A/C service units
Most A/C machines are separate units combined into a Refrigerant Management System (RMS). An RMS contains a recovery machine, vacuum pump, electronic scales and an LCD control panel. The unit often includes very simple programmes allowing the technician to select automatic recovery, recycle, evacuation and charging procedures. Some units include databases for
vehicle information and flushing capabilities.
The unit’s pipe work, connectors, switches and gauges are generally colour coded:
Red for the High Pressure (HP) side, also referred as ‘liquid side’ of the A/C system.
Blue for the Low Pressure (LP) side, also referred as the ‘vapour side’ of the A/C system.
Other colours are used to assist in service operations directing the technician to carry out certain procedures, e.g. yellow service hose.When the unit is not in use all valves should be closed.
An A/C system can be charged with refrigerant in liquid or vapour form. The charging process is carried out by measuring the refrigerant charge weight. This is the only approved method of charging an A/C system.
The RMS (Refrigerant Management Station) unit is capable of automatic refrigerant recovery, cycle, evacuation and recharging. The unit also has a database on the oil quantities for different manufactured vehicles, which is updatable. The unit can also flush A/C systems using appropriate adapters.
Some RMS units can be used with multiple refrigerants.
Main components of a service station
Low pressure gauge
This gauge is a compound gauge and measures the low pressure side of the air-conditioning system and is coloured blue. It will also give an indication of the vacuum depression when the system is being evacuated. If the vehicle has a suspected leak that is very slight, this gauge may not be able to detect it.
High pressure gauge
This gauge measures the high pressure side of the air-conditioning system, and is coloured red. When the air-conditioning system operation is being checked, the various modes can be seen to operate on this gauge with the different pressure alterations being indicated as they are switched. The relationship between the pressure readings on the two gauges provides a reliable guide to the functioning of the system and an indication of when problems exist.
Note – on R12 systems the couplings for connection to the service connectors are the same. Therefore, it is imperative to note the colour coding. Incorrect connection would damage components of the servicing unit. In R134a systems different couplings are used. The connectors on the high pressure and low pressure sides are of different diameters.This prevents incorrect connection or different refrigerants being mixed with one another.
Low pressure valve
This should be coloured blue to signify it is part of the low pressure side of the charging station system. It must not be overtightened. It should be turned using the fingertips until an impedance is felt (signifying it is closed) and then gently pinched to lock it.When the valve is reopened the operator should be able to ‘crack’ the locking torque without using undue pressure (with the fingertips).
High pressure valve
This should be coloured red to signify it is part of the high pressure side of the charging station. This valve must be tightened and unlocked using the same method as the other valves.
High pressure pipe
Colour coded red, it is usually of a suitable length so that a clearance can be maintained between the charging station and the vehicle. Minimising the possibility of the charging station damaging the vehicle paint work.The end connection to the vehicle system has a Schrader type valve depressor built into it.
Note – at no time should the pipe be uncoupled at the charging station while it is connected to the vehicle.
There is a hose gasket fitted to provide a good seal between the vehicle system and the charging trolley.This should be inspected for wear or damage at frequent intervals.The sealing properties of this gasket can be prolonged with the application of a small amount of refrigerant oil before use.
Low pressure pipe
Coloured blue to signify that it is part of the low pressure circuit. It is usually the same length as the high pressure pipe with the same type of end connector fitted.
The hose gasket fitted into the connector requires the same attention and maintenance as for the high pressure pipe gaskets.
Manifold assembly
This unit allows the charging station to evacuate, recharge and test the vehicle air-conditioning system in situ without disconnecting any hoses. For discharge purposes the pipe connecting the manifold to the charging station must be disconnected so that the refrigerant 12 can be discharged into a recovery station.This enables the operator to measure the amount of refrigerant oil lost from the air-conditioning system during the discharge operation.
