So there I found myself, driving around with intermittent heat. Fine around town, but as soon as I hit the highway - freezing! I thought it odd that if I was going faster, and the engine running hotter, that I would lose all heat. It would not just cool down, it was completely gone.

I surfed the news groups and read up a bit (rec.auto.mercedes) and the general consensus was: "Need to replace the Monovalve." I even found someone else that had the same symptoms - Heat in the city, no heat above 50 mph, and heat did not return when you slowed, you had to re-start the car to get any heat once it had gone out. The Fix - New Monovalve.

Repair is is so easy, it takes longer to put together this WWW page than the repair.

The Monovalve is the sensor/valve that allows coolant from the engine to get to the heat exchanger. As I understand it, it is the gateway to engage the heat exchanger to allow some of the engine's warm goodness to be shared with the occupants of the vehicle. I bought mine at the local dealership here in Salem for $36.00 (Note, this is for just the 'insert' not the whole valve assembly.) But then again, I really don't know. I am a lawyer by trade and many will agree that being a lawyer makes one generally incompetent.

Where is it: Between the firewalls and next to the battery. See the arrow in the lead photo on this page, as well, the below side view.