AC problem
 

I think 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 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 makes a noise). 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.

As summer approaches I'd like to put this right. I'd appreciate your input before going to the dealer...

Your evaporator is icing up....textbook case. The compressor should cycle on and off when the evap temp gets low enough...apparently its not doing such.

Now the question is what's causing this...weeeel...I don't know any of the specifics of 500SL air conditioning, but most cars nowadays have a thermoswtich that shuts off the compressor momentarily when the evaporator gets to a certain temperature...usually 35F or so, then back on around 40F. It could also have a temp sensor on the evaporator, and the climate control computer monitors the temp sensor, then shuts off the compressor...but like I said, I don't know the specifics of this car.

That is very helpful; thank you very much. What you describe corresponds with my car's symptoms: the small amount of air that does come through is ice cold. The car is under warranty, so I'm not too concerned if the dealer simply wants to replace the whole unit!!

Well, an entirely different take on the problem. If the fan runs but no air comes from the vents, it might also be a vacuum actuator problem. These actuators manipulate the air doors and ducts that control what air goes where. It's all very complex and you're right to seek out the services of a competent Mercedes shop. Tod

This was my first line of thought, but the problem only occurs whent he A/C is switched on. It does not happen in Economy mode; in fact the problem is (temporarily) cured by switching to Economy mode. Also, it is not something that happens immediately.

It is going to the shop on Thursday. I'm not sure whether they are competent though...