To avoid the possibility of refrigerant entering the vacuum pump during the system discharging operation, ensure the vacuum pump valve is closed.The manifold is designed so that when the high and low pressure pipes are connected to the vehicle air-conditioning system, with both of the valves in the ‘off’ position, the gauges will read the vehicle system pressure. When the manifold is set to this position, the charging station is isolated from the air-conditioning system. The valves can then be opened, connecting the charging station to the air-conditioning system as required.
Vacuum pump valve
The vacuum pump valve’s function is to switch the vacuum pump depression to the manifold, and to isolate the vacuum pump when refrigerant is being circulated through the charging system.
Refrigerant control valve to the charging cylinder
This valve, which is normally coloured red, connects the refrigerant bottle (fitted on the rear of the charging station) to the charging cylinder. It can be finely controlled so that refrigerant can be slowly measured into the charging cylinder.
Refrigerant control valve to the manifold assembly
This red coloured valve must be treated with the same consideration as the other valves. Its function is to control the refrigerant flow to the vehicle, via the manifold. If refrigerant is allowed to flow too quickly it will boil and vaporise, reducing the vacuum depression, which is required to draw the refrigerant into the vehicle system.
Charging cylinder gauge
The gauge is connected to the top of the charging cylinder sight glass. Its function is to measure the pressure variations of the refrigerant in the charging cylinder that can then be calibrated to a reading on the plastic shroud that surrounds the cylinder. This can be done by taking a reading from the charging cylinder pressure gauge and comparing it to the table that is marked around the top of the plastic shroud. Line this reading up with the charging cylinder to indicate the pressure of the refrigerant in the measuring cylinder. There are different types of refrigerant, with the plastic shroud depicting different tables for each of the most common types.
Charging cylinder
This charging cylinder can store and deliver a specific amount of refrigerant to the vehicle airconditioning system with an accuracy of plus or minus 7 grams (4 oz). Dial-a-charge charging cylinders allow the operator to compensate for refrigerant volume fluctuations resulting from temperature variations.This allows the purchase of more economical drums of refrigerant. It also provides a greater degree of accuracy when charging the air-conditioning system, eliminating over- and undercharging, which can lead to problems when the vehicle has gone back into service.
These cylinders are provided with heating elements that allow the operator to overcome equalisation of pressure between the air-conditioning system and the charging cylinder, which reduces the time required to recharge the system. Simply ‘dial’ the calibrated plastic shroud so that the pressure reading at the top of the shroud corresponds to the gauge reading. The calibrated column that is in line with the sight glass will then show the amount of refrigerant that is in the sight glass at that pressure. The charging cylinder is now ready for the transfer of an accurately measured amount of refrigerant 12 to the vehicle air-conditioning system.
Vacuum pump
This is a very robust pump that enables the vehicle air-conditioning system to be evacuated quickly and effectively.
It requires very little routine maintenance, having its own oil circulation system that requires periodic checking according to the manufacturer’s instructions.
Vacuum pump to manifold connecting pipe
This pipe connects the vacuum pump to the vacuum pump valve on the manifold. It requires a good seal so that there are no leaks between the vacuum pump and the manifold.The connections should be periodically checked for tightness.
System evacuation
A vacuum pump may be purchased and used as a separate item. The centre or yellow service hose connects to the vacuum pump and the two hoses on the manifold gauge connect to the low pressure (LP) and high pressure (HP) sides of the system. The vacuum pump should only be attached when system gauge pressure is zero or showing a vacuum. Before using the vacuum pump check the oil via the pump sight glass is satisfactory. Start the pump and open the LP and HP service connectors to apply a vacuum.The reading on the LP gauge should steadily go into vacuum. If the vacuum pump fails to draw a vacuum then the system may have a leak. Periodically check the gauge reading and if unsatisfactory carry out a leak test. If satisfactory progress is made then evacuate for 30 minutes to a pressure of 1.006 bar. Upon satisfactory evacuation of the system shut the manifold gauge valve to the pump and check that the pressure is maintained for 10 minutes minimum, overnight if required. A slight pressure increase may be experienced if trapped refrigerant in the oil boils off.
