Compressor runs constantly

Causes, most likely first

The order is approximate and based on common faults in the field. Always measure before replacing anything.

1. 1
   Flowchart D

   The load is higher than the system is designed for.

   Remedy: Reduce the load: doors, seals, insulation and products. Recalculate the load if the problem persists.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Check doors, seals and the cold-room envelope

   Warm, moist air that leaks into a cold room gives ice on the evaporator, frost on the walls and a higher load. Doors and penetrations are the usual weak points.

   1. Test the inside release first. Before you work inside, check from inside that the door opens with the emergency release, even when it is locked from outside. Test the person-trapped alarm button if there is one.

      ✓ Check: The door opens from inside at once, and the alarm sounds where it should.

   2. Inspect the gaskets. Look for tears, flattened or hard sections and ice on the gaskets. Close the door on a strip of paper at several points around the frame and pull it out.

      ✓ Check: The paper is held firmly all the way around; places where it slides out easily need a new or adjusted gasket.

   3. Check closing and door alarm. Open the door and let it go: check that the closer or self-closing hinges shut and latch it. Leave it open and check that the door-open alarm sounds after its set delay.

      ✓ Check: The door closes and latches by itself, and the alarm works.

   4. Check frame and relief port heating. Feel the door frame and, on freezer rooms, the pressure relief port: they should be warm and free of ice and condensation. If not, test the heater circuit (isolate, lock off and prove dead before you touch its terminals; see the guide on heater cables).

      ✓ Check: Frame and relief port are warm and dry, and the relief port flap moves freely.

   5. Check strip curtains. Check that the strips overlap, hang down to the floor and are not missing, torn or tied aside.

      ✓ Check: The curtain closes the whole opening.

   6. Inspect panel joints and penetrations. Look for frost or ice on the inside and damp on the outside at panel joints and pipe or cable penetrations; an IR camera makes this quicker. Check that penetrations are sealed and that the vapour barrier on the warm side is intact.

      ✓ Check: No frost spots or damp patches, and every penetration is sealed.

   Compare cooling load with system capacity

   If a room never reaches its temperature, or the plant cycles all the time, compare the actual load with what the plant can deliver at the conditions you measure.

   1. Record conditions. Measure room temperature and humidity, the surrounding temperature outside the room, and the temperature of products as they come in. Note evaporating and condensing temperatures.

      ✓ Check: You have a set of values measured at the same time.

   2. Collect room and use data. Note room size, insulation type and thickness, product quantity per day and its entry temperature, door openings, people, lighting, fans and defrost.

      ✓ Check: Every load source has a value or a reasoned estimate.

   3. Calculate the load. Calculate the load per 24 hours with the Kylbehov app or a recognised method (for example the cooling load chapter in the ASHRAE Refrigeration Handbook), and divide by the planned run time per day, which leaves time for defrost.

      ✓ Check: You have the required capacity in kW at the room's design conditions.

   4. Find the actual capacity. Read evaporator capacity at the actual temperature difference between room air and evaporation, and compressor capacity at the measured evaporating and condensing temperatures, from the manufacturers' data. Note that rated capacities apply to standard conditions.

      ✓ Check: You have evaporator and compressor capacity at the real conditions; the lower one limits the system.

   5. Compare with run time. Read compressor run time and starts per hour from the controller log. Compare required and actual capacity.

      ✓ Check: Capacity below the load, or near-continuous running, shows undersizing or excess load; very short cycles and many starts show oversizing.

   6. Decide on measures. Reduce the load first (doors, curtains, product temperature, envelope), then consider more capacity or a different control strategy. Write down the comparison.

      ✓ Check: A written comparison of load and capacity with the chosen measure.

2. 2
   Flowchart B

   Too little refrigerant reaches the evaporator.

   Remedy: Troubleshoot the low pressure.

   Continue with: System trips on low pressure →

3. 3
   Flowchart E

   The evaporator is iced up, dirty, has a stopped fan or is too small.

   Remedy: Troubleshoot the iced evaporator, check the fans and clean the coil. Check the evaporator capacity against the load.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Defrost and clean an evaporator

   A heavily iced or dirty evaporator lets too little air through, so the room gets warm and the coil ices up further. Defrost it gently and clean it without damaging fins or tubes.

