The symptom: No (or intermittent) heat in the front seats of a 2007 W221 S600.
- The vents blow cool to cold air most of the time.
- Changing the temperature zone setting has no effect.
- Turning the climate control system OFF for a minute or so, then back ON will result in warm air blowing briefly, cooling off rapidly — as if there is hot water in the heat exchanger, but no flow of hot water from the engine.
- XENTRY/DAS diagnostics show all temperature sensors working.
- Replaced AC water valve. The old one was in need of replacement; the top water hose neck snapped off during removal and the rack & pinion gear was jammed with some debris, so it likely wasn’t working. The valve is a non-Mercedes sourced part, unbranded, likely Chinese sourced like everything else. $40, quality appears to be quite good and the fit was perfect.
- Tested valve operation with XENTRY/DAS. Using the actuations top open and close the valve result in hot air (CLOSED) and cool air (OPEN), so the valve is mechanically and electrically operational.
- Used diagnostics to teach-in and normalize all actuators, flaps, and air control potentiometers. This had no effect on the symptoms.
- I wanted to reverse the wires to the valve at the plug. My hypothesis was that the motor was simply installed backward, resulting in the valve running opposite the direction commanded by the HVAC system. Since it’s an open loop system, the front SAM would have no idea that this was happening. Unfortunately, I had no convenient place to swap the wires. I ended up determining that it was indeed wired backwards internally. A new part from a Mercedes dealer (thanks, Husker Auto Group!) fixed it.
Results: Fully working heat!!