Ice Formation on Evaporator, Localizing Malfunction

Ice Formation on Evaporator, Localizing Malfunction

• The A/C compressor regulator valve (N280) is activated by Climatronic control module (J255) so that temperature of air downstream of evaporator reaches the specified value (approximately 2 to 5 °C) (35 to 41 °F):

• A value greater than 0.55A is displayed after vehicle start, depending on temperature, engine speed and vehicle voltage measured. As soon as the temperature measured by the evaporator vent temperature sensor (G263) nears the target value, the activation, and thus the compressor output, is reduced.

Under certain operating conditions, residual moisture in the refrigerant circuit can lead to an ice build-up at A/C compressor regulator valve (and at expansion valve). A/C compressor control is reduced by this ice build-up, evaporator is cooled too intensely and freezes. The freeze-up of the evaporator can be the cause for the following customer complaints:

- After a long drive, A/C system repeatedly or sporadically fails (no cooling or heating performance), after switching off vehicle and after a short time, A/C function is OK again.

- The windows fog up from the inside after driving for a long time; also, when using the "Defrost" button on the Climatronic control module and the windows do not clear up right away, after turning off the vehicle (or after turning off the A/C) and waiting a short time, the A/C system is OK again.

Corrective action:

- Check the measured value from the evaporator vent temperature sensor with the "Read measuring value block" function using the Vehicle diagnostic tester in "Guided Fault Finding".

- If the measured value of the Evaporator Vent Temperature Sensor is too high under the conditions described by the customer (although the A/C is working properly, depending on the outside temperature greater than 10 °C (50 °F) for example) check the Evaporator Vent Temperature Sensor (an incorrect measured value can cause the evaporator to ice up).

- Is the measured value from the evaporator vent temperature sensor is below the adjustment requirement described by the customer (at a temperature above 0 °C (32 °F), longer when it is lower than 0 °C (32 °F)). Bring vehicle to a workshop that has the necessary tools and in which the work can be performed accordingly by qualified personnel. Bring the problems determined to the attention of the workshop.

- Check the refrigerant line "low pressure side" (wide line between the quick-release coupling and the A/C compressor), refer to => [ Refrigerant Circuit Component Overview ] Refrigerant Circuit Component Overview with the engine running. If this line is thickly iced-up when a problem occurs (a very thin layer of ice is permitted), this indicates that the temperature in the evaporator is too low. Bring vehicle to a workshop that has the necessary tools and in which the work can be performed accordingly by qualified personnel. (it is possible that A/C compressor regulation is not OK). Bring the problems determined to the attention of the workshop.

• Ice build up can occur on the evaporator under extreme circumstances for particularly unfavorable A/C system settings (for example airflow direction is set to the instrument panel vents and these vents are closed, set to MAX AC for a minimal fresh air blower speed). With this setting, there is no air flowing through the evaporator making the temperature at the evaporator vent temperature sensor higher than the actual evaporator temperature. It can be assumed for the A/C system regulator, that the measured value of the evaporator vent temperature sensor has the actual evaporator temperature and continues to activate the A/C compressor regulator valve and the evaporator cools too much.