86' 500SE, replaced both monovalves and still no heat
 

Here's a riddle hopefully you guys can help me solve. my 500SE has never had heat from the main air vents since I had the car and I always suspected the monovalves (for some reason if I closed all the main vents I could get a decent flow of hot air through the small upper vents on the end. Between that and heated seats I never got cold enough to check it) I recently replaced the radiator and while I was at it I replaced both valves since the car is dual zone. Sure enough both old valves were completely destroyed so I figured it was solved however I still have no heat. The only thing I can think of is there is still a lot of air in the system since it was partially drained for the radiator and the valve box was empty by the time I pulled the lid off. Is there a way to bleed the system or could something more sinister be the cause?

If you unplug the valve assembly, you should get very hot air from the side vents & defrost.

Center vent is not designed to allow for hot air.

the vents in the middle of the dash are not designed to allow for hot air? Is that something euro specific?

No. The center vents are designed to blow cool/cold air. When your AC is running or using climate control cool/cold air will come out of the center vents to maintain proper cabin temperature.

this is unlike any other MB I've ever owned, I was under the impression that with the heat on there would be warm air out of all the vents...

That would be incorrect. It ffunctions the same way on my '91and '92 124's.

I'm not saying you're wrong, but my 87 and 88 300Es and 78 240D blew hot from all vents with the heat on. I will unplug the connector from the valve assembly when I get home and see if heat comes out of the other vents.

I've been wrong before...lol. Although I'm pretty certain about this problem.

dablk1 is correct, the center vents are closed off when heat is on to force flow to the floor vents. Failed actuator(s) at flaps allows the heat to blow thru the center vents. A PITA to repair so many just live with hot face and cold feet.
I would suspect a thorough bleed of coolant circuits will reawaken the heat. Warm car, turn heat to max and add coolant as needed. Takes a bit of time but the system will self bleed.

just to be clear, this is a euro spec model with a separate dial to control where the air goes to, and one to control fan speed instead of the usual push button system. The vents are controlled by sliders to shut them off.

On a W126 the center vent was not designed for heated air.

tried the heat again today, the only vents that get any heat are the upper end vents, the ones that are mostly for defrost, I do not feel any heat from the main end vents or from the main defrost. I don't think there's any air left in the system either since the coolant has leveled off. The knob to adjust which vents output air doesn't seem to be changing anything but I figure there should still be heat coming from somewhere...