   1. Protect the goods and isolate. Move or cover food near the evaporator. Isolate, lock off and prove dead the fans and defrost heaters: fans can start automatically.

      ✓ Check: The meter shows 0 V on the fan and heater terminals and the fans cannot start.

   2. Melt the ice. Start a manual defrost or melt the ice with warm water at low pressure. Never chip the ice with a screwdriver or sharp tool: punctured tubes release refrigerant. Make sure the tray and drain can take the melt water.

      ✓ Check: The coil is free of ice and no tubes or fins are damaged.

   3. Clean the coil. Spray a food-safe coil cleaner on the fins, let it work for the time on the data sheet and rinse with low-pressure water in the direction of the fins. Do not use a high-pressure washer. Straighten bent fins with a fin comb.

      ✓ Check: Light passes evenly through the coil and the rinse water runs clean.

   4. Clean the drip tray and drain. Clean and rinse the drip tray and check that the drain runs freely. Sanitise surfaces that can drip onto food.

      ✓ Check: Water poured into the tray drains away quickly.

   5. Restore power and check the fans. Refit the covers, remove your lock and switch on. Check that all fans run smoothly in the right direction without touching the shroud.

      ✓ Check: All fans run quietly and blow air through the coil in the right direction.

   6. Check the result and find the cause. Watch the room temperature come down and check the next automatic defrost. If the coil ices again, check the defrost system, the door seals and the drain heater.

      ✓ Check: The room reaches its setpoint and the coil is clear after the next defrost.

   Check a condenser or evaporator fan motor

   A fan that stops, runs slowly or turns the wrong way gives high pressure on the condenser side, or low pressure and ice on the evaporator side. Check the supply, the control signal, the motor and the fan itself.

   1. Look and listen with the unit running. Watch the fan from a safe distance: does it run, at what speed, and in the right direction (air through the coil the way the arrow on the housing or the manual shows)? Listen for bearing noise and look for ice, dirt or a damaged blade.

      ✓ Check: You know whether the fan is stopped, slow, noisy or turning the wrong way.

   2. Check the supply and the control signal. Measure the voltage at the motor terminals or the EC fan connector with the fan called for. For an EC fan also measure the control signal (0–10 V or PWM) and read any status lamp or fault code on the fan electronics. Check fan delay settings and thermostats in the controller: a fan that is held off on purpose is not faulty.

      ✓ Check: Supply voltage is within the rated range on the nameplate, and the control signal asks for the fan to run; otherwise the fault is upstream (contactor, fuse, controller output, wiring).

   3. Isolate, lock off and make sure the fan cannot start. Switch off and lock the isolator, prove the circuit dead with a tested two-pole tester, and wait for the fan to stop completely. Remember that an EC fan can restart on its own when the signal returns, and that a fan wind-milling in a draught can generate voltage.

      ✓ Check: 0 V on the motor terminals and the blades are blocked or stationary.

   4. Turn the fan by hand and check it mechanically. Turn the impeller by hand: it should spin freely and evenly, without scraping, axial play or a rough bearing. Check that the blades are tight on the shaft and not cracked, and that the guard and the coil are free of ice, dirt and loose objects.

      ✓ Check: The fan turns freely and quietly and the blades are undamaged; a stiff or rough motor must be replaced.

   5. Measure the motor (AC motors). With the motor disconnected, measure the winding resistance between the terminals and compare the windings with each other and with the data sheet; test the run capacitor (see the capacitor guide). Measure the insulation resistance to earth with an insulation tester – never with the fan electronics or an EC motor connected. Check that the motor's internal thermal protection is closed when the motor is cold.

      ✓ Check: Windings are equal where they should be and match the data sheet, insulation resistance is well above the limit your electrical rules require, and the thermal protection is closed when cold.

   6. Restore, run and measure the current. Reconnect from your photo, fit guards and covers, remove the lock and start. Measure the running current on each phase and compare it with the nameplate, and check the direction and the air flow.

      ✓ Check: The fan runs smoothly in the right direction, the current is at or below the nameplate value and the pressures settle (head pressure falls on a condenser, suction pressure rises and ice clears on an evaporator).

   Compare cooling load with system capacity

   If a room never reaches its temperature, or the plant cycles all the time, compare the actual load with what the plant can deliver at the conditions you measure.

   1. Record conditions. Measure room temperature and humidity, the surrounding temperature outside the room, and the temperature of products as they come in. Note evaporating and condensing temperatures.

