixing them up reverses loading |
| Pilot-operated diaphragm | Larger drain and dryer valves | Minimum operating differential, often a few tenths of a bar; it will not shift at zero pressure |
| Timer or level-controlled drain | Receivers, aftercoolers, dryers | Open time, interval, strainer condition, level probe |
Coil checks with a multimeter:
- Voltage at the plug. Measure at the plug when the controller commands the valve, not at the panel. Coils are commonly rated for about −15% to +10% of nominal voltage. That is 20.4–26.4 V for a 24 V DC coil and 195.5–253 V for a 230 V AC coil. Low-voltage periods on rural feeders can leave a coil too weak to pull in.
- DC coil resistance. The resistance should be close to V² ÷ P. A 24 V, 8 W coil gives 24 × 24 ÷ 8 = 72 Ω, and a 24 V, 12 W coil gives 48 Ω. An open circuit means the coil has burnt out. A reading well below the calculated figure means shorted turns.
- AC coil resistance. The DC resistance of an AC coil is far below V² ÷ VA, because inductance does most of the current limiting. Compare it with a known good coil of the same part number instead.
- Type mismatch. An AC coil fed with DC overheats and burns out. A DC coil fed with AC buzzes and may not hold. An AC coil whose plunger is jammed keeps drawing inrush current and burns out within minutes, so free the plunger before you fit a new coil.
- Magnetic pull. With the coil energised, a steel screwdriver tip held at the coil nut should be pulled firmly. A coil that is energised and pulling while the valve still does not shift points to a stuck plunger, a contaminated seat or too little differential for a pilot-operated valve.
Fault-finding by measurement
The symptom table above tells you which device to suspect. This table gives the reading that confirms the suspicion before you buy a part.
| Symptom | Suspect | Measure | Healthy | Points to the device |
|---|---|---|---|---|
| “Pressure sensor fault” alarm | Pressure transducer loop | Loop current | 4.00 mA when vented | 0 mA, or below about 3.6 mA, with supply present |
| Pressure reads 0.3–1 bar high or low | Transducer drift or wrong range | Current compared with a test gauge | Within ±0.08 mA of 4 + 16 × P ÷ span | Steady offset at all pressures (zero drift), or offset growing with pressure (wrong span) |
| Temperature shows maximum or a sensor fault | RTD open circuit | Resistance at the sensor leads | About 110 Ω at 25 °C (PT100) | OL on the meter |
| Temperature 2–5 °C above a separate thermometer | Connection resistance | Resistance at the sensor compared with resistance at the controller end | Difference below 0.5 Ω | Each extra 1 Ω adds about 2.6 °C |
| Will not load | Loading solenoid | Voltage at the plug on a load command, then coil resistance | Rated voltage within −15% to +10%; resistance close to V² ÷ P for DC | Voltage present but the coil is open circuit, or pulls in without shifting the valve |
| Air escaping from the blowdown silencer while loaded | Blowdown solenoid | Coil state when loaded, compared with the valve’s normal state | An NO valve is energised when loaded | Correct signal but still venting: worn seat or seal |
| Drain never discharges | Drain solenoid or timer | Voltage pulse at the set interval | Pulse arrives and the valve clicks | Pulse present but no discharge: blocked strainer or failed valve |
| Dew point alarm while downstream equipment stays dry | Dew point sensor drift | Comparison with a calibrated portable meter at the same point | Within the sensor’s stated accuracy | Sensor reads wet while the portable meter reads dry |
| Separator differential alarm with a new element | Differential pressure switch or sensing line | Gauge readings either side of the element | Below the alarm setting | Real differential low: switch or blocked sensing line |
Fit a sensor, solenoid or coil only once its reading has been confirmed as wrong. For control boards, see compressor controllers, and for drain valves and filter elements, see filter elements and drains.
How Reddy Compressors helps
We are an independent multi-brand compressor specialist based at Tarnaka, Secunderabad, working on Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic, Gardner Denver, Sullair, Doosan, Kirloskar, Bobcat and many other makes. We are not an authorised distributor of these brands; we identify parts from your machine and confirm fit before quoting. We have tie-ups with all major and reputed brands.
- Fault finding on site to separate a device failure from wiring, supply, controller or pneumatic causes.
- Identification of pressure transducers, temperature sensors, pressure switches, solenoid valves and coils from the nameplate, controller and old part.
- Supply of matched devices, with the range, signal and connector confirmed before quoting.
- Fitting, controller set-up where required, and verification of the reading after replacement.
- Inclusion of device checks in service visits and annual maintenance contracts.
Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply spare parts across India. If a machine must stay down while a device is sourced, a rental compressor through our temporary air service can keep production running.
What to send us
- Compressor make, model and serial number, with a clear photo of the nameplate.
- Controller model or a photo of the display, plus any fault code shown.
- Photos of the old device from several sides, showing the label, connector and thread.
- For solenoids, the voltage and part number printed on the coil and valve body.
- Running hours and a short description of what the machine is doing.
- Your site location and whether you need on-site diagnosis or the part only.
Frequently asked questions
How do I know if my compressor pressure sensor is faulty?
Compare the controller reading with a calibrated gauge fitted at the same point. If they disagree by more than a small margin, or the controller shows a sensor fault, check the supply voltage, cable and connector before blaming the transducer. Measuring the loop current on a 4–20 mA sensor shows quickly whether the device is producing a sensible signal for the actual pressure.
