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Rear A/C System, Testing Cooling Performance






Rear A/C System, Testing Cooling Performance

Special tools, testers and auxiliary items required

• Diagnostic operation system (VAS 5051 A) (or Vehicle Diagnosis & Service System (VAS 5052) )

• On vehicles with Basic version A/C system, a conventional thermometer (for measuring temperature, if necessary use a thermometer with 2 measuring probes for simultaneous measurement for example, temperature on right and left sides)

• On vehicles with Comfort version A/C system, the specified temperatures themselves can be measured or read out via the "Read measuring value block" function using. Vehicle Diagnosis, Testing and Information System VAS 5051 in "Guided Fault Finding".

• On vehicles with a rear heating & A/C unit, cooling performance in front heating & A/C unit must be checked first => [ Test Requirements ] Test Requirements, only when this is OK can the cooling performance of rear heating & A/C unit can be checked => [ Rear A/C System, Testing Cooling Performance ] Rear A/C System, Testing Cooling Performance.

• If there is a complaint at only the evaporator in rear heating & A/C unit on vehicles with two evaporators, first perform cooling performance testing at front A/C system => [ Test Requirements ] Test Requirements. If cooling performance at front is OK, check cooling performance at rear A/C system. If, during cooling performance testing on rear A/C system, it is detected that performance is not sufficient and then no cause for the complaint can be found, check line connection between evaporator in question and exit of line connection at distribution point of refrigerant lines in left front wheel housing (for example, for constriction). If the fault cannot be found, bring vehicle to a workshop that has the necessary tools and in which the work can be performed accordingly by qualified personnel.

• There is no sensor that measures air temperature downstream of evaporator installed in rear heating & A/C unit, compressor performance control is performed therefore only via the measured value of Evaporator Vent Temperature Sensor (G263) which is installed in front heating & A/C unit. Under certain unfavorable conditions, it may occur therefore because of this that the temperature set on the rear heating & A/C unit is not attained during normal regulated operation even though the A/C system is OK. Also it may occur that for example, on an A/C system on which the specified outflow temperature has already been attained in the front heating & A/C unit and maximum cooling output is requested at rear A/C system, the rear outflow temperature is a few degrees °C higher than when a high cooling output is requested at front A/C system (if necessary increase front cooling output via setting at front).

Test Prerequisites

• Cooling performance of front A/C system was checked and is OK => [ Test Requirements ] Test Requirements.

• Test requirements for testing front cooling performance are met => [ Test Requirements ] Test Requirements.

• Air intake for Rear Fresh Air Blower (V80) via the air intake grille installed in left luggage compartment lining is not obstructed by objects, refer to => [ Air Duct In Rear Heating & A/C Unit ] Air Duct In Rear Heating & A/C Unit.

Rear Cooling Output, Checking

- Perform test of front cooling performance, refer to => [ A/C System Cooling Performance, Checking ] A/C System Cooling Performance, Checking.

• Measured values of temperature sensors ( Left Rear Vent Temperature Sensor (G405) and Right Rear Vent Temperature Sensor (G406) ) installed in air duct are evaluated for cooling performance test of rear A/C system. By using these measured values, it is then not necessary to remove components for the test.

• For the cooling performance test at front and rear, the same settings on Climatronic Control Module (J255) and on Rear A/C Display Control Head (E265) are required (Auto operating mode, temperature preselect Lo).

- Initiate On Board Diagnostic (OBD) for rear A/C display control head of A/C system using Vehicle Diagnosis, Testing and Information System VAS 5051 in "Guided Fault Finding" function.

- In the rear A/C display control head select "read measured values block" the display group with the measured values from the Left Rear Vent Temperature Sensor (G405) and Right Rear Vent Temperature Sensor (G406) using Vehicle Diagnosis, Testing and Information System VAS 5051 in "Guided Fault Finding".

- Compare the displayed value (of the left rear vent temperature sensor and right rear vent temperature sensor) with the values in the diagram.






A Vent temperature on the evaporator, depending on the outside air temperature (measured air temperature on left rear vent temperature sensor and right rear vent temperature sensor)
B Ambient temperature
C Permissible tolerance range

The air temperature measured by left rear vent temperature sensor and right rear vent temperature sensor must be the same or a maximum of 5 °C (41 °F) higher after 5 minutes than the specified vent temperature at evaporator, depending on ambient temperature (see diagram).

• If the A/C system is OK and the same temperature is reached on both evaporators (inside the front and rear A/C), then the temperature on the front evaporator is displayed in the measured values block for the Climatronic control module (measured value from the Evaporator Vent Temperature Sensor (G263) ).

• The air travelling from evaporator to vents is heated slightly by heat radiated by heater core in heating & A/C unit and by leaking air which flows by at temperature doors through the heater core.

- If the required values are not reached in either display fields, then check the measured values from the left rear vent temperature sensor and right rear vent temperature sensor and the activation of the Right Rear Temperature Door Motor (V314) and Left Rear Temperature Door Motor (V313) using Vehicle Diagnosis, Testing and Information System VAS 5051 in "Guided Fault Finding". If the fault cannot be found, bring vehicle to a workshop that has the necessary tools and in which the work can be performed accordingly by qualified personnel (there is a fault in the refrigerant circuit).

- If the values required are obtained only on one side, check measured value of sensor, Left Rear Vent Temperature Sensor (G405) or Right Rear Vent Temperature Sensor (G406) , which has the measured value not in the permissible range. If measured value is OK, check function of control motor and temperature door operation for the side where the measured value is not OK (Right Rear Temperature door Motor, refer to => [ Right Rear Temperature Door Motor (V314) ] Right Rear Temperature Door Motor (V314) or Left Rear Temperature door Motor, refer to => [ Left Rear Temperature Door Motor (V313) ] Left Rear Temperature Door Motor (V313)) using Vehicle Diagnosis, Testing and Information System VAS5051 in the "Guided Fault Finding" function. If no malfunctions can be determined here, check air duct in rear heating & A/C unit, refer to => [ Air Duct In Rear Heating & A/C Unit ] Air Duct In Rear Heating & A/C Unit.

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