Air conditioner problem
 

Had the first warm day this spring where I decided to put on the air conditioner to cool the vehicle down after being outside in the sun. I had no cold air coming out of the vents and when I turned the a/c off there was a burst of cold air for a few seconds. Fan and heater functions work perfectly.

Looking through wiring diagram info etc I came across the following:
Evaporator control (compressor control)

Evaporator control (compressor control)
Evaporator controlis activated by pressing the A/C button in the operating unit. A/C standby mode is indicated by the LED in the A/C button. If controlled compressors are installed, the cutout threshold is dependent on the outside temperature and may be between 1 °C and 3 °C. If a standard compressor is installed, the threshold is reduced by 1 °C at high outside temperatures (> 30°C). Disengagement of the compressor clutch is scarcely achieved by the mechanical control of the compressor output on the controlled compressor . The A/C compressor is controlled directly by the heating/air conditioning control unit and the auxiliary fan by the digital motor or diesel electronics (DME/DDE) . Requests for A/C compressor and auxiliary fan are sent via the CAN/K-bus to the DME/DDE. The pressure sensor also monitors the refrigerant pressure and switches the A/C compressor off if the pressure is too high or too low. A pressostat that checks the refrigerant pressure and, if necessary, interrupts the supply line to the compressor clutch is only installed on the M51 diesel engine.
Switching conditions for the magnetic clutch
ON:A/C button ON and evaporator temperature > 4°C (depending on the outside temperature) and coolant temperature < 117°C and terminal 30 at control unit > 9.7 Volt.
OFF: A/C button OFF or evaporator temperature < 3°C (depending on outside temperature) or blower stage 0 or coolant temperature > 120°C or terminal 15 OFF or terminal 30 < 9 Volt.
DME_AC idle speed increase
When the A/C button is pressed, the heating/air conditioning system requests an idle speed boost via the K-bus in order to provide adequate cooling capacity at idle speed.
DME_KO compressor activation
The DME receives the instruction to switch on the compressor via the K-bus . If no full load cutout is set a feedback signal enables the DMD/DDE to activate the compressor clutch. The load moment of the A/C compressor is derived from the pressure sensor signal and passed on via the K-bus to the DME.
Infinitely variable auxiliary fan control
The refrigerant pressure in the air conditioning is registered by means of a pressure sensor and converted by means of a table into an auxiliary fan stage which is then sent to the DME/DDE via the K-bus/CAN bus . From this, the electronic engine control system then generates the control voltage for the infinitely variable auxiliary fan motor .


Is it possible the fault is simply low refrigerant pressure? Anyone had a similar problem?

Have you verified the correct operation of the auxiliary fan? If the aux. fan isn't working, the A/C won't get cold unless the car is moving briskly.

You can check the aux. fan operation with INPA or just have a look through the grill when the car is idling (stationary, obviously!) and the A/C is on. Aux. fan should be spinning.

No I haven't checked that. Thanks for the tip. I'm coming up for a few days off so I will scan with INPA and see what comes up and do a visual inspection.

You mentioned moving briskly. When I tried the a/c I was travelling at 100kph (62mph).

Yep - 100kph is what I'd define as "brisk". :D

May not be an aux. fan issue but still worth looking into.

A fan fault for the aux fan will be logged in the DME module, not the AC system module.

It could be a number of issues, but most common failure is the aux fan behind the front bumper. Turn the AC on and check outside the car to see if the fan turns on.

Here's an aux fan DIY to help with replacement if you end up needing one!

https://www.youtube.com/watch?v=N_K0KPHsteA

It's right the A/C will only work if the aux fan is working, but as you quoted in your first post: if the refrigerant level is too low the a/c won't work and the aux fan won't start spinning...
On my previous e39 I wasn't even able to trigger the aux fan via INPA but the only fault was a leaky high pressure hose...

Air Conditioning problem SOLVED!!!
 

Problem Solved!! Well at least I'm pretty sure it is. I changed the thermostat today and while I had the fan shroud and the thermatic fan off I noticed that the air cond belt had thrown. I put it back on but it threw straight off again. I pulled the belt out and saw a chunk missing from the ribs. Bought a new belt today and will fit tomorrow morning.

Air Conditioning problem SOLVED!!!
 

As above. Problem solved. Replaced the belt and now have cold air.

Thanks for the follow-up and well done! :thumbup: