Have a 2003 W220 S600 with an intermittent AC problem. All climate controls and functions work but the problem is that the system seems to decide on its own when to produce cold air and when not too. In general, when the car is stationary it'll produce nice cold air but when moving it may or may not put out cold air.

I have the full Star diagnostic system. No trouble codes. All tests work fine. System pressure is 7.0+ BAR at rest and up to 20 BAR (but usually around 15 BAR) when producing really cold air.

It almost seems like a sensor problem. However, with only one exception all sensors appear to be working fine. (See below.)

Both interior sensor measurements (dash (N22b1 and overhead N70b1) seem right on as does the Sun Sensor (B32/2) and the multifunction sensor on the air intake under the hood (B31). The various coolant system sensors, (evap and core B10 2,3, &6) all seems fine as do the various other sensors such as engine and refrigerant temps.

The one sensor reading that seems odd is the outside temp reading displayed as Nr. 2 on the self diagnostics. When stationary (and the AC working great) this reading can be as high as 50C! but when the car starts moving, it'll drop to around 27-30C (which is what it should be here in Key West Florida.) But when the temp readout drops, I usually but not always see the BARs drop and the cold air taper off. This reading is supposed to come to the AC control unit from the outside sensor (B14) through the Left SAM module. The kicker is that B14 also feeds the dash temp gauge and that gauge always seems to be exactly where it should be. I installed both a new sensor and SAM module just to see if things would correct themselves but everything stayed the same.

One other thing, the AC controller passes all tests. And I see it calling for AC when I'm not getting cold air (when using my Mercedes diagnostic system). Something seems to be overriding the call for cold air and it may or may not have anything to do with the outside air temp abnormality I am seeing.

Any thoughts/Suggestions?

Thanks in advance.

 

Any thoughts, suggestions?

 

I re-connected my STAR diagnostic system and went through everything again. (I'm still learning it.)

I found was that there was a separate controller diagnosis section for the SAM's. The was in the section called BODY. Checked for faults on the three SAMS and found under the front left SAM that I have error B1640. "Component or signal line to A9 (compressor) has open circuit or short circuit." I went through the 2 tests. Test one actuates the compressor for 10 seconds. The compressor is supposed to raise the refrigerant pressure and lower the refrigerant temp. And it does...I did test two which is to check the resistant between a pin at the SAM which goes to the compressor. The acceptable range is 4 to 12 ohms. My reading was 14.3 ohms. The STAR system tells me if this is the case, either the line or the compressor is defective.

I will check the connection in the morning (when the motor cold.) I'll unplug and re-plug it. And then retest the system.

As a note, I went back into the AAC section and found that I'm now failing the refrigerant circuit test and that I have to clear the B1640 error before it'll work. But still no error codes in the ACC section of STAR.

 

Thanks for the update of this thread. Interesting challenge- looks like your use of the STAR may be isolating the possible point of failure.