[Don Shorty]

My 2008 GL450 blows cold air from passenger vents, warm air from driver's side vents with A/C set to cool. It has been doing this for a couple of weeks with no change. I tried to search for this but can't get search terms specific enough to get results. Anyone seen or had this problem? Any suggestions how to diagnose? No warning lights are on. Thanks.

 

[Don Shorty]

Ok. I seem to have solved my problem. I googled "Mercedes A/C blows hot air on left side and cold air on right side". I found several threads that said low refrigerant could cause this problem on Mercs a few years older than mine. Sometimes this answer would be questioned as to why low refrigerant would cool on one side but not the other. No answer to this question was ever given that I could find other than "they were designed to do that". So I didn't really believe that could be the problem. But.....I checked the refrigerant pressure yesterday and found that it was indeed low. Added a can of 134A this morning and pressure was still a little low but now it is blowing cold on both sides. Don't know why but it fixed the problem but it did. I will check pressure again to see if it is holding steady or if I have a leak that needs to be dealt with.

 

[shadow00026]

My best guess is that there was enough to feed one side of the vehicle and not the other. I bet the dual zone climate control plays a role in that.

 

[moosejaw5.0]

Another cause of this is the heater mixing valve. This valve has a proportional control so it can get stuck or stop working while open. This is a common failure on the W211 and a MB Star scan will show an error but nothing will come up in OBD II scans.

 

[Don Shorty]

It is still working fine after adding refrigerant. Pressures are saying constant (adjusting for ambient). When it was not working, the left side air was outside air (not heated air) and the right side was cooled air. Both sides had good air volume coming out. My guess is that some sensors determine the temp of the liquid refrigerant downstream of the expansion or the temp of the air across the evaporator. Then, if the temp is not low enough, the left side is ducted to outside air resulting in less air volume comes across the evaporator with this cool air only being ducted to the right side. I wonder why the driver's side was not given priority in cooling?????

 

[moosejaw5.0]

If the heater valve is stuck you won't get heat. You might go from 45F vent temp to 75-80F vent temperature. By the time the water gets part way through the heater core it is cooled down. HVAC sends the outside air into the evaporator coil first and then into the heater core (for proper humidity removal). The heater valve is a common item and will exhibit these very symptoms. Once again, a MB Star scan will show if there is any stored codes on the HVAC system.

I don't recommend just adding refrigerant without knowing how much is in the system (leaked down or full charge). It is really easy to overcharge an AC system and you won't know it until it blows out the compressor discharge o-ring on a hot day.

 

[Don Shorty]

If the heater valve is stuck you won't get heat. You might go from 45F vent temp to 75-80F vent temperature. By the time the water gets part way through the heater core it is cooled down. HVAC sends the outside air into the evaporator coil first and then into the heater core (for proper humidity removal). The heater valve is a common item and will exhibit these very symptoms. Once again, a MB Star scan will show if there is any stored codes on the HVAC system.

I don't recommend just adding refrigerant without knowing how much is in the system (leaked down or full charge). It is really easy to overcharge an AC system and you won't know it until it blows out the compressor discharge o-ring on a hot day.

Thanks Moosejaw5.0 for your advice and knowledge. My A/C is still working properly with similar vent temps on both sides after addition of refrigerant to the previously under pressured system. If the operating suction and discharge pressures are within range for a specific ambient temperature, how can it be overcharged? Overcharge would result in a high head pressure wouldn't it? Assuming there is no blockage or failure in the system which would show up in pressures being out of range on one or both sides? Since it is a closed system and PVT relationships for 134a apply, why do you need to know the remaining weight of refrigerant? Won't the pressures tell you when you have added the correct amount to the system for it to function as designed more precisely than just adding a set amount without regard to pressures? When I work on older cars, the likelihood of one or many components not being of original design and capacity is very high so you can't rely on "factory specs" for volume needed for a completely evacuated system.

 

[JCDrew]

2012 ML350... I thought I had low refrigerant, as I was not getting cool air when 95F+ outside. Checked the level and I was at 40lbs... right where it should be. Thought I'd just deal with it later since summer is about over. My wife then noticed that the passenger side IS cooling properly. Very definite difference between driver (left) and passenger sides. Also tried change air distribution to the floor/vents to see what would happen. Same problem. Continuing to investigate.

 

[wallyp]

"Not being hampered by facts, I can discuss the subject freely."

I know very little about the ML-Class - but on the W220 S-Class, that would likely be the "duovalve", a computer-controlled dual-solenoid valve that controls the flow of coolant to the left and right sides. The valve can often be disassembled and cleaned to solve the problem. Perhaps this will at least give you a place to start searching...