Mercedes ML55 (W163) A/C cuts off in hot weather

[mattmisanthropy]

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?

[JohnHere]

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?

[Cusser]

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.

[johnarden]

Hey, you’ve done a great job piecing this together — your logic is actually pretty sound.

What you're describing does sound like it could be related to either overcharging or a cooling fan issue, possibly even both. When an A/C system is overcharged, it can definitely build excessive pressure quickly, especially in higher ambient temps. If the pressure gets too high, the high-pressure switch will cut off the compressor to protect the system — and that would explain why it stops working once the engine warms up and temps climb.

Now, in cooler weather or lower load situations (like cruising or light driving), the pressure may not spike as much, so the compressor keeps running — which matches what you're seeing.

A few things to consider/check:
Overcharge?
Yes, a system that's just a bit overcharged might still behave “okay” in cooler temps but act up in the heat. It doesn’t always shut down constantly — it depends on how much over. You might want to get the high-side pressure readings checked again with proper gauges just to rule this out.

Cooling fans working properly?
If the fans aren’t kicking in when they should (especially the high-speed setting), pressure in the A/C system can build rapidly. So even if refrigerant charge is fine, bad fan performance can still mimic overcharge symptoms. Worth checking that both fans run when the A/C is on and engine temp is up.

Fan speed issues (1-4 being weak):
That sounds like a blower motor resistor problem. On many cars, if the resistor fails, only the highest speed (which bypasses the resistor) will work properly. It’s not related to refrigerant charge, but definitely worth replacing — they’re usually cheap and easy to get to.

Smell when A/C stops working:
That could be a sign of mold or refrigerant oil, or possibly the system just going into protection mode and venting pressure internally. Hard to say without sniffing it myself, but it might help a mechanic pinpoint the issue.

Pulley bearing noise:
If it's constant with or without A/C on, it’s likely the pulley bearing as you said — not the compressor clutch. Still worth keeping an ear on, but probably not your main A/C issue.

TL;DR:
Yes, it could be overcharged, but it could just as easily be airflow/cooling related — like fans not kicking on properly under load. If it were me, I’d:

Get the system pressure tested again (static and running, high and low sides).

Check that the condenser fans are running at full speed once the A/C is on and temps rise.

Replace the blower resistor to fix the cabin airflow issue — unrelated but worth doing.

Let us know what you find! Troubles like this can be super frustrating but you're definitely on the right track.