W163 ML320 2003 - Intermittent AC Problem, Engine speed related

[John5788]

Hello,


I have a strange problem on my 2003 ML320.


AC used to work fine until recently. Refrigerant charge has been double checked and refilled (pulled vacuum and recharged).


It seems like the compressor does not engage while driving, but will start to blow cold air when stopped at a red light, or coasting to a stop from exiting the highway or something. Once the engine is not at idle RPM, the cold air stops blowing.


I also tested this on the freeway at speed. When traveling around 80 mph, the cold air does not blow, but once I throw the transmission into neutral and let the engine drop to idle, the cold air will begin to blow while I am coasting at highway speeds in neutral.


Any ideas before I start replacing parts? I already got a spare compressor off a 2004 ML350 ready to swap in if necessary.

[John5788]

I went out for a drive and collected some data with my handheld logger. Outside ambient temperature is around 57F, set the AC controls to 60F.

When the AC is not working, it looks like A9 (AC Compressor) is at 100%, like it is trying to call for full compressor, but nothing is happening. The high side sensor is saying we are stuck at 5 bar, which is validation nothing is happening.

When the AC kicks in, the control unit stats regulating the compressor by reducing to 55% and my evaporator gets cold, cold air comes in through the vents, and the high side increased to 7 bar.

What is going on? Compressor dead?

[John5788]

I replaced the compressor today, added 20cc of PAG46 to the compressor, vacuumed the system and charged it back up. AC is working perfectly fine on the highway again.



I drove the exact same route as last night and was getting cold air out of the vents throughout the test drive.

Side note / observation: With the live data stream going, I was able to see that the AC system tries to maintain the evaporator at a cold temperature (6 degree C) since the compressor is clutchless and always engaged. Even when you are calling for heat, the evaporator remains cold. When the temperature knob is adjusted for cold air, instant cold air out the vents since the evaporator was maintaining temperature. The only time I noticed the evaporator raise temperature is when the AC Off/Rest button is pressed (and also when my old compressor wasn't working).

Thanks for all your help and feedback guys!