I think this was the post that solved it for me .Its the evaporator temperature sensor. Or at least it was on mine see below
R129 Climate problem
Hi all,
My turn to give something back to the forum, for a change. Hopefully this might solve your air-con problem as it did mine.
For a year, I battled with an air conditioning problem. Air stopped venting into the vehicle after 30 minutes or thereabouts, even though the fan was running and I could feel hot or cold behind the vents when I changed the temperature.
To cut a long story short, I got the following advice. Turned out to be a sensor. My local garage were a bit reluctant when I told them exactly what I wanted doing, but below is the advice I was given - it saved me changing the heater control panel (garage advice) and it has worked fine ever since:
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Service operation: 83 8054 01 "Temperature sensor on evaporator replace"
Part number: A 210 830 07 72 "Temperature sensor"
Sounds like your evaporator temperature sensor has failed. This causes the evaporator to ice up. You can check it on the pushbutton control module. There is also a technical bulletin about this problem.
It is a very cheap and easy repair. New sensor is about US$50 and can be fitted in the blink of an eye.
You can test the evap temp sensor using the diagnostic feature on the a/c push button control unit.
Come to your car in the morning or when it has been standing for a few hours (so the engine and heater are cold). Turn the ignition on, without firing the engine. Run the self-diagnostic routine on the air con pushbutton control panel and check the evaporator temperature reading. If the sensor is faulty, you will have a logically impossible value:
1. Turn ignition on (without firing engine)
2. Set the air con temp to 72 deg F or 22 deg C
3. Press "REST" for more than 6 seconds
4. The left side of the display will alternately display "01" (which is the test number) and the cabin interior temperature (eg 72 deg F or whatever the temp in the car happens to be).
5. Press the fan speed "+" button to advance to the next test (eg "02").
6. The evaporator temperature is test number "05". The working temperature of the evaporator is between 2 deg C and 12 deg C. Obviously, if your car has been left overnight and has not been started, the value returned should be consistent with the ambient exterior temperature. So if it is about 12 deg C outside, the sensor should return about 12 deg C also. If the evaporator temperature sensor is broken it will return a logically inconsistent value, eg 40 deg C.
7. Now turn on the car and run the a/c for a few minutes. Repeat the procedure above. Test "05" which is the evap temp sensor, should return a value of 2 to 12 deg C if it is working. If it is broken, it will return an impossible value, such as 40 deg C.
The other tests on the push button control unit are great:
01: in car temp sensor
02: outside temp sensor
03: heater core temp sensor
05: evap temp sensor
06: ECT temp sensor
07: refrigerant pressure meassured in BAR
08: refrigerant temp sensor
21: engine speed
22: vehicle speed
There are loads more.
Part No. 2108300572….or 1298300072
Heating and ventilation …heater case
Service operation: 83 8054 01 "Temperature sensor on evaporator replace"
Part number: A 210 830 07 72 "Temperature sensor"
Nr. Description Unit
01 In-car temperature sensor °C
02 Outside temperature sensor °C
03 Left heat exchanger temp. sensor °C
05 Evaporator temperature sensor °C
06 Coolant temperature sensor °C
07 Refrigerant pressure bar
08 Refrigerant temperature sensor °C
09 Not assigned --
10 Blower control voltage Volt
20 Auxiliary fan control current mA
21 Engine Speed x100 RPM
22 Vehicle Speed km/h
23 Circuit 58d (% of batt. Voltage) %
24 Battery Voltage Volt
40 Software staus
41 Software staus
42 Variant Code 1st numerical value
43 Variant Code 2nd numerical value
50 Not assigned
51 Not assigned
52 Not assignd
60 Roof OPE/CLO
61 Left side air outlet, knurled wheel
potentiometer Volts
62 Vacuum actuator 46, feedback
potentiometer Volts
63 Center outlet, knurled wheel
potentiometer Volts
64 Vacuum actuator 47, feedback
potentiometer Volts
65 Right side air outlet, knurled wheel
potentiometer Volts
66 Vacuum actuator 48, feedback
potentiometer Volts
Item 63 is the input signal from the thumbwheel control for the center vent, and 64 is the feedback signal to the controller giving it the position of the vent flap. As you move the thumbwheel these values should continously vary from a minimum of roughly .7 to 4.5.
The original part number was A1298300072 - it has been superceded by A2108300572 which is £28. It is located under the dash on the drivers side by the centre console housing - you just need to drop the trim panel for access.
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