Several days ago, one of my friends asked me to check, what happened with the AC of his car. AC didn’t work, IHKA blew quite warm air, independently of temperature set (obviously – the compressor don’t work).

As the specialists of local repair centers were not familiar with the performance of AC and didn’t know the parameters, required for it, and after refilling of the system it didn’t start to blow cold air, I decided to investigate, what are the characteristic parameters for AC of E60/90 and similar vehicles?


In this entry, I will mention basic things about the “producer” of cold air – the compressor. The system or valves (distribution) is simple, I believe, even a not very experienced specialist can handle it.

The structure of the AC is simple. During the time, it has “grown up” with different sensors, the motor of the compressor – with the clutch, which regulates its power, but the basic principle of performance hasn’t changed.

How does the compressor works, you can read, for example, here:


How to work with BMW diagnostic tools?

Start a new session in ISTA D, perform vehicle identification and diagnostics. Choose IHKA in the Device tree and press Call-up ECU.


In the Menu choose diagnosis scan sub-menu, then – choose parameters, which are interesting for us.

To understand, if the conditioning cycle is working correctly, choose the following basic parameters:

  • outside temperature
  • inside (saloon) temperature
  • load of the compressor
  • the pressure of the system (before valve, high-pressure contour)
  • the temperature of the evaporator (evaporator, low-pressure contour)

If CAN communication works correctly, the outside temperature has to match with temperature, displayed by KOMBI. The inside (saloon) temperature has to match approximate temperature in the saloon. The temperature sensor is placed in IHKA panel (location: on the left side of main management button, behind small “ventilation” openings).


Before the expansion valve, the back valve with an approximate opening threshold of 10 bar is placed. It means – if the pressure in high-pressure contour drops below this threshold, expansion valve doesn’t inject the freon in the low-pressure contour.


The pressure in high-pressure contour typically varies from 10 to 20 bar (where the lower threshold corresponds to back valve pressure value), so the evaporator temperature would be +2 .. 3 oC. Feedback coefficient of the Closed loop system is chosen in following way: when the temperature of evaporator changes in range 0 .. 5 oC, high-pressure pressure changes from approximate 10 bar (back valve closed – injection don’t happen) to 18 bar (compressor activated with 80+% power).


If the AC is turned on, the clutch of the compressor works, the temperature indications of outside and inside temperature and also the pressure sensor indications are correct, the compressor is supposed to increase the pressure in high-pressure contour. It means:

  • high-pressure line pressure increases above 10 bar;
  • the temperature of the evaporator  drops down (below outside temperature) and during several tens of seconds becomes close to +3 oC marking.


For correctly working system:

after switching off the engine, the pressure in high-pressure contour drops down during several tens of seconds and stays in level around 10 bar (it means, valve closing pressure).

If this “remained” presusre is significantly different – check the indications of the pressure sensor (compare with indications of manometer).


For a vehicle, working in idle, by approximate conditions:

  • outside temperature around 26 oC
  • set temperature in the saloon: +16 oC (min) for both sides
  • power of saloon fan: 100 %
  • maintaining mode – medium
  • all windows and doors open

performance of correctly working AC is characterized by following approximate parameters:

  • used power of fan: 85 .. 90 %
  • evaporator temperature: +6 oC
  • high-pressure contour pressure: around 18 .. 19 bar


For example, evenly driving with speed 70 .. 90 km/h; outside temperature around 26 oC; all Windows and doors closed; set temperature +19 oC on both sides; maintaining mode – medium:

  • used power of compressor: 45 .. 50 %
  • evaporator temperature: +1 .. 3 oC
  • High-pressure contour pressure: around 10 .. 11 bar

Accordingly: if the pressure in high-pressure contour is significantly higher, or the load of the compressor is higher – clean the AC radiator (in front of the main radiator, bottom part). If AC has problems to maintain necessary evaporator temperature, check the amount of freon in the system, if necessary – refill it.

If, for example, the evaporator temperature doesn’t drop down, even the pressure sensor shows above 10 bar in high-pressure contour:

  • check the indications of the pressure sensor (compare to indications of manometer);
  • check the back valve and expansion valve (possible, some piece of rubbish has gotten inside).

Still interesting, that ISTA displays the voltage supply of the pressure sensor (it has to be close to 5.00 V), but don’t display indications of the sensor itself (in V – Volts). As we can think, IHKA management module measures (the pressure sensor is an analogue sensor, which is supplied with 5.00 V reference-voltage, mentioned before, via current supply source) difference of voltages between reference voltage (which is displayed by IHKA test module) and voltage, which is “received” from pressure sensor.

Accordingly – if the reference-voltage is close to 5.00 V, but the indications of the pressure sensor are significantly different from indications, measured by manometer – the pressure sensor has to be replaced.