Mercedes-Benz W126 heater not heating or heat disappears while driving
W126 heater blows warm air only briefly or heat disappears while driving, what is the fault?
The most common cause is the water valve: its rubber diaphragm has torn or the valve internals are broken, so warm coolant does not circulate properly. Sometimes the symptom is that heat only comes at idle. There is no full agreement on the cause; worn valve springs and air in the cooling system have also been suspected.
Where are the W126 water valves and circulation pump and how are they checked?
The water valve (solenoid valve) is next to the battery, and the circulation pump is on the bulkhead below the valves, by the hoses. Opening the valve cover shows immediately whether the diaphragm is broken. The pump can be tested by feeding it power from the battery. The grille and water channels should be cleaned of leaves and needles.
What should be noted when replacing a water valve?
One heater control wheel means one valve, two wheels mean two. Four screws in the cover mean one repair kit and six screws mean two. The magnet part number is A0008350644. Valve parts from another model, for example the W123, do not fit the W126.
What else can prevent heat from coming out in the W126?
The temperature sensors in the roof and in the footwell may be dirty or faulty. If only a little air comes out, there may be a leak or a loose hose in the vacuum that controls the air flaps. The blower relay or fuses also affect air flow. The valves regulate temperature, not air volume.