System vapour charging
Vapour charging is carried out on the low side (blue hose) of the A/C system where during normal A/C operation vapour is flowing into the compressor.When vapour charging the compressor is running to draw vapour in. The refrigerant cylinder must be positioned so only vapour can leave which is generally the upright position with the blue valve open (vapour valve).A small amount of air is bled (purged) off from the service hoses at the manifold gauge end to ensure that air present in the hoses is removed. Some automated machines do not require air purging.The refrigerant cylinder is placed on electronic scales and weighed at the start and throughout the process of charging the system to monitor that the correct quantity of refrigerant is delivered. The A/C system is initially evacuated to achieve a deep vacuum; the refrigerant vapour will fill the system producing an initial charge level.This produces enough pressure to shut the low pressure switch/sensor, which allows the A/C compressor to engage and the A/C system to operate and draw the rest of the vapour into the system.
System liquid charging
Liquid charging is a much more dangerous process and is carried out on the high pressure (HP) side of the A/C. The HP connector is generally positioned as follows: on the compressor (early models), on the condenser, between the condenser and receiver-drier (TXV) or between the condenser and FOV. Systems that have the HP valve fitted to the compressors should not be liquid charged. This is because there is a danger of the compressor being internally damaged. The engine is not running during liquid charging. A liquid charging process uses high pressure to charge the A/C system which is obtained using either a charging unit or a charging cylinder.
System charging using a charging cylinder
The refrigerant is transferred from its cylinder to a charging cylinder. A charging cylinder as previously discussed has a calibrated shroud used for temperature compensation which is rotated until the graduated number for that refrigerant volume at that pressure is next to the sight column.The refrigerant is transferred to the charging unit by warming the source refrigerant bottle (max. 45°C) and by applying a vacuum to the charging cylinder. The heated source bottle is connected to the charging cylinder. The red ‘liquid’ valve on the source bottle is opened and then air is carefully purged out of the hoses. The valve on the charging cylinder is opened and liquid refrigerant can flow into the unit. Once the correct level is obtained the valve is closed. When the charging cylinder is filled to the required level plug the unit into a 240 V supply and heat the cylinder to about 35°C which is noted on the thermometer. The system can now be filled via the high pressure connection/liquid line. The refrigerant will flow from the red liquid valve on the charging cylinder straight into the A/C system on the high pressure side (normally into the condenser).
Note – never liquid charge into the compressor because of damage to the valve plate.
Section 1 Service preparation and precautions
When handling refrigerant or carrying out repairs to an automotive air-conditioning system it is recommended that eye protection and gloves are worn. Extreme caution must be taken not to allow any refrigerant to come into contact with the skin and especially the eyes. Liquid refrigerant (R134a) evaporates at approximately 26.3°C, and because it evaporates quickly, it freezes anything it comes into contact with. Care must also be taken to avoid breathing refrigerant or system lubricant vapour. Exposure may irritate eyes, nose and throat.
Use only approved automotive air-conditioning service equipment. Avoid carrying out airconditioning service repair work in any small unventilated area to avoid asphyxiation. We strongly recommend that all servicing technicians refer to their appropriate COSHH file for more detailed information. Ensure that protective covers are applied to the vehicle before commencing any work. The battery must be disconnected to prevent accidental starting of the engine and the possibility of personal injury if access to service connectors are close to fans, belts etc. Make sure that tools, measuring equipment and parts to be fitted are clean and dry.
