Ice Formation on Evaporator, Localizing Malfunction

Ice Formation on Evaporator, Localizing Malfunction

The A/C Compressor Regulator Valve (N280) is activated by the Climatronic Control Module (J255) so that the temperature of the air downstream from evaporator reaches the specified value (approximately 2 to 5 °C): After vehicle start, depending on temperature, engine speed and vehicle voltage in the measured value block, a value greater than 75% (0.55 A) is displayed for activation of the A/C compressor regulator valve. As soon as temperature measured by the Evaporator Vent Temperature Sensor (G263) approximates the specified value, the activation is rescinded and thus the compressor output is reduced. Check using Vehicle Diagnosis, Testing and Information System VAS 5051/ under "Guided Fault Finding".

Under certain operating conditions, residual moisture in the refrigerant circuit can cause ice to build up on the A/C compressor regulator 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.

• After a long drive, the windows fog up from the inside and they do not clear after pressing the Defrost button. The A/C function is OK again after switching the vehicle off for a short time.

Corrective action:

- Check the measured value of the evaporator vent temperature sensor in "read measured values block" using Vehicle Diagnosis, Testing and Information System VAS 5051/ under "Guided Fault Finding".

- If the sensor measured is too high under the operating conditions reported by the customer (even though the A/C system is operating without difficulty at greater than e.g. 10 °C, depending on ambient temperature), check the evaporator vent temperature sensor (an incorrect measured value may cause evaporator the ice up).

- If the sensor measured value under conditions reported by the customer is too low (at an ambient temperature above 0 °C, colder than 0 °C for an extended period of time), transfer the vehicle to workshop that has the required tools and qualified personnel to perform the work. Bring the problems determined to the attention of the workshop.

- Check refrigerant line from evaporator to A/C compressor (thick tube, low pressure side) with engine running. If this is heavily iced when the problem occurs (a think layer of ice is acceptable), this shows 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. Bring the problems determined to the attention of the workshop.