Air conditioning preconditions

[Roy32]

In case this is any help to the community, this is copied from BMW Ista about compressor control:

Evaporator control (compressor control)

Evaporator control is 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 .

[Crowz]

Originally Posted by blaubenz

I am going with what I know about general a/c (not specific to e53) but...



well if the wire(s) to the clutch is disconnected the clutch will not engage and the light will still many be on. Did you do all you can to see everything is still connected?? If the outside temperature is below 60 deg f the clutch may not engage and the light will still be on. You are in the east cost is it like 50 deg outside when you tried.



IF you have zero freon the clutch may not engage. Were you able to confirm that you have some pressure in your a/c system??? The freon is added through a valve like your tire (except bigger). If you don't have any guages you can see if you have at least some freon by removing the dust cap and press the valve stem to see if some freon is coming out (like you tire). Or you can get one of those cheap 134a charging kit that have a guage on it and it will have instructions on now to check and charge you a/c system. But I don't think low pressure is you problem. The clutch should still engage in low freon.

Regular ac stuff doesn't cover the e53 that well since unlike tradition ac the e53's dme computer controls the entire system. There are no low or high side switches like in a traditional ac system. Just a pressure sensor.

[Roy32]

I am going to test the compressor clutch today, and post my results.

[Roy32]

i was able to test the compressor clutch by applying 12V to the signal wire, and it definitely moves. I even went a little risky and ran the engine with the clutch activated. I could tell the difference in load on the engine, so i'm thinking the clutch is good. Clutch releases as soon as I take away the 12V signal, so it's being overridden by some condition. Hopefully I didn't ruin the system by running it dry if that's in fact the issue.
It seems like the only things left are:

1. Low refrigerant pressure (have not checked yet, need to check INPA values or actual gauge readings)
2. Faulty pressure sensor (no faults stored but still could be reading wrong yet plausible)
3. Faulty temperature sensor (need to check values in INPA, but coolant temp and ambient temps seem correct. not sure if there is another sensor more relevant to the AC system)

Next step is probably get my hands on INPA one more time and gather more data to see whats keeping my AC off. My gut tells me it's low on refrigerant. Hoping to figure it out soon, its getting warm in NJ!

[Crowz]

Well if you impatient and want to know if its low on Freon you can just see how cold the low pressure line gets when you manually run the compressor by applying power.

If it doesn't get cold then more than likely its out of Freon

You wont damage the compressor by running it with the Freon low on a sealed system. It would have to be completely dry which is unlikely in your situation.

[Roy32]

I ended up buying a can of refrigerant with the gauge on it. Nothing read on the gauge when I connected it. I added about half the can, and at that point the compressor engaged. I filled it to the proper amount on the gauge, and I have somewhat cold air. I'm now thinking I should spend the money and have a shop properly evacuate and recharge the system. also curious if there is a decent sized leak somewhere, which would be apparent when they vacuum the system.

[Crowz]

Not a bad idea seeing as it had to go somewhere.