Change air conditioning compressor

Air conditioning compressor replacement instructions - comprehensive technical information

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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.

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

How the air conditioning compressor works

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!

Compressors without magnetic clutch

General information

For some years now, so-called "clutchless", externally controlled, variable compressors have been in use. All reputable compressor manufacturers use a wide range of basic types. The most common types on the market are listed here: Denso, with types 6SEU and 7SEU; Sanden, with types PXE 13 and PXE 16. However, Delphi/Harrison is also represented with the CVC7 series, which is very similar to the design of the V5 compressor. This generation of compressors is used by almost all vehicle manufacturers (Audi, BMW, Citroen, Seat, VW, Opel....).

Externally controlled means that the displacement of the compressor is determined via a built-in control valve controlled by the air-conditioning control unit, depending on the most diverse system parameters, such as outside/desired temperature, high/low pressure, speed or engine load. "Clutchless" means that the compressor no longer has an electromagnetic clutch. And this in turn means that the compressor is permanently driven via the pulley and works even when the air-conditioning system is switched off. However, the performance is then reduced to just a few %.

Function example — Functional example (drawing)

Function

The pulley unit of the compressor consists, for example, of a drive plate and the actual pulley (drawing). The drive plate consists of a rubber element and forms the connection between the pulley and the compressor shaft. Firstly, it acts as a vibration damper and also protects the compressor and other driven units from overload or damage. If the compressor blocks, for example, the transmission forces between the pulley and the drive plate greatly increase in the area of the rubber element.

Compressors without magnetic clutch — Figure 1

Depending on the compressor manufacturer or on the type of compressor, the connection is interrupted either by deformation of the rubber element or by triggering of the "overload protection". The pulley then only runs idle. This prevents damage to the belt or to other units driven by the belt.

The control valve (Fig. 1) is located in the compressor and receives its pulse width modulated signals (PWM) from the air-conditioning control unit. The current that is fed from the control unit to the control valve and that ultimately determines the performance of the compressor can, by using a diagnostic device, be displayed as a measured value block.

Compressors without magnetic clutch — Figure 2

Clutchless compressors also have a safety valve (Fig. 2), which is designed to protect the compressor and the other components of the air-conditioning system from excessive pressure. The valve usually triggers between 35 and 45 bar (depending on the compressor manufacturer). The valve only opens until the excess pressure has been released. It then closes again to prevent the entire amount of refrigerant from being released into the atmosphere. If the foil of the valve is damaged, it can be assumed that the valve has "triggered".

Diagnostics

The pulleys and their rubber elements designed as "overload protection" are constructed differently, depending on the type of compressor. And depending on the particular type, different methods can be used to determine whether the "overload protection" has been triggered:

1

Rubber abrasions are visible on the inside of the pulley (Fig. 3). The compressor shaft is no longer being driven. The pulley or rubber element can be replaced if the compressor can be easily turned.

2

The overload protection has triggered the drive plate (Fig. 4). The drive plate or rubber element can be individually replaced. Prerequisite: Compressor can be turned easily.

3

A triggered torque limiter cannot always be visually noticeable. In order to check whether the limiter has triggered, the compressor shaft is to be held with a suitable tool (Fig. 5) and the pulley turned to the left at the same time. If the pulley can be turned to the left, the limiter has triggered and the compressor must be replaced. It is not possible to replace the torque limiter on Sanden PXE 13 and PXE 16 compressors.

The maximum current sent from the control unit to the control valve is approx. 0.65 A in the Audi A3, for example, at the lowest temperature setting. This is also when the compressor reaches its maximum output. During normal operation, an average current of 0.3 A flows. The problem with newer vehicles, however, is that diagnosis outside the engine management area is not yet possible with many test devices. The use of an oscilloscope is ideal here. With the aid of suitable test probes, the PWM signal can be recorded at the plug connection of the compressor.

Diagnose

The oscilloscope should be set to 5 V/div and 0.5 ms/div. The individual operating modes can now be displayed on the oscilloscope screen while the engine is running.
At the lowest temperature setting ("Lo"), a square wave signal with a duty cycle of approx. 75% is displayed (Fig. 7). The duty cycle results from the ratio of the pulse width band to the signal interval -C- (in this case 75% duty cycle, 25% switch-off time).

