Heater control E300D W124

[freebird]

My heater is uncontrollable and constantly blows hot air even when both controls are set to cold. Is this a common fault and does anyone know how to rectify the problem.
I have had this car for ten years and intend to keep it. I would like to add cruise control, is this possible.

 

[jberks]

Seems to becoming a common thread.
3 possible faults apparently - you may want to look these up.
1) Duovalve
2) circuit board behind the controls
3) ambient temperature fan and sensor.

It will probably be one of those. Start with gently tapping the duovalve, they get stuck apparently. After that, you're into the electrical stuff.

 

[MERCYBIRD]

Heater and other electrical Gremlins

Older Merc`s can all suffer from electrical faults that can be traced to "dirty"contacts in the fuse box, between the ends of the fuse and the holder.

 

[joe bloggs]

First test, pull the 3 pin plug off the duo valve and check for an ignition supply to the centre socket, can you also check the resistance of the valve between the centre pin and both outer pins (2 readings) and post back, have a look at the fuses as mentioned, The heater valve defaults to fully hot when no voltage is present.
The reason for the resistance test is that often one side of the duo valve resistance drops fairly low (internal short/corrosion etc) this increases the load and therefore the current the ecu is being asked to supply, as the ecu has a failsafe cut-out when excessive current is drawn this is the reason both valves lose supply (spring open).