      ✓ Check: You have a set of values measured at the same time.

   2. Collect room and use data. Note room size, insulation type and thickness, product quantity per day and its entry temperature, door openings, people, lighting, fans and defrost.

      ✓ Check: Every load source has a value or a reasoned estimate.

   3. Calculate the load. Calculate the load per 24 hours with the Kylbehov app or a recognised method (for example the cooling load chapter in the ASHRAE Refrigeration Handbook), and divide by the planned run time per day, which leaves time for defrost.

      ✓ Check: You have the required capacity in kW at the room's design conditions.

   4. Find the actual capacity. Read evaporator capacity at the actual temperature difference between room air and evaporation, and compressor capacity at the measured evaporating and condensing temperatures, from the manufacturers' data. Note that rated capacities apply to standard conditions.

      ✓ Check: You have evaporator and compressor capacity at the real conditions; the lower one limits the system.

   5. Compare with run time. Read compressor run time and starts per hour from the controller log. Compare required and actual capacity.

      ✓ Check: Capacity below the load, or near-continuous running, shows undersizing or excess load; very short cycles and many starts show oversizing.

   6. Decide on measures. Reduce the load first (doors, curtains, product temperature, envelope), then consider more capacity or a different control strategy. Write down the comparison.

      ✓ Check: A written comparison of load and capacity with the chosen measure.

   Continue with: Evaporator keeps icing up →

4. 4

   The condenser is dirty or its fans do not run, so the condensing pressure is high and the capacity drops.

   Remedy: Measure the condensing pressure and compare with the outdoor temperature. Clean the condenser and check the fans.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Connect gauges and read pressures

   Correct pressures are the basis of almost every fault diagnosis. Connect the gauges so that no refrigerant is released and the reading can be trusted.

   1. Choose gauges and protection. Check the refrigerant and the system PS on the nameplate. Choose gauges and hoses rated above PS and suitable for the refrigerant; CO2 needs at least 130 bar on the high side. Use tools rated for flammable refrigerants on A2L/A3 systems. Wear goggles and gloves.

      ✓ Check: Gauges and hoses are marked for the refrigerant and a pressure above the system PS.

   2. Inspect hoses and couplers. Check hose seals and that the low-loss couplers are closed. Zero the digital manifold at atmospheric pressure.

      ✓ Check: Seals are intact, couplers closed and the gauges read zero when open to air.

   3. Connect to the service ports. Remove the caps and fit the couplers to the suction port and the discharge or liquid port by hand, then open them. Keep away from ignition sources on A2L/A3 systems.

      ✓ Check: The couplers are tight and a leak detector finds nothing at the connections.

   4. Remove air from the hoses. Evacuate the hoses with a vacuum pump, or use short low-loss hoses that do not need purging. Never purge hoses by blowing refrigerant to the air.

      ✓ Check: No air is in the hoses and no refrigerant has been released.

   5. Read and convert. Read the pressures in stable operation and note whether they are gauge or absolute. Convert to saturation temperature for the right refrigerant: dew point for suction and bubble point for liquid with zeotropic blends. On CO2 above 31 °C / 73.8 bar (the critical point) there is no saturation temperature on the high side.

      ✓ Check: You have pressures and saturation temperatures for the correct refrigerant.

   6. Disconnect without loss. Close the couplers, remove them and refit the caps, which are a secondary seal. Leak test the ports.

      ✓ Check: The caps are on and no leak is detected at the ports.

   Clean a condenser or gas cooler

   A dirty condenser or gas cooler raises the condensing pressure, the power consumption and the risk of high pressure trips. Clean it, straighten the fins and check that air can flow freely.

   1. Measure before you clean. With the system running, note the condensing pressure (or gas cooler outlet temperature on CO2) and the air-on temperature, so that you can see the effect afterwards.

      ✓ Check: Pressure and air temperature are recorded.

   2. Isolate and lock off. Isolate the condensing unit and fans, lock and tag the isolator and prove dead at the fan motor terminals. Fans can otherwise start automatically. On roofs, use fall protection.

      ✓ Check: 0 V at the fan terminals and the fans have stopped.

   3. Remove loose dirt. Brush off leaves and fluff from the air-entering side with a soft brush along the fins. Blow or rinse from the air-leaving side, against the normal airflow, so dirt is pushed out rather than in.

