Changing the air conditioning compressor - troubleshooting & instructions

Here you will find useful basic information and handy tips relating to the air conditioning compressor in vehicles.

The air conditioning compressor isn't just the heart of the air-conditioning system, it's also the component which is placed under the greatest strain. This page explains how the air conditioning compressor works and why it could fail. You will also find out how to diagnose faults and key points to note when replacing the air conditioning compressor in order to avoid consequential damage.

Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their work. The information provided on this website is intended for use by suitably qualified personnel only.


The air conditioning compressor is driven by the vehicle engine via a V-ribbed belt or ribbed V-belt. It compresses and transports the refrigerant in the system. There are different types of compressor design.

Operating principle

The refrigerant is sucked in in gaseous form under low pressure and low temperature from the evaporator; it is then compressed and forwarded to the condenser in gaseous form under high temperature and high pressure.


Before mounting a new compressor, always check, and if necessary top up, the oil quantity and the viscosity according to the manufacturer's instructions!


A damaged or failed compressor can manifest itself as follows:

  • Leak
  • Noise development
  • Insufficient or no cooling performance
  • Fault code in the air-conditioning control unit or engine/body control unit


There are a number of possible causes of failure:

  • Bearing damage caused by a defective clamping device or wear
  • Leaks in the compressor shaft or the housing
  • Mechanical damage to the compressor housing
  • Contact (electrical connections)
  • Electrical control valve
  • Lack of refrigerant oil
  • Lack of refrigerant
  • Solids (e.g. chips)
  • Humidity (corrosion etc.)
  • Defective tensioning elements and ancillary units



  • Does the compressor switch on, is the connector securely in place, is there voltage?
  • Check the electrical control valve and/or the control circuit.
  • Check that the drive belt is positioned correctly, undamaged, and that it is tensioned correctly.
  • Conduct a visual inspection for leaks.
  • Check that refrigerant hoses are securely in place.
  • Compare the pressures on the high and low-pressure sides.
  • Read out the fault memory in newer systems.

Troubleshooting for externally regulated compressors

Step-by-step diagnostics for an externally regulated compressor.


03:48 min


Notes on troubleshooting and replacing the compressor

It is essential to observe the following points when troubleshooting noise sources and prior to replacing the compressor:

Troubleshooting on the retaining clamps or fixing points

Check all retaining clamps and fixing points for breaks or cracks and any missing nuts or bolts.


Any vibration resulting from this may be the cause of excessive compressor noise. Observe whether the noise changes if you apply force onto the retaining clamps or fixing points using an assembly lever, for example (Figure 1). If the noise changes, the noises are most likely not being caused by the compressor.

Troubleshooting on hoses or lines

Check the hoses and lines to determine whether vibrations from the engine or pulsing refrigerant are being transmitted to the vehicle interior.


For this purpose, hold them with one hand and observe possible changes in the noise (Figure 2).

Troubleshooting on the V-belt, clamping device, tensioner pulleys, alternator freewheel clutch, and pulleys

Check the V-belt, clamping device, tensioner pulleys, alternator freewheel clutch, and pulleys for smooth running, play, and alignment.


Excessive tolerances caused by worn parts can cause noise.

Troubleshooting for excessive high pressure

Excessive high pressure (Figure 3) can cause unusual compressor noises.

If the high pressure service connection is located behind a blockage in the system, the high pressure may be higher than that indicated on the manometer. In order to diagnose such a problem, it is useful to measure the temperatures at the condenser.

Contaminated refrigerant

Excess or contaminated refrigerant causes excessive high pressure, which, in turn, may cause compressor noise.


The same applies to refrigerant where the content of non-condensable gases (air) is too high.

Troubleshooting for excessive high pressure

The condenser can also be considered a cause of unusual noise.

If insufficient air is routed through the condenser, the refrigerant cannot sufficiently condense and the high pressure increases excessively. This can result in unusual noises. Therefore, check whether the fan(s) transport(s) sufficient air through the condenser. Check the condenser and radiator fins for possible contamination as well (Figure 4).

Troubleshooting for excessive high pressure

Noise can often also be caused by contaminated expansion valves (Figure 5) or throttle valves.

This can be caused by contamination in the form of metal abrasion, for example. This reduces the refrigerant flow and causes excessive high pressure. "Defective" expansion valves can generate various buzzing, whistling, or droning noises, for example, which are clearly audible in the vehicle interior.


Noise development in the area of the air conditioning compressor

This technical clip covers where the noise comes from and how to localise and remedy the fault.


03:35 min


Workflow for fault analysis and replacement

Thorough flushing

Particles of dirt in the air-conditioning circuit can only be removed by flushing the entire system thoroughly. Depending on the contamination level, refrigerant R134a or a special flushing solution, as provided by Hella Gutmann Solutions, are suitable for flushing. Compressors, dryers (accumulators), expansion valves, and throttle valves cannot be flushed. Since system contamination (abrasion, chips) must always be assumed in the case of a compressor defect and cannot be ruled out, it is absolutely essential that the system be flushed when replacing this component.

Refrigerant oils

Follow the manufacturer’s instructions and instruction leaflets/note the viscosity.


1. Distribution of the oil
There is refrigerant oil in every component of the air-conditioning system. The oil is removed with the replaced component during repairs. It is therefore essential to refill the appropriate quantity of oil. The graphic on the right shows the average distribution of the quantities of oil within the system.


2. Observe the quantity and specification of the oil
Before mounting a new compressor or when refilling refrigerant oil, always check the oil quantity and the viscosity in the vehicle manufacturer’s instructions.


3. Correct quantity of system oil in the compressor
As one and the same compressor can potentially be used for different vehicles or systems, it is essential that the oil filling quantity be checked/corrected before mounting the compressor. For this purpose, all the oil must be siphoned off and collected. The compressor must then be refilled with the entire oil quantity specified by the vehicle manufacturer (system oil quantity). To ensure that the oil is distributed equally, the compressor must be turned 10 times by hand before installation. This also corresponds to
the instructions from the compressor manufacturer Sanden;
specifications from the vehicle manufacturer
must be observed separately in each case.

Compressor filter screen

Every air-conditioning system must be flushed when the compressor is replaced in order to remove contamination and foreign bodies from the system. If there is still contamination in the circuit despite flushing, damage can be prevented by the use of filter screens in the suction line – see the Hella Gutmann Solutions tool catalog.

Filling the air-conditioning system with refrigerant

Run-in specification for the compressor:

  • Only fill the refrigerant using the air-conditioning service station via the high-pressure side service connection to prevent pressure surges of refrigerant in the compressor.
  • Only the correct refrigerant in the quantity/specification defined by the vehicle manufacturer may be used.
  • Set the air distribution to “centre vents” and open all centre vents.
  • Set the switch for the fresh air fan to medium.
  • Set the temperature to maximum cooling.
  • Start the engine (without running the air-conditioning system) and run the engine for at least 2 minutes without interruption at idle speed.
  • While at idle speed, turn on the air-conditioning system for approx. 10 seconds, then turn off the air-conditioning system for approx. 10 seconds. Repeat this procedure at least 5 times.
  • Carry out a system check.

Leak detector

Compressor damage is also caused by a lack of refrigerant. It is therefore recommended that the air-conditioning system be maintained regularly and that contrast agent be added to the system if necessary. There are various methods of doing this – see the Hella Gutmann Solutions tool catalog. Document the use of contrast agent in the vehicle. This prevents the system from being overfilled – in extreme cases, overfilling could damage the compressor.


Always replace all O-rings and coat with refrigerant oil before installation. Before mounting a new compressor, always check, and if necessary top up, the oil quantity and the viscosity according to the manufacturer's instructions! The entire air-conditioning system must be flushed and the consumables and non-flushable components must be replaced when the compressor is replaced!


Oil cannot be drained from Denso 5SE/5SL and Visteon VS16 compressors due to their design. These are prefilled with the required system oil fill quantity. Observe the special/separate product and installation notes.


Compressor replacement – the expert way

Changing/replacing an air conditioning compressor in the vehicle air conditioning system.

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