The air-conditioning in my 1999 E300 has been acting strangely lately. For a week or two it was erratic. It mostly worked fine but occasionally it just stopped working. That is, the air was blowing but it was no longer cold. Start the car again a few minutes later and it worked fine for quite a while. I was scratching my head as to what could explain this. I have had compressors fail in other cars, but once they stop working they don’t start up again! I found that pressing the EC (Economy) button on the controls and then pressing it again a minute later seemed to restore normal operation. But today I tried that and now I cannot get the EC button to toggle off. No matter what I do the red light stays on. I was originally thinking there was something amiss with the clutch for the compressor. But this odd behavior of the control unit makes me wonder if the problem is not mechanical but electronic. The control unit looks like it pops out of the dash like the stereo. Is there something I can check out in the unit? My abilities and tools are limited although I can certainly pop the thing out and fiddle around with it. Any ideas?

I have notes from Blake's post from 2010 re AC diagnostics but with the EC light permanently on I am not sure it will work!

 

you have a leak someplace and are low on r134. when the freon gets too low. the red EC light will come on and the ac wont work.

 

Thanks!

 

The MB dealer said they found an error code indicating a blown fuse, apparently in the dashboard control unit, and replaced it (for $100)(Fuse might have been free; reading the code was probably $100!). AC worked fine for maybe an hour. My local garage sized it up and said it was the fan motor for the AC radiator. They replaced it twice (first one was apparently a dud). Said they checked the entire system, presumably including the refrigerant. AC worked fine on the drive home. After dinner it worked for a few miles then quit. That AC radiator motor is not turning over; the system blows no cold air; and the EC light remains on no matter what I do.

Data from 8/1/2012 about 2:00 a.m.. Car temp readout shows 79 degrees:
1- 76
2- 80
3- 78
4- 80
5-134
6-155
7- 06
8- 87

Codes:
E61-419
E61-241
E-FF

My comments: Refrigerant pressure (#7) seems quite low. The evap temp (#5) seems way hotter than the ambient air, particularly with the engine not running long enough to get hot. Not sure how that's possible unless the sensor is kaput.

Any help would be appreciated.

 

I also have an air conditioning problem: In my 1993 300d when I am up to a decent road speed (50+ mph) the a/c works fine, but when I stop for a stop light it starts getting warm. The MBZ house (not official Mercedes) told me that there is an rpm sensor (on the A/C?) which disengages the clutch when I slow down. They said I need to replace the compressor ($1200+). Does this sound right to you?

 

I agree with ForcedInduction. From what little I know about that particular system in that 93 300d. The switches and fans were a common issue. If your AC blowing cold while your driving I would guess that most likely the compressor is working just fine. From my experience sometimes just a low R134 charge can cause poor cooling at idle to.

 

So - No rpm switch on the 93 300d? Also, I think the 93 used r12 - how about the conversion process for r134a?

 

I appreciate your comments. Since I am an electrical engineer (retired) with more time than money, I was wondering if you could tell me where I could find a wiring diagram for the A/C since you told me RPMs are not directly involved.

 

Thanks for your response. Could you tell me what switch(I don't have a wiring diagram) and where I could find it on the chassis? One other indicator: when I am sitting at idle, the a/c clutch does not engage.

Any advice would be appreciated!

 

Thanks for the diagram. It is very clear, but I'm a little confused: You said there's no RPM sensor (maybe I called it a switch), but the wiring diagram shows one. Is it internal to the compressor or somewhere else?