92 300SL Air Conditioning is erratic

[45jake]

My SL has 90K miles and I have replaced the evaporator. It blows nice cold air, but occasionally (more often lately) it will stop blowing cold air and actually blow hot air. Sometimes, the hot air comes from the side vents in the dash, while cold air blows from the center vents. Question is, where should I begin troubleshooting?
Thanks.

[tecqboy]

I have a 91 with the same HVAC controller that you have. To say the least, it's very confusing to operate. There are all those "arrows" on the controller itself and then there are the two buttons ( red and blue ) in the center dash air outlet. I undersatnd that MB wanted a method to blow cold air in your face while keeping your feet warm and various other combinations. Anyhow, in my car, the red and blue switches sometimes "forget" where they were set and need to be re-pressed. Also, this controller is known to have issues with the solder joints on the switches breaking down over extended years of use. If you use the search tool of this forum and look for information on the HVAC controller, you'll get a lot of info.

[bobterry99]

Originally Posted by 45jake

Sometimes, the hot air comes from the side vents in the dash, while cold air blows from the center vents. Question is, where should I begin troubleshooting?

The system apparently has no control over the flow of hot coolant into the heater. This is likely a faulty with the in-dash control unit or monovalve. I would begin by checking for stored fault codes as explained in this manual: http://www.benzworld.org/forums/r129-sl-class/1266535-diagnostics-sl-pre-94-a.html?.#post2342164. Begin on page 86.