Automotive Air Conditioning Information Forum

Hi there,

The A/C will initially work quite well in hot weather (today for example: 37c/98f), up until the engine temperature reaches about 85c/185f. So in about 5 minutes of driving. Once it reaches this, it will only blow ambient air for the rest of the drive. It has a smell about it too when this happens.
If outside temperatures are about the 34c/93f mark and below, the A/C will stay on for the entire drive... unless I'm really giving it the beans.
So it sounds like it's related to engine temps.
I've had the high pressure sensor replaced and the refrigerant has been recharged.

My question is, could this be a symptom of over-charging? My logic (correct me if I'm wrong): In high heat, the system reaches the high pressure limit very quickly, triggering the compressor to shut down. In lower heat, it perhaps doesn't quite reach that point? Or does an over-charged system always produce too much pressure (in high or low heat), shutting down the compressor?

I know an over-charged system can also cause noise from the compressor. My vehicle does make a noise, but I think is a worn pulley bearing (next to the compressor). I think the noise is present whether I have the A/C on of off. I'll have to check this again.

I've also read that it can effect how much air blows through your vents. I have noticed by manually controlling the fan output, settings 1-4 are basically the same (barely blowing), and 5 suddenly blows at full speed.

Any ideas?

Normally, I don't work on M-B's. But in looking-up your model (with U.S. specs), the information I have indicates that your car was made from about 2000 to 2003 as an ML55 AMG. The specs further show that your car came from the factory with either a fixed-displacement compressor (with a clutch), or a direct-drive variable-displacement compressor (without a clutch).

If your car has high mileage on it, the compressor might be getting "tired," especially if it's the original unit. Usually what happens is (if your car has a fixed-displacement compressor), either the clutch gap widens, or the clutch coil starts to fail. In either case, the compressor usually won't run once the unit heats up.

If your car has a direct-drive variable-displacement compressor, the electronic control valve could be at fault.

An overcharge can trip the High Pressure Cut-Off and subsequently shut-down the compressor on a hot day. But if you recently had the system re-charged with the correct amount and type of refrigerant, that possibility seems unlikely.

What year is your car, how much mileage has it logged, and is it a European-spec vehicle?

When my 1998 Frontier experienced similar symptoms about 2019, I tested it and found that when it wasn't cooling, the compressor clutch was not engaged but was getting voltage. For me the fix was to remove an AC clutch shim. Took me 30 minutes, didn't even need to remove my AC belt; and AC works fine still.