As he said, unplug monovalve and run it to see what happens. Also, as stated, I've yet to see a non MB monovalve work for any length of time.

I got a shop set up to repair these cars. Unfortunately it is about 2 hours to Soperton from Tybee but let me know if I can help.

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Jim

Unplugged the mono valve...still blasts out cold air. Will order new mono valve (good one) and change it out myself. Still, if the fail safe position is full on heat, there must be something else wrong.
HELP!

The aftermarket valves are reputed to have a spring that's weak enough that the engine coolant pump can suck them closed. Considering the relatively weak flow through the heater core, that's a pretty wimpy spring.

Another possibility is a failed or clogged auxiliary water pump if your vehicle has one, though you should still get heat even if it is seized.

Start with the Monovalve cartridge. It is the most common cause of no-heat and one of the cheapest parts you can replace. DO NOT waste money on the aftermarket brands. Go with OEM or don't bother.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Also, put your hand on the tubing after the mono valve and before the heater core to see if it's cold. If that section is hot then there could be a clog at the heater core.

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1990 190E 3.0L

Moral of this story is to use MB parts. It was easy to do myself and works great. Thanks for advice.