I have spent months trying to diagnose this problem, and spent quite a bit of time searching the AC threads on this grate site... Each time I think I have found the source of the problem, it turns out not to be it. So, here is the LONG version of the story:

I purchased this 1988 300TE (w124.090, engine 103.983) to be my primary commuter car when I lived in Kansas. It worked great, and I had no complaints. Then I moved. Two hours down the road, the AC cuts out. The cold dry air coming out the vents changed over to cool moist air, and after a little while, to hot dry air.

The first diagnosis was low freon. I could not find anyone in NW Iowa (where I was many miles from a major metro area) who was willing/able to actually do the recharge on this car in the time I had in the area. A few days later, I headed north to visit family in Northern MN, and the AC worked the whole trip (Cloudy day, relatively cool dry air). I got the system recharged up there - but I found out later they managed to botch the job, and didn't get the fittings properly sealed, about half my R134a leaked out within about two weeks - but by that point I was in Virginia, and had to pay someone else to re-do the fitting and recharge. On my way out of MN, it was a hot humid day, and the AC system cut out after about 20 minutes on the road. Further drive testing determined that at full blast, the AC would only run for about 20 minutes (from a cold engine), but on a minimum cool setting, it would run indefininately (at least at first - before the refrigerant started to get low a week or so later). By the time I was commuting in Virginia, the AC would only run for about 5-10 minutes (much hotter and more humid than on the Prarie or Plains). I took it into a shop in Virginia, and they very quickly diagnosed the leak - it was almost visible to the naked eye (even I could see the yellow dye staining everything around the low pressure fitting). They drained the system, replaced the fitting, and recharged it. That's where things start to get interesting.........

I picked up the car and drove it to another place in the county to register it (I'd only lived in the couny for about a week), and sure enough, about 20 minutes after I left the shop, the AC stopped blowing cold dry and started blowing cool moist. I turned right around and went back to the shop. They kept the car all day the next day - running the car on idle most of the day trying to force the AC to fail. It didn't. Then they got in the car to drive it, and it cut out within a block. Temps both days were high, and the humidity similar. It was the edge of a long weekend, and they woudn't be able to do much for the weekend anyway, so I was going to take the car and drop it off again the next week - except it wouldn't start. It seems the battery was dead. A quick jump-start and check of the electrical system with a voltometer, and it becomes immediately apparent that the Alternator is only putting out about 12.2V even at high RPM (e.g. not enough to run/recharge everything properly). A replaced alternator later, everything but the AC is running much better (I had been having a few other minor annoyances that were evidently electrical in nature).

So, I take the car home, and pull the wiring diagrams off the shop manuals, and start trying to trace a fault in the control lines (I'm an Engineer myself, so while this vehicle is unfamiliar, the concepts are not). I cannot find one. All the electrical lines seem to be working properly - the compressor is getting shut off after about 20 minutes of operation, and I'm stumped as to why. As I look more and more into the operation of the AC system, I suddenly realize I'm low on engine coolant - so I top it off, and immeidately take a test drive. The engine is now getting cooled better, so the engine coolant switch doesn't cut off the compressor as often, and for about an hour I can recognize the compressor cutting out at high engine temp, and turning back on once the engine cools back down. No problem. It seems to be fixed. It was a HOT and dry day.

The next day it's cool and rainy, and the AC cuts out after about 20 minutes.


So, I start checking all sorts of things, even asked a family member who is a retired GM mechanic (whom I had not spoken with in over a dozen years). I explain the whole problem to him, (as I have above), and he thinks it's either a) the dryer crystals are shot, and I need a new dryer, or b) my condensation drain may be plugged. The Humidity (both inside and out) is what he keyed off of. If I have a plugged condensation drain, then the water will back up and reduce the air-flow over the system, and not permit the proper heat-exchange, thus freezing the system and forcing the compressor to shut down. He suggested I put a temperature probe in the vent to see what temp the air coming out is. I did so the next morning, and every day since. The temp runs down to about 43.0°F at minimum. I have seen it as low as 41.7°F. The AC seems to operate between about 40°F and 50°F, but once it gets much above 50°F it has lost compression, and rapidly climbs to the ambient outside air temp, as well as becoming massively humid.

As soon as he mentioned a possible water build up, I remembered that I had heard on occasion what sounded like water sloshing around in my dashboard - but I have found no mositure inside that could be associated with it (I have found some moisture near the hatch in the back, but that is following rain and is coming in through a crack in my antenna base). I also have not seen any water dripping beneath the car, at all. Ever. He suggested I take a coat hanger or other wire and push it up through the drain hose to see if I can clear the blockage.

I did some checking, and found what looked just like a section of bycycle tire inner tube (wide and flat) coming down from about the midline of the car, directly beneath the windshield, and routing around the top of the transmission to the driver's side. It does not appear to be a straight-shot from the hose opening to the top, and I am really nervous about the idea of probing around in there with a coat hangar. I did attempt to probe it with a 1/4" plastic/nylon hose (clear, so I can see immediately what's in it, but soft enough so that I couldn't do any real damage to anything with it). I was unsuccessful.

Searching on this site seems to tell me that indeed I have a blocked condensation drain, but mine does not appear to have the insulation on it that I have seen described elsewhere, nor do I have a clear path forward from here. My read on it is this: I have a clogged condensation drain, and I need to get that water out of there, regardless of if it fixes the AC problems or not. I am an Engineer, but I do electronics and software, not mechanical, so this is kinda outside my realm of expertise. I do understand the basic concepts, and with a good set of instructions I can probably do the repair myself, but I'm missing a disk from my shop manuals on the 124 series (Gotta love moving and having everything in boxes), so I do not have any idea where to start on going in from the pasenger compartment side, nor do I really feel good about poking with a coat-hangar from the undersdie. I do understand that I may have a wrong diagnosis, but I do need to get that water out regardless - unless I am supposed to have a water-sloshing sound in my dashboard in this particular model year (1988 300TE).

It has been both hot and warm, but consistantly humid every day since the system worked for as long as I tested it; and has consistantly cut out at between 18 and 25 minutes after first getting the vent temps down into the 40s. I did have one day where after the system cut out, the humidity in the car built up to over 95% (I have a humidity meter on my probe thermometer), so I get both temp and humidity).

If you've gotten this far, thank you for your patience in reading my ramblings, and thanks in advance for any advice you can give me. I will stand ready to answer any questions, and I throw myself upon the expertise of this site.

CTM

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE