Help me troubleshoot an HVAC issue, please
Thread Starter
Super Member
Joined: May 2019
Posts: 671
Likes: 288
From: Omaha, NE
2007 S600
Help me troubleshoot an HVAC issue, please
The car: 2007 S600, 49K miles.
The symptom: It's summer. Over 90 F every day here. When you start the car after for the first time each day, the AC blows HOT air. Not warm, not even outside temp, but HOT air for a while. Like, 5-10 minutes or more. Driving or idling. The air gets hotter as the car warms up. At some point, if you're lucky, it will go from HOT to fairly cold air and work fine from that point forward -- until the next day.
What I've done: I checked the refrigerant; it was OK, I added a little. It's full, not over-filled. No change in symptoms. I had the dealer diagnose it; they found the code that says the left center vent control pot is shorted or open. That's an intermittent problem. Did not have them fix it for $800+. I have checked the actual values, it's fine in all but the fully closed position. I have a new vent assembly on the way, but I'm 99% sure that won't fix this. I also used DAS to run ALL of the AC checks. All passed. No fault codes. Blew cold air like it should. Of course no failure when it was testing...
What I suspect: The only thing that I noticed that was odd was the lowest evaporator sensor reading ever shown by DAS was 15.2 C. That's almost 60 Fahrenheit. Seems not too cool for an A/C system. I suspect there's a valve or flapper or something that's running the heater at some level... but I don't know enough about the system to know where to look, or how to test that theory. Anyone else have that knowledge?
Oh -- and my son's 2005 E550 is doing almost the same thing. Also ran DAS checks on it, also no trouble or fault codes found but it's blowing either very HOT or at best room temperature air. His was recently serviced by a local German car shop with a new AC compressor and the refrigerant is full. I've read a lot of posts on line about people with what sounds like the same issue, but frustratingly enough so far have yet to find a report of a successful fix.
The symptom: It's summer. Over 90 F every day here. When you start the car after for the first time each day, the AC blows HOT air. Not warm, not even outside temp, but HOT air for a while. Like, 5-10 minutes or more. Driving or idling. The air gets hotter as the car warms up. At some point, if you're lucky, it will go from HOT to fairly cold air and work fine from that point forward -- until the next day.
What I've done: I checked the refrigerant; it was OK, I added a little. It's full, not over-filled. No change in symptoms. I had the dealer diagnose it; they found the code that says the left center vent control pot is shorted or open. That's an intermittent problem. Did not have them fix it for $800+. I have checked the actual values, it's fine in all but the fully closed position. I have a new vent assembly on the way, but I'm 99% sure that won't fix this. I also used DAS to run ALL of the AC checks. All passed. No fault codes. Blew cold air like it should. Of course no failure when it was testing...
What I suspect: The only thing that I noticed that was odd was the lowest evaporator sensor reading ever shown by DAS was 15.2 C. That's almost 60 Fahrenheit. Seems not too cool for an A/C system. I suspect there's a valve or flapper or something that's running the heater at some level... but I don't know enough about the system to know where to look, or how to test that theory. Anyone else have that knowledge?
Oh -- and my son's 2005 E550 is doing almost the same thing. Also ran DAS checks on it, also no trouble or fault codes found but it's blowing either very HOT or at best room temperature air. His was recently serviced by a local German car shop with a new AC compressor and the refrigerant is full. I've read a lot of posts on line about people with what sounds like the same issue, but frustratingly enough so far have yet to find a report of a successful fix.
