New problem on my wifes car. Its a 2005 C240 4Matic sedan. Not sure what the type of AC is called but it has separate Left and right AC temp settings. The problem started yesterday, and got worse today. It is Blowing wonderful ice cold air on the passenger side, room temp air on the drivers side. (my wife says it is hot air.. lets say its noticeably not as cold as the passenger side) This is true for both front and rear vents. Any ideas on what to look for? I've never encountered a problem like this on any of the previous cars (of any brand) I've owned.

 

 

Read the high side pressure, of course ambient temperature is a factor but at 87F there needs to be at least 200PSI, higher pressure with higher temp. You should not rely on low pressure reading only, do not overcharge the system.

In case you don't know, high side pressure is the output of the compressor, low side is the input of the compressor.

Purge hoses with 134a if you recharge the AC system yourself.

 

Very helpful answers both of you. Thank you. But I've been suspicious of the AC controllers as one of the problems. The Air is blowing plenty cold on the right side, yet not cold at all on the right. If there was a problem with the compressor system wouldn't it fail to blow cold on either side? I was thinking that perhaps there was a flaw with the dual temp controls, the passenger side working, the drivers side not.

While searching for parts, I came across the air temperature climate control sensor. If there are two of those (one for each side) then the problem could be caused by the drivers side going bad.

I'd love help, I'm new to Mercs and this one is driving me nuts.

 

But did you check the A/C refrigerant pressure????? Obviously not if its still driving you nuts. Forget about your logic thinking the pressure must be ok if its blowing cold on the right side. What you are not aware of is that Mercedes engineers didn't subscribe to your typical logic. They have given you advanced warning via software that you have a low pressure problem. And the warning is "blow ambient on the driver side when there is low pressure in the A/C system"!!!!!!!!!!!!! Maybe you have a different problem but I doubt it so check your pressure according to my earlier reply, now this is not saying you don't have a leak, but 134A will slowly leak out of any system and since you still have cold air chances are you just have to add some 134A. But when adding you cannot let any air get into your system which means you have to purge the charging hoses and if you don't you will end up with an expensive repair bill. So you need to add the 134A with manifold gauges, where you can read the high side and can easily purge the air out of the hoses. The cans of 134A you buy at the auto parts store that have only a low side gauge attached is not the proper way to add 134A. Usually you can get the gauges as a loaner tool from AutoZone. Red hose goes to the high side, blue to the low side, yellow to the 134A tank/can. If you do use a can, purge the included blue hose, by exhausting some 134A through it, then close the end valve on the hose so the 134A is trapped in the hose. Then you can just use the manifold gauge to connect the red hose and first make sure all valves are closed on the manifold gauge set before connecting, you only need to open the valve at the high side port, don't open any valves on the gauge set if you are using a can that has the blue hose already connected to it. Charge the high side to 200psi when the ambient is 85F, although it won't hurt to bring it up to 225psi. DO NOT INVERT THE CAN TO ALLOW LIQUID INTO THE SYSTEM. CHARGE IT SLOWLY, DO NOT OVER CHARGE THE SYSTEM. The advanced warning idea, can work in favor of a shop who might falsely charge you unnecessary for parts and the labor associated when you only needed $10 dollars of 134A. Yes owner it was the Knueton controller which will only cost you $450 because we are running a special this week. Its buried in the dash so it took us 3.5 hours of labor to get to it.

 

Blend motors to the flapper valves would be my suspect. STAR can verify operation of the flapper valves. The two-zone and four-zone systems are highly networked systems. On my older W210 it was a broken vacuum line that caused failure of one side of the air distribution. I don't have anything on the W203 but here's something similar from the W211 class. This should give you a starting point. Start with M2/6 or W203 equivalent.
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Thank you both for the much more detailed answers. I'll check the pressure on the system first, as I already have the manifold gauges for that. If I get good readings on that I'll look into the vacuum systems, before buying any new parts. I have had a chance to study the symptoms in more detail. The problem (the difference between the left and right temperatures) is greatest when it is hottest outside. in the evenings/mornings we get cool air on both sides, as it gets hotter outdoors the difference becomes much greater.

 

can you properly adjust hot cold temp on both sides without AC? that would verify if your blend flaps work properly

But it may well be low refrigerant or leak... similar symptoms on other MB models... The freon evaporates first on the passener side part of the evaporator... not enough left when it gets to the drivers side...