      ✓ Check: The surface of the coil is free of loose dirt.

   4. Clean with coil cleaner and water. Protect fan motors and electrical parts from water. Apply a coil cleaner suitable for the fin material, follow its safety data sheet (gloves and goggles) and rinse with low-pressure water against the normal airflow. Never use a high-pressure washer, which flattens the fins.

      ✓ Check: The rinse water runs clear and the fins are undamaged.

   5. Straighten fins and check clearances. Straighten bent fins with a fin comb of the right pitch. Check that nothing blocks the air inlet or outlet and that warm air cannot recirculate to the inlet (walls, snow, other units).

      ✓ Check: Fins are straight and the free distances around the unit match the installation instructions.

   6. Restart and compare. Remove your lock, restore power and let the system stabilise. Compare the condensing pressure and the air-on temperature with the values before cleaning.

      ✓ Check: The condensing temperature is lower at a similar air temperature.

   Check a condenser or evaporator fan motor

   A fan that stops, runs slowly or turns the wrong way gives high pressure on the condenser side, or low pressure and ice on the evaporator side. Check the supply, the control signal, the motor and the fan itself.

   1. Look and listen with the unit running. Watch the fan from a safe distance: does it run, at what speed, and in the right direction (air through the coil the way the arrow on the housing or the manual shows)? Listen for bearing noise and look for ice, dirt or a damaged blade.

      ✓ Check: You know whether the fan is stopped, slow, noisy or turning the wrong way.

   2. Check the supply and the control signal. Measure the voltage at the motor terminals or the EC fan connector with the fan called for. For an EC fan also measure the control signal (0–10 V or PWM) and read any status lamp or fault code on the fan electronics. Check fan delay settings and thermostats in the controller: a fan that is held off on purpose is not faulty.

      ✓ Check: Supply voltage is within the rated range on the nameplate, and the control signal asks for the fan to run; otherwise the fault is upstream (contactor, fuse, controller output, wiring).

   3. Isolate, lock off and make sure the fan cannot start. Switch off and lock the isolator, prove the circuit dead with a tested two-pole tester, and wait for the fan to stop completely. Remember that an EC fan can restart on its own when the signal returns, and that a fan wind-milling in a draught can generate voltage.

      ✓ Check: 0 V on the motor terminals and the blades are blocked or stationary.

   4. Turn the fan by hand and check it mechanically. Turn the impeller by hand: it should spin freely and evenly, without scraping, axial play or a rough bearing. Check that the blades are tight on the shaft and not cracked, and that the guard and the coil are free of ice, dirt and loose objects.

      ✓ Check: The fan turns freely and quietly and the blades are undamaged; a stiff or rough motor must be replaced.

   5. Measure the motor (AC motors). With the motor disconnected, measure the winding resistance between the terminals and compare the windings with each other and with the data sheet; test the run capacitor (see the capacitor guide). Measure the insulation resistance to earth with an insulation tester – never with the fan electronics or an EC motor connected. Check that the motor's internal thermal protection is closed when the motor is cold.

      ✓ Check: Windings are equal where they should be and match the data sheet, insulation resistance is well above the limit your electrical rules require, and the thermal protection is closed when cold.

   6. Restore, run and measure the current. Reconnect from your photo, fit guards and covers, remove the lock and start. Measure the running current on each phase and compare it with the nameplate, and check the direction and the air flow.

      ✓ Check: The fan runs smoothly in the right direction, the current is at or below the nameplate value and the pressures settle (head pressure falls on a condenser, suction pressure rises and ice clears on an evaporator).

   Continue with: System trips on high pressure →

5. 5
   Flowchart A

   Wrong setpoint or faulty sensor (on a cold wall, in a draught or defective).

   Remedy: Compare the sensor with a calibrated thermometer and move or replace it. Check the setpoint and neutral zone.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Check a sensor against a reference

   A temperature or pressure sensor that reads wrong makes the controller regulate wrong: wrong superheat, wrong defrost, false alarms. Compare it with a calibrated reference before changing settings or parts.

   1. Fit the reference next to the sensor. Fasten a calibrated thermometer probe beside the temperature sensor on the same pipe or in the same air stream, with heat-conducting paste and insulation. For a pressure sensor, connect a calibrated gauge to a service port at the same point in the circuit. Wear goggles and gloves when connecting to the refrigerant circuit.

