System Component Functions
A/C REFRIGERANT SYSTEM (R-134A), COMPONENT FUNCTIONSCOMPRESSOR
The compressor is driven via a belt on the engine when the A/C clutch engages (A/C ON).
Low pressure refrigerant gas from the evaporator is compressed by the compressor. After compression, the refrigerant gas (now high pressure) flows to the condenser.
Note!
^ The compressor contains refrigerant oil (PAG oil) that is mixable under all temperatures with R-134a refrigerant.
^ A label on the compressor indicates that compressor is for R-134a systems only
CONDENSER
The condenser transfers heat from the compressed refrigerant gas to the outside air which causes the refrigerant to change state from a gas to a liquid.
Note!
Illustration shows the condenser from an Audi 100 model.
EVAPORATOR
Liquid refrigerant entering the evaporator absorbs heat from air passing through the evaporator rings and cools the air. As the refrigerant absorbs heat it turns to vapor and then is suctioned by the compressor.
Note!
Illustration shows the evaporator from an Audi 90 model.
ACCUMULATOR
The accumulator traps the mixture of refrigerant vapor and gas coming from the evaporator. While in the accumulator, the refrigerant vapor turns to gas and, along with the refrigerant oil1 is then suctioned by the compressor.
An oil extraction passage is incorporated into the accumulator to ensure that refrigerant oil flowing with the refrigerant does not remain in the accumulator.
Any moisture in the system is absorbed by the desiccant in the accumulator.
Note!
Do not remove caps -A- and -B- until ready to ins tall new accumulator. If caps are removed too soon, the desiccant becomes saturated with moisture after a very short time. If this occurs, the accumulator must be replaced.
RESTRICTOR
The restrictor restricts and controls refrigerant flow to the evaporator thus lowering refrigerant temperature and pressure.
Upstream of the restrictor, the refrigerant is hot and under high pressure. Downstream of the restrictor, the refrigerant is cold and under low pressure.
A strainer located upstream of the restrictor filters out any dirt or particles. Another strainer located downstream of the restrictor atomizes the refrigerant before it enters the evaporator.
Note!
Insert restrictor into evaporator in direction of Arrow -A-. Illustration shows the evaporator from an Audi 90 model.
O-RINGS
O-rings seal the connections between components of the A/C system.
Use only new red or green O-rings which are compatible with R-134a refrigerant and refrigerant (PAG) oil.
Always use correct size O-rings (dimensions -a- and -b-). Note!
^ Do not reuse O-rings, always replace.
^ Lubricate O-rings with refrigerant (PAG) oil be fore installing.
PRESSURE RELIEF VALVE
The pressure relief valve is mounted on the compressor. At approx. 38 bar (551 psi), the valve opens to vent excessive pressure. When system pressure is reduced to approx. 30-35 bar (435-508 psi), the valve closes to prevent total refrigerant loss.
A/C SYSTEM HOSES AND LINES
The mixture of refrigerant oil (PAG oil) and refrigerant R-134a attacks some metals and alloys (for example, copper) and breaks down certain hose material. Use only hoses and lines which are identified with a green mark (stripe) or the lettering "R-134a".
Hoses and lines are secured with threaded connections. Always properly torque hose and line connections.
A/C REFRIGERANT SYSTEM (R-134A), HIGH PRESSURE SERVICE VALVE
- Use only the specified R-134a quick coupling connectors when servicing the refrigerant system.
- Before removing the valve insert from the service valve, discharge A/C refrigerant system.
- Always reinstall cap -D- over service valve.
High pressure service valve1 assembly (87-1405)
A - Service valve connection (soldered to front of condenser)
B - Schrader valve insert
C - O-ring seal
D - Cap with seal
Note!
Illustration shows the condenser and service valve of an Audi 100 model.
Only a high pressure service valve is used for A/C system servicing. A low pressure service valve is not installed in the A/C system. Perform all A/C system service operations (i.e discharging, evacuating, and charging) through the high pressure service valve only.
A/C REFRIGERANT SYSTEM (R-134A), VALVES/SWITCHES
Note!
The following A/C system switches are available in different versions. Always consult the proper parts information to ensure correct switch application.
A/C REFRIGERANT HIGH PRESSURE SWITCH
Function
Switches coolant fan (V7) to second speed when A/C system pressure increases.
For switching pressures and removing/installing switch, see switches, Function/Removing/Installing section.
A/C REFRIGERANT HIGH PRESSURE SWITCH
Function
Switches A/C clutch OFF if system pressure is too high.
For switching pressures and removing/installing switch, see switches, Function/Removing/Installing section.
A/C REFRIGERANT LOW PRESSURE SWITCH
Function
Switches A/C clutch (N 25) 0FF it system suction pressure is too low (low refrigerant charge). This switch protects the compressor it the refrigerant system is empty.
A/C SYSTEM SWITCHES, VALVE CONNECTION
A Schrader valve is installed under the A/C system switches so the switches can be replaced without discharging the A/C system.
- A/C system low and high pressure switches use different size threads.
- Use only the correct switches and O-ring seals specified for use with R-134a refrigerant and PAG oil.
A - Switch connection (soldered)
B - O-ring seal
C - Schrader valve insert
A/C COMPRESSOR SPEED SENSOR (87-1276)
- inductive sensor (4 pulses per compressor rotation)
- determines A/C compressor speed; the A/C compressor clutch control module (Manual A/C) or the A/C control head (Automatic climate control) compares compressor speed to engine speed and calculates belt slippage (as a percentage) and switches compressor OFF if slippage is too large (for example, if belt is too loose.)