Has anyone experienced any problems with the heating system in their W140? I have a 93 400SEL and all controls appear to work properly, only thing is, cold air blows out at all temp settings. This past summer I had a problem with the A/C blower motor. I replaced a electronic module on the blower unit and fixed the problem. Now it's time to use the heater and all I get is cold air. I thought maybe the "heater core" has gone out but, I'm not experiencing any other symptoms of a bad heater core. If anyone has experienced this or know of anything that can be checked out, please let me know.
It's a really good looking car, it runs and drives great. I don't have any really good pictures yet but, I will soon. I've made a few interior and exterior upgrades. I've added ADR 20x9 wheels and a mobile video/navi system.

 

The duovalve is a valve assembly with an electric pump which opens and closes valves to regulate hot coolant to the heater core. There are 2 valves (hence, the name duovalve), one for the right and and one for the left side of the car. The main duovalve is mounted just behind the left side headlamp (at least for US cars. I don't know if right hand drive cars are opposite on this). Unfortunately, it's pretty common for the duovalves to have problems. If you have the rear climate control option, your car will have a second duovalve mounted low behind the right wheel well.

For pics of the rear duovalve, see pcmaher's rebuild on his web site:
http://v12uberalles.com/Rear_AC_System.htm

The main duovalve looks nearly identical sitting behind the left headlamp.

Brett