      ✓ Check: The reference reading is steady for a few minutes.

   2. Compare with the controller reading. Read the live value of the sensor in the controller display or service tool at the same time as the reference.

      ✓ Check: The difference is within the tolerance given by the sensor or controller manufacturer (for a temperature sensor typically within about 1 K).

   3. Check offsets and sensor type. In the controller parameter list, check that the sensor type (e.g. NTC, PT1000, 0–10 V, 4–20 mA), the pressure range and any calibration offset match the fitted sensor. Record the original values before changing anything.

      ✓ Check: Sensor type, range and offset in the controller match the sensor data sheet.

   4. Measure the sensor resistance. Isolate and lock off the controller, disconnect the sensor and measure its resistance with a multimeter while the reference shows the temperature at the sensor. Compare with the manufacturer's resistance table. A PT1000 reads 1000 Ω at 0 °C; an ice-water bath gives a simple 0 °C check point.

      ✓ Check: The resistance matches the table value for the reference temperature; OL or 0 Ω means a broken or shorted sensor or cable.

   5. Check the sensor location and mounting. Inspect the pipe clamp or pocket: firm metal-to-metal contact, heat-conducting paste, insulation against the surrounding air and the correct position (e.g. evaporator outlet, not in the air stream of a fan).

      ✓ Check: The sensor sits firmly at the correct point and is insulated.

   6. Reconnect and confirm. Reconnect the sensor, restore power and compare the reading with the reference once more. Replace a sensor that is out of tolerance with one of the same type.

      ✓ Check: Controller and reference agree within the tolerance.

   Check controller settings and alarm log

   Many faults that look like hardware faults are wrong settings or alarms that keep repeating. Read the alarm log and compare the settings with the manufacturer's parameter list before you change anything.

   1. Read the alarm log. Open the alarm menu with the keypad or service tool and note every recent alarm code with date and time. Look up each code in the manufacturer's manual.

      ✓ Check: You have a list of alarms with times and know what each code means.

   2. Check the clock. Check the controller's date and time. A wrong clock shifts scheduled defrosts and night setback and makes the alarm log misleading.

      ✓ Check: The controller clock shows the correct local date and time.

   3. Record all settings before changing anything. Write down, photograph or export setpoints, differentials, neutral zone, minimum run and off times, anti-short-cycle timer, defrost interval and duration, and the operating mode.

      ✓ Check: The service report holds a complete copy of the original settings.

   4. Compare with the parameter list. Compare each value with the manufacturer's parameter list and the plant documentation. Look in particular for a differential that is too small (short cycling), timers set to zero and setpoints that do not match the application.

      ✓ Check: Every deviation is noted with the original and the recommended value.

   5. Check the safety limits. Compare low and high pressure cut-outs and temperature limits in the controller with the manufacturer's values. Never set a safety limit beyond the manufacturer's values, and the high pressure cut-out must stay below the safety valve set pressure.

      ✓ Check: All safety limits are within the manufacturer's values.

   6. Change one value at a time and test. Correct one setting at a time, check the operating mode, and let the system run long enough to see the effect. Record every change in the service report.

      ✓ Check: The system runs as expected and the alarm does not return.

6. 6

   The contactor contacts have welded, so the compressor runs although the controller says stop.

   Remedy: Isolate the supply and replace the contactor. Find out why it was overloaded.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Safe isolation and lock-off

   Before you open a panel or touch electrical parts, isolate every supply, lock it off and prove that the circuit is dead. Electrical work is for qualified personnel only.

   1. Identify all supplies. Use the wiring diagram to find every supply: main feed, separate control supply, heaters, alarm and UPS supplies, and inverters that can feed back.

      ✓ Check: You know every isolation point for the equipment you will work on.

   2. Switch off, lock and tag. Switch each isolator to OFF, fit your own padlock and attach a tag with your name and date. Keep the key on you.

      ✓ Check: Every isolator is locked in OFF and cannot be switched on by someone else.

   3. Prove the tester. Check the two-pole voltage tester (EN 61243-3) on a proving unit or a known live source.

      ✓ Check: The tester shows the voltage of the known source.

   4. Prove dead. Measure phase to phase, each phase to neutral and each phase to earth at the point of work, on both line and load side.

      ✓ Check: The tester shows 0 V on every combination.

