I have a problem with the AC on my 1999 R129 SL500.

It runs fine in "EC" (Economy) mode, ie without AC. However if I turn the AC on, the air supply dies after about 10 to 15 mins. The fan speed increases (as the in-car sensors detect the increasing interior temperature), but no air comes out of the vents. This happens even if I override the "Auto" setting and turn the fan up to max and demand air from the vents: still no air blows (although the fan speed increases). The problem is temporarily cured by switching back to "EC" mode for a few minutes; the air then blows strong again. However, this means regular fiddling with the aircon controls.

It has been suggested to me the evaporator is icing up, but I don't know what this means or how it might be fixed. Any help would be aprreciated...

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JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190

excellent description of evaporator icing.

Caused by faulty anti-ice switch--which is supposed to turn off the compressor when the sensor detects lower than about 36F in the evaporator. The theromstatic sensor capillary cracks and slowly leaks out the working fluid or the switch mechanically fails..

I bet the incabin filters are dirty!!!

Now I've only just had a charcoal filter changed as part of a major service; are there more filters I should know about?

As for the evaporator, while I was searching on the forum, I read that after 1996 MB evaporators do not cycle but rather the temperature is controlled by a variable pressure system--whatever that is. Does that mean anything to anyone? Even so, presumably any variable pressure system would need a temp sensor to know how to regulate itself?

The car is still in warranty, so maybe the big evaporator job will need doing!

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JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190

Hi

There is a cabin filter just below the windscreen wiper arm, it is quite wide and accessed by two philips screws I think. I'm not sure if this is the charcoal filter. I thought the charcoal filter was part of the emmission control system.

My filter was extremely dirty, my symptoms were total lack of air coming in

Regards

After scouring this forum and reading Steve Brotherton's article for Import Car I've run the diagnostic tests and pulled the fault codes from the push button control unit.

Evaporator temperature is 40 deg C. I gather it should be about 4 deg C. According to Steve Brotherton, the push button module is programmed to show an unrealistic value, eg 40 deg C, if an open circuit to the evap temp sensor or a short to ground is detected. This leads me to think my evaporator sensor is broken.

The push button module also shows intermittent fault code 419. I think this refers to the electro-magnetic clutch. I do not know what the fault could be. I have cleared the fault memory and will see if it comes back.

I gather the evap temp sensor is difficult to change, so I'm glad the car is under warranty still!

Any further ideas?

__________________
JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190