Keep all necessary equipment and tools within easy reach so that the system is not left open any longer than is absolutely necessary. Before undoing any refrigerant lines, joints or connectors, clean off any dirt, moisture, oil etc. in order to prevent contamination of the system.All open connections should be capped or plugged (air tight) immediately to stop dirt, air or moisture getting into the system. Air inside the circuit will damage the system and reduce the cooling effect as it contains moisture.Any O-rings disturbed by undoing unions must always be renewed after lubricating with refrigerant oil prior to fitment. When removing O-rings from couplings, care must be taken not to scratch the sealing face. It is recommended that the receiver-drier/accumulator is replaced if the system has been open to the atmosphere for more than 4 hours (depending on the manufacturer), is physically damaged or has been in service for more than 2 years. Do not remove plugs from new components until each component is ready to be installed into the A/C system, this will limit the amount of air and moisture entering the system. When adding refrigerant oil, ensure that any filling equipment (hose, container etc.) is clean and dry.The oil container must be sealed immediately after use. To ensure the system works correctly after servicing, the system must be evacuated (vacuumed) for a minimum of 30 minutes before recharging.This will remove (by dehydration) any moisture from the system. One of the most important requirements when filling the air-conditioning system is to use clean refrigerant. Any foreign matter including air, moisture, dirt etc. in the air-conditioning circuit will have an adverse effect on refrigerant pressures and impair the system performance. After every repair or service procedure, the system must be leak checked to identify any leaks that may be present.If any leaks are found,the refrigerant within the system must immediately be recovered and the leak repaired.
Section 2 Initial system test
Step 1
Position car in workshop bay.
Switch off engine.
Open bonnet.
Remove charge port caps.
Note – it is recommended that a refrigerant identifier is used to ascertain system refrigerant
type, percentage of air and whether the refrigerant is contaminated prior to commencing.
Refrigerant identifier
A refrigerant vapour sample flows via the low side of the A/C system to an infrared sensor capable of sensing a range of refrigerants and blends. Audible and visual alarms indicate the presence of hydrocarbon-based refrigerants. The percentage of purity as a level of contamination is provided on an LCD screen.A printer port allows the connection to a printer to provide the customer with a detailed report.
Note – if the refrigerant is unknown, established not to be R12 or R134a, then the customer should be informed and advised of appropriate action. The refrigerant should be recovered into a waste bottle and the system oil and receiver/accumulator must be replaced. A system flush could also be carried out before recharging. When sampling a blend a reading will appear showing the different refrigerants and percentages e.g. the percentage of R12, percentage of R134a and % of hydrocarbons. OEMs in the UK do not recommend the use of blends but the US have a range of blends on the market. If blends are used then a fingerprint (percentage of each refrigerant) should be sampled before charging the system so the correct blend percentage of each refrigerant is known.
With the introduction of new CO2 refrigerants the analysers will eventually be updated or superseded.
Refrigerant labels
A simple method of identifying a refrigerant is to look for the A/C system manufacturer’s original label or retrofit label. These are often lost due to the vehicle engine bays being stream cleaned or body repair work being carried out. Service ports often provide evidence of whether the system is R12 or R134a but this cannot be treated as conclusive.
A comparator
A comparator and thermometer with a set of pressure gauges can be used to identify a refrigerant.The comparator or slide rule uses the pressure and temperature relationship of the refrigerant in a saturated liquid/vapour window. This method is not always accurate due to some refrigerants’ performance characteristics being closely related.
With the use of a refrigerant identifier record the refrigerant type and percentage of air in the system. Use a printer if available to connect to the analyser providing the customer with a detailed report.
Reminder
If the refrigerant NCG is 98% or greater then recover and recycle. If below then reclaim using a separate A/C machine (avoid contamination of A/C machine).
Step 2
Once the refrigerant has been identified the correct service equipment should be used.
Note – technicians must ensure that there is sufficient space for the refrigerant in the recovery or waste bottle. A simple calculation must be made. The amount of refrigerant in the system must be added to the space available inside the bottle without exceeding 80% capacity.
Disconnect the refrigerant analyser from the low side of the A/C system. Make sure the low and high side connectors are shut off. Connect the low pressure and high pressure hoses to the A/C system’s low pressure and high pressure service ports.Open hose connectors to allow refrigerant to flow to the RMS (Refrigerant Management System) (Fig. 5.10).
Record system pressures (A/C Off)
Note – if system pressure is at 0 bar go to Section 4 Evacuation.
Step 3
Using a digital thermometer take the ambient temperature reading (Fig. 5.11) near the front of the vehicle and record for reference during system performance tests.
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