1990 560 SEL Heater Problem - Not the Mono Valve
 

If the car is started from cold with the heater control set to max, as the car warms up it has great heat. Makes no difference whether I'm on the highway, driving around town or idling, the heater puts out good, hot heat! When I turn the control dial to anything less than max the heat goes away, or is very minimal.

I've changed the mono valve twice, first to a, presumably good, used Bosch unit, then to a new MTC unit, but the symptoms are always the same. Even turning the car off then back on with the dial set to max does not give me heat. It's only when the car is started from cold with the dial set to max that I have heat again.

Could this be a fault in the control unit? It works perfectly for every setting except heat and it works fine if the car is started from a cold engine with the heat set to max.

Hope someone has a clue where to look next. It's getting cold! Thanks.

It does sound rather like the control unit has an issue. One thing that effects just about every module on a w126 in some way or another is bad solder joints. I cannot even count how many issues I've fixed on my cars by simply removing a module, opening it up, and re-soldering any questionable looking solder joint.

I would start there first, and if that doesn't pan out, you'll probably need to get a meter and start checking for continuity, etc. The control module is supposed to control the monovalve like a pulse width modulator; it should continuously open and close the monovalve to achieve what it thinks is the correct air temperature. When you set the wheel to "Max" and it clicks, it is supposed to override the pulsing and make the monovalve wide open.

Again, hate to say it but it does sound like the control module. Don't despair until you've opened it up and checked all the solder joints - be particularly suspicious of the joints at the edges of the board, where connections to other components like switches and buttons are made.

Good luck.


-tp