You didn't mention which model car you have. The 123 amd 126 cars with ACC (probably others, too, but I'm not sure) have an auxiliary water pump that activates when you turn on the heater. If this pump fails you will have poor coolant flow to the heater core at low RPMs. If this is the problem the air should get warmer at higