Mercedes-Benz W211 heater not heating, water valve and heater matrix
Why does the heater on a W211 not heat or blow air at the wrong temperature?
The most common cause is a seized heater water valve. It can keep the air hot even when cold is requested, or prevent heat altogether. Another cause is the circuit of the front SAM control unit that drives the valve, in which case a new valve alone does not help. A third is a blocked heater matrix.
How do you check the operation of the water valve and the SAM control unit?
The valve can be opened and closed with the Star diagnostic tool. If warmth starts to come, the valve is fine. Fault code 9026 points to the valve's motor and 9027 to a short circuit. For the resistance measurement, disconnect SAM connector M4 and measure between pins 9 and 12: less than 10 ohms means the valve is replaced.
How is the water valve replaced and the heater matrix cleaned?
Remove the windscreen wipers and the plastic panel under them; the valve is behind a rubber cover. The hoses can be clamped shut with pliers, and coolant has to be topped up. Check the wiper motor connector, because it can oxidise. A blocked heater matrix can be flushed by circulating water through it, and the flushing may have to be done twice.