Mercedes-Benz W126 heater blows only hot air
W126 heater blows only hot air and the temperature cannot be adjusted, what is the fault?
Usually the fault is in the heater control solenoid valves ("jay's eggs"). Without power the valves stay open and hot coolant flows all the time. The cause is often a broken rubber diaphragm in the valve or a lack of electrical power.
Where are the W126 heater control solenoid valves and how many are there?
The valves are at the front of the engine bay on the passenger side, next to the battery. The valve block comes off when you undo a couple of screws and lift it out. Automatic climate control has one valve, manual has two.
What should be checked when the W126 heater control does not work?
First check the valve fuse and the connector, which may have come loose behind the heater control panel. Then open the valve and look at the diaphragm: a broken diaphragm is renewed or the whole valve is replaced. The coil wires break easily, so open it carefully.
What else could it be if the temperature will not adjust even though the valves are fine?
Cold air may only come out with the control in the zero position, in which case the sliding surfaces of the control knob may need cleaning. The blower motor brushes may also be worn out or the control roller linkages poor.