W124 max heating, like it or not

[DavidBlake]

My 300CE heating will only deliver maximum heat, even with the temp selector wheels on the climate control set to minimum, or off.
Does any body have experience of this and know the gremlin?

 

[C220GJS]

The heating valves default to fully open when power to the duovalve/monovalve solenoid controls is lost , check your heater fuses, the bullet type fuses fitted to your car are a known problem area , if you find this is the problem it's best to replace all the fuses and clean up the contacts in the box while you are at it.
Cheers Geo.

 

[DavidBlake]

Thanks, but fuses are OK.

 

[C220GJS]

Well that's one thing ruled out, but I still think the cause is a lack of power to the duo-valve, can you disconnect the plug from it and check if there is any power reaching the plug.

Check this previous thread,http://forums.mercedesclub.org.uk/showthread.php?t=17868, Geo.

 

[DavidBlake]

Thanks for your help, the problem is now intermittent, so I think I have a dodgy supply to the duo valve. It always happens to both sides which seems to rule out a mechanical problem with the valve. I will check all the connections and the supply.

 

[Bolide]

Max Power heating

Most of the time it's the infamous Duovalve but there's another possibility too: there's a sensor up in the interior light assembly with an air pump that pulls air past the sensor. If this fails or is blocked then the heating tends to go full hot

Use a wallpaper stripping scraper on the right hand side of the light assembly to push back the spring clips and the unit will come out of the headlining. You'll see a rubber tube going off into the headlining - it goes down the A pillar to the pump in the dash. Clean the sensor vent & check the pump is working

But I still think it's the Duovalve..

Nick Froome
www.w124.co.uk