Ice Formation on Evaporator

Ice Formation on Evaporator

• The A/C compressor regulator valve (N280) is activated by the Climatronic control module (J255) , so that the temperature of air downstream from the evaporator in the front A/C unit reaches the specified value (approximately 2 to 5°C (35.6 to 41 °F)).

• After vehicle start, a value greater than 0.55 A is displayed in the relevant measured value of the Climatronic control module depending on the measured temperature, the engine speed, and the vehicle voltage using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function. As soon as the temperature measured by the evaporator vent temperature sensor nears the target value, the activation, and thus the compressor output, is reduced.

• No temperature sensor is installed after the evaporator in the rear A/C unit. If the refrigerant flow through this evaporator is too high (for example because the rear expansion valve is open too wide), the evaporator in the rear A/C unit can ice up. Since the air flowing through the evaporator in the rear A/C unit was already dried in the front A/C unit, it usually does not cause icing of this evaporator or only after a long time. If the evaporator in the rear A/C unit does ice up, the air flow from the rear vents will decrease until there is no longer any cooling/heating output in the rear.

Under certain operating conditions, moisture in the refrigerant circuit can lead to icing on the A/C compressor regulator valve (and on the expansion valves). A/C compressor control is impaired by this icing. The affected evaporator is cooled too intensely and ices up. The freeze-up of the evaporator can be the cause for the following customer complaints:

In the case of icing of the evaporator in the front A/C unit:

- 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 windshield, rear window and/or door windows fog up on the inside. Pressing the Defrost button of the Climatronic control module, does not clear the windshield, rear window, and/or door windows. After turning off the vehicle and waiting a short period, the function of the A/C system is OK again.

• 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 (G263) 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.

Checking:

- In the read measured values function, check the measured value of the evaporator vent temperature sensor using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function

- If the measured value of the Evaporator Vent Temperature Sensor 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 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 evaporator vent temperature sensor is below the adjustment requirement described by the customer (at an 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 => Refrigerant
R134a Servicing 00 General Information. Bring the problems determined to the attention of the workshop.

- Check the refrigerant line low pressure side (wide line between the expansion valve and the refrigerant line with the inner heat exchanger), refer to => [ Refrigerant Circuit Components Overview ] Refrigerant Circuit Components 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.

In The Case Of Icing Of The Evaporator In The Rear A/C Unit:

- After a long drive, the rear A/C system repeatedly or sporadically fails (no cooling or heating output). After turning off the vehicle and waiting a short period, the A/C system function is OK again.

Check The Front (And Rear) Evaporator If Icing Is Suspected:

- Open the vent in rear center console

- Open the vent in the left and right B-pillars

- Start the engine.

- Switch the A/C compressor on by selecting the Auto mode on the Climatronic control module (the indicator lamps in the AUTO buttons and the AC button illuminate).

- Via the rotary switch(es) of the Climatronic control module, the temperature preselection is set to cold (for the driver and front passenger side).

- On the Climatronic control module set the lowest possible speed for the fresh air blower (V2) (lowest possible cooling output of the front A/C unit).

- On the rear A/C display control head set the temperature preselection to HI (for the left and right sides).

- On the rear A/C display control head set the air flow direction via the rotary switches on the rear A/C display control head to the vents in the rear center console.

- On the rear A/C display control head set a medium speed for the rear fresh air blower (V80).

- Set the engine speed to 2000 RPM.

- In the "read measured values" function, read out the measured value of the evaporator vent temperature sensor using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function

- Use a thermometer to measure the temperature of the air from the vent in the rear center console.

- Do the measured temperatures for the rear correspond to the temperature measured at the evaporator vent temperature sensor, maximum permissible deviation from the measured value at the front +6°C/- 3°C (42.8/26.6 °F) but not less than +1°C (33.8 °F))?

- If the measured value for the air from the vents in the rear center console is too low (at an ambient 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 rear expansion valve and the refrigerant line with the inner heat exchanger), refer to => [ Refrigerant Circuit Components Overview ] Refrigerant Circuit Components Overview with the engine running. If this line is severely iced up when a problem occurs (a thin layer of ice is permitted), this is an indication that the temperature in the rear evaporator is too low. Bring the 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 the rear expansion valve control is not OK). Bring the problems determined to the attention of the workshop.

• If the measured value for the air from the vents in the rear center console is too high, perform the cooling output test, refer to => [ Cooling Output Of Front A/C System Is OK But Not In Rear ] Cooling Output Of Front A/C System Is OK But Not In Rear.

In The Case Of Icing In An Expansion Valve:

- After a long drive, the A/C system repeatedly or sporadically fails (no cooling or heating output via the A/C unit upstream from this expansion tank). After turning off the vehicle and waiting a short period, A/C system function is OK again.

- Bring the vehicle to a workshop that has the necessary tools and where the work can be performed by qualified personnel.

- Bring the problems determined to the attention of the workshop.