W201 Tempmatic Chaos?
 

After dancing the victory dance about my 87 190D Batt Drain issue, I have had strange partial failure of the Tempmatic System. A few nights ago I was traveling in a HUGE snowstorm, the Blower and Temp on Hi. As the car fought the blizzard, I got blasted by the depressing "blast of Burnt Electrical $omething" ! Not far from home, I parked it and planned a Troubleshooting session. Symptoms? The system went to default, full heat / DEF. After doing some button pushing today, I discovered that the System worked as advertised,(ie Temp Regulation, DEF/Floor/Side vents OK) EXCEPT on one button, the Floor Heat. It trips the default, until I shut off the car, that's all I get. Restart the car, and everything is OK, until I hit Floor, then the system defaulted to Hi Heat, DEF. Turn off car for a reset, same thing! 1) Can the CCU "break" in just one Button position? What can cause it? 2) Can the Switch block only fail? 3) The stench was TERRIBLE! Can just a partial failure, with no blown fuses, (lower Amp circuit)cause such a stench? Any thoughts? Thanks! BTW the PBU was a Reman installed 4 years ago....

Guess you need to pull the glove box & look for melted wiring?

Your HVAC control box is full of switches. They do get dirty.

I also do electronics repairs.

While I had the heater ripped apart on my 190E, I cleaned up and did a fair amount of circuit board soldering on my control unit. I found a cold solder joint at the fuse itself on my unit, for starters.

Mine is working fine now.