   5. Prove the tester again. Check the tester on the proving unit once more, to be sure it did not fail during the measurement.

      ✓ Check: The tester again shows the voltage of the known source.

   6. Deal with stored energy. On an inverter, wait the discharge time printed on the drive and measure the DC bus. Discharge start and run capacitors through a resistor of about 20 kΩ, 5 W with insulated leads, never with a screwdriver.

      ✓ Check: 0 V DC on the DC bus and across every capacitor.

   Test a contactor

   Worn, welded or loosely connected contactor contacts cause single-phasing, overheating and motor damage. Check the contacts, the coil and the voltage drop, and replace a worn contactor. Electrical work is for qualified personnel only.

   1. Isolate and lock off. Switch off and lock the isolator, attach a tag and prove dead with a two-pole voltage tester on all line and load terminals, also the coil supply, which can come from a separate control circuit.

      ✓ Check: 0 V on all poles, the coil terminals and to earth.

   2. Inspect the contacts and the mechanism. Remove the arc cover if the design allows. Look for heavily pitted, burnt or welded contacts, and discoloured terminals or melted plastic. Press the armature by hand (power off) and check that it moves freely and springs back.

      ✓ Check: Contacts are without heavy pitting or welds, and the armature moves freely.

   3. Check the coil. Disconnect one coil lead and measure the coil resistance. Compare with the manufacturer's data or a new contactor of the same type.

      ✓ Check: The coil is neither open (OL) nor near 0 Ω.

   4. Check the coil voltage and pull-in. Reconnect, remove your lock, restore power and use the CAT III/IV meter to measure the voltage across the coil when the controller calls for operation.

      ✓ Check: The coil voltage matches the rating printed on the coil, and the contactor pulls in firmly without humming or chattering.

   5. Measure the voltage drop across each pole. With the motor running, measure from line to load terminal across each closed pole.

      ✓ Check: The drop is close to 0 V and about equal on all poles; a pole with clearly higher drop has worn contacts.

   6. Tighten or replace. Isolate and lock off again. Tighten all terminals to the manufacturer's torque. Replace a worn contactor with one of the same coil voltage and at least the same motor rating (AC-3), then restart and measure the current on each phase.

      ✓ Check: Terminals are tight, and the motor current is about equal on all phases after restart.

7. 7

   A relay or output on the control board is stuck closed.

   Remedy: Measure the output with the controller in stop. Replace the relay or the board.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Trace the control and safety chain

   When a compressor or fan does not get a start signal, follow the control voltage through the safety chain in the wiring diagram to the point where it disappears. Live measurements in an open panel are for qualified personnel only.

   1. Get the wiring diagram. Find the diagram for the unit and mark the path from the control supply through fuses, safety devices (HP/LP switches, motor protection, thermal protection) and controller outputs to the contactor coil. Use a CAT III/IV meter with insulated probes.

      ✓ Check: The diagram matches the unit's model and serial plate, and you know the order of the devices.

   2. Check the control supply. Measure the voltage at the secondary of the control transformer or power supply and check the control fuses.

      ✓ Check: The rated control voltage (e.g. 24 V AC or 230 V AC) is present and the fuses are intact.

   3. Follow the chain device by device. Measure from each terminal in the chain to neutral or control common, in the order of the diagram. Never bridge a safety switch to make the system run.

      ✓ Check: You find the point where voltage is present on the input but 0 V on the output: that device is open.

   4. Find out why the device is open. Read the device: is the HP switch tripped and does it have a manual reset, is the motor protection tripped, is a door or flow switch open? Check the cause (pressure, current, temperature) before resetting.

      ✓ Check: The cause of the trip is known, not only the device.

   5. Check the controller output and relays. With a call for operation active, measure the output at the controller relay terminal. A relay that is energised but has no voltage on its output, or that stays closed without a call, is faulty.

      ✓ Check: The output follows the call: voltage when the controller calls and none when it does not.

   6. Repair and test. Isolate and lock off before replacing a part or tightening terminals. Restore power, reset and check that the chain closes and the contactor pulls in.

      ✓ Check: The system starts on call and stops when each safety device opens.

8. 8
   Flowchart C

   The compressor does not pump as it should.

   Remedy: Troubleshoot the compressor that runs but does not cool.

   Continue with: Compressor runs but does not cool →