Can I use any 4–20 mA pressure transducer on my compressor?
Only if its measuring range, supply, connector, pinout, thread and wetted materials all match what the controller expects. The controller scales the 4–20 mA signal to a set range, so a transducer with a different range will give a wrong reading even though it fits and works. Some controllers also accept only specific device types or require a parameter change.
Why does my screw compressor trip on high temperature when it is not hot?
A failed temperature sensor, a damaged cable, a loose connector or a sensor of the wrong type can all cause a false high-temperature trip. Before replacing anything, confirm the actual discharge temperature with an independent instrument. If the machine really is hot, check the oil level, cooler cleanliness, thermostatic valve and room ventilation instead.
Why will my air compressor not load?
Common causes are a failed load solenoid coil, a stuck plunger, a blocked control-air line, a sticking inlet valve or no output from the controller. Check whether the controller is commanding load, whether voltage reaches the coil and whether control air passes when the solenoid is energised. That sequence usually isolates the fault without guesswork.
What is the difference between a normally open and normally closed solenoid valve?
A normally closed valve stays shut until its coil is energised, while a normally open valve stays open until energised. On compressors the choice is often safety-related: a blow-down valve may be arranged to vent the vessel when power fails. Replacing one type with the other can stop the machine unloading or leave pressure trapped after shutdown.
Can you supply compressor sensors and solenoids outside Hyderabad?
Yes. We supply spare parts across India once the part has been identified from your nameplate, the old device and the controller details. Our service teams cover all of Telangana and Andhra Pradesh for on-site diagnosis and fitting, and project-based site support elsewhere in India can be arranged where feasible.
What does the solenoid switch on an air compressor do?
On a screw compressor, the solenoid valves are electrically operated air valves. The controller uses them to load and unload the machine, open the blowdown vent, and fire the condensate drains. On small piston compressors, “solenoid switch” usually means the pressure switch and its unloader valve. The switch stops the motor at cut-out pressure, and the unloader vents the delivery pipe so the motor restarts easily. If the unloader sticks, the motor stalls or trips on restart. If the switch contacts weld, the safety valve lifts. Check the coil voltage and the switch setting before replacing either part.
How do I test a compressor solenoid valve with a multimeter?
First isolate the machine and vent it. Unplug the coil and measure its resistance. For a DC coil it should be close to the supply voltage squared divided by the coil wattage, which is about 72 Ω for a 24 V, 8 W coil. An open circuit means the coil has burnt out. Reconnect the coil, then measure the voltage at the plug while the controller commands the valve. If the correct voltage is present and the coil pulls a screwdriver tip but the valve does not shift, clean or replace the valve body. A new coil will not fix that.
What should a 4–20 mA pressure transducer read at 7 bar?
It depends on the transducer’s range. Use I = 4 + 16 × P ÷ span. A 0–16 bar transducer reads 11.0 mA at 7 bar(g), a 0–25 bar transducer reads 8.48 mA and a 0–10 bar transducer reads 15.2 mA. With the transducer vented to atmosphere, every range should read 4.00 mA. A good device is within about ±0.5% of span, or ±0.08 mA. If the current is correct but the display is wrong, the controller’s range setting is the fault, not the transducer.
What resistance should a PT100 compressor temperature sensor read?
A PT100 reads 100.00 Ω at 0 °C, about 109.7 Ω at 25 °C, 132.8 Ω at 85 °C and 138.5 Ω at 100 °C. A PT1000 reads ten times these values. Measure with the sensor disconnected, and compare against a separate thermometer at the same point. Open circuit means the element or wire is broken. Close to 0 Ω means a short. On two-wire sensors, each ohm of terminal or cable resistance makes the reading about 2.6 °C high, which can cause early high-temperature trips.
Related pages
Field guide
Sensors and solenoids: Small parts, big consequences
Pressure transducers, temperature probes, pressure switches and solenoid valves tell the controller what is happening and carry out its decisions. A wrong reading can stop the machine, or worse, let it run outside safe limits. Replace them with the correct part and verify the reading afterwards.
| Part | Typical failure | Effect |
|---|---|---|
| Pressure transducer | Drift, water ingress, damaged cable | Wrong load and unload points, false high-pressure trips |
| Temperature probe | Open circuit, poor contact, wrong type fitted | False high-temperature trips, or no protection |
| Pressure switch | Contact wear, drifting set point | Short cycling on piston compressors, wrong pressure band |
| Loading solenoid | Burnt coil, sticking spool, blocked orifice | Machine will not load or will not unload |
| Blow-down solenoid | Leaking seat | Loss of pressure at stop, hard restart |
| Drain solenoid | Blocked strainer, burnt coil | Water carried into the system, or air wasted |
Getting the right replacement
- Match range, signal type and connection: a 0 to 16 bar 4 to 20 mA transducer is not interchangeable with a 0 to 10 V or different range unit.
- Match coil voltage (AC or DC), port size, pressure rating and normally open or normally closed function for solenoids.
- Use the correct probe type for temperature sensors; the controller is set for one type.
- After fitting, compare the controller reading with a calibrated gauge or thermometer and test the trip function.
What to send for a quotation
- Compressor make, model and serial number.
- Photo of the part showing its label, connector and fittings.
- Controller reading and the fault text.
- Coil voltage for solenoids.
Send it through find my part or the spare parts enquiry, or call +91 99590 60718 or +91 92468 88812.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.