W124300d

Hello, this may be a car from hell? It doesn't matter how I set the temperatre or which directional button I push, hot air blows powerfully out of the vents. The O setting does keep it off. The a/c of course is dead, but I can't get normal outdoor air into the car. One clue -- someone has been working on the sun roof. Is there a temperature sensor in the roof somewhere that they may have damaged??

rivcal4life

The temperature sensor is located inside the dome light assembly. The motor for the sensor (that sucks air past it) is in the dash on the passenger side.

http://www.500eboard.com/forums/show...quot-%29-motor

However, this does not sound like your problem. You likely have a broken vacuum pod behind the passenger airbag (or glovebox if thats what you have).

http://www.500eboard.com/forums/show...AC-vacuum-pods

abaddsm

I had the same problem with my 88 300e and with a swap of another HVAC button set the problem no longer existed. I bought it used on ebay for like 12.

neanderthal

Might also be duovalve.

DW124

+1 on the duo valve

jshumphrey

I had that problem. I fixed it by replacing the "monovalve" that controls whether or not water flows through the heater core. When it gets a 12V signal, it's supposed to open. Troubleshooting is simple. Check the voltage across the electrical connector when the heater is off. It should be 11V or more. If not, your problem is most likely in the push-button control unit. If voltage is there, it's most likely the valve. You can apply 12 directly to the valve and listen for a click. No click = bad valve.