PeachParts Mercedes-Benz Forum - View Single Post - Being nickled and domed to death ($1000+ so far) over heater problem.

Diseasel300 · #2 12-31-2017, 11:16 AM

There is no damper in the ductwork. Heating is controlled by the monovalve. It opens and closes to allow coolant into the heater core. The heater core is ALWAYS exposed to the air stream, it's temperature is controlled by coolant flowing through it.

The definitive way to test the monovalve is to simply unplug it. It energizes to CLOSE, not to open. If you unplug the monovalve, you should have full heat 100% of the time.

If you don't, the monovalve cartridge is bad (you didn't say if they used genuine MB or aftermarket), or the heater core is clogged. It's really that simple. If they used a crappy aftermarket cartridge in the monovalve, shell out the $35 or so it takes to get an OEM one from the dealership or from this forum's sponsor and change it out. It's 4 screws and very easily DIY.