Diagnose

At the same time, the level of the vehicle electrical system voltage (approx. 13.5 V) can be read from the volt divisions (A=5 V). The voltage value displayed as a number (9.8 V) is only an average value. The pulse width depends on the required refrigerating capacity and on the vehicle electrical system voltage. The current to the control valve is "regulated" by the control unit via the section marked -B-. Depending on the setting of the operating unit and the ambient influences (e.g. outside temperature), the pulse width of the square wave signal is changed or the control valve is controlled in such a way that the compressor output required to reach the desired temperature is achieved. Fig. 8 shows how the compressor at the "High" temperature setting is reduced.

Diagnose

Fig. 9 was pictured in "Econ" mode (compressor off) and shows no signal. This method can be used to determine the extent to which a signal change has been made by the control unit. If there is a plausible change in the signals, but no change in the discharge temperature or reduction in the interior temperature, there is probably a compressor fault.

There are also diagnostic devices on the market with which it is possible to generate a PWM signal with various pulse durations. This makes it possible to determine whether activation of the compressor leads to a change in the refrigerant pressure. This serves as a basis for determining the extent to which the compressor is still working properly.

Diagnostic device

A function test using a PWM signal can also be carried out with a function generator
(Fig. 10). However, in order to perform such a test, it is absolutely essential to connect a "load" on the control unit side of the air-conditioning system that corresponds to that of an electronic control valve. Otherwise, the control unit recognises an error in the system and saves it in the error memory. This can lead to malfunctions or to failure of the system. In such a case, the error memory has to be read out and deleted using a diagnostic device.

Compressors are often the subject of premature complaints in connection with noises and other problems associated with the air-conditioning system. In many cases, it turns out that the compressor is OK or that the cause of the fault does not actually come from the compressor itself. For this reason, all components of the system should always be included as the cause of the fault. Noise can be caused not only by the compressor, but also by its mounting, the drive, the expansion valve or piping and cabling. The wrong amount of refrigerant can also be responsible for various noises.

Furthermore, the oil provides important information about any possible damage:

If the oil in the compressor or in the system has taken on a red colour, this may be due to the presence of too much moisture.
Black-coloured oil indicates a defective compressor.
Silver-grey oil should be checked for metal chips. The greyish discolouration is a sign of metal abrasion.

As the system oil quantities are becoming smaller and smaller (sometimes only 80 ml), monitoring and maintaining the oil quantity (e.g. when servicing the air-conditioning system and replacing components) are of the utmost importance.

Repair of clutchless compressors is only possible to a limited extent. And it must always be carried out with the aid of suitable tools and repair information and instructions.

It goes without saying that the assessment of the system pressures is of particular importance in the diagnosis. The default values provided by the vehicle manufacturer should be used in such instances. This also applies to the discharge temperature.

Points of reference for the assessment of the system pressures can be derived from the table below:

Assessment of the system pressures
High pressure	Low pressure	Symptoms	Possible cause	Potential solution
Normal	Normal	→ Outflowing air does not get cold	→ Too much oil in the air-conditioning system → Air or moisture in the air-conditioning system	→ Vacuum, flush and refill the air-conditioning system with oil and refrigerant → Vacuum the air-conditioning system, replace and refill the dryer
High	High	→ Low pressure line colder than evaporator → High pressure decreases when the capacitor is cooled with water → High and low pressure balance themselves out as soon as the compressor switches off and pulsate as soon as it switches on	→ Expansion valve open too wide → Too much refrigerant in the system → Capacitor dirty/blocked → Fan problems → Problem with the compressor (outlet valve/sealing)	→ Replace expansion valve → Vacuum and refill air-conditioning system → Check and clean/replace capacitor → Check fan → Check compressor, replace if necessary
Low	Low	→ Outflowing air does not get cold → Suction line is colder than the evaporator	→ Too little refrigerant in the system → Blockage on the suction side	→ Vacuum and refill air-conditioning system → Check pipework and connections, replace if necessary
High	Low	→ Ice formation on the liquid line → Ice formation on the dryer	→ Line/dryer blocked	→ Check dryer/line, replace if necessary

Air conditioning compressor faulty

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

Checking the air conditioning compressor

Function test and pressure management on the system

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.

Video on the subject

Troubleshooting for externally regulated compressors

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

03:48 min

Noises from the air conditioning compressor

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:

1

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.

Figure 1: Troubleshooting on the retaining clamps or fixing points

2

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).

Figure 2: Troubleshooting on hoses or lines

3

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.

4

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.

Figure 3: Troubleshooting for excessive high pressure

5

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.

6

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).

Figure 4: Troubleshooting on the condenser

7

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.

Figure 5: Troubleshooting on contaminated expansion valves

Video on the subject

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

Air conditioning compressor replacement

Workflow for fault analysis and replacement

1

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.

2

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.

3

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.

4

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.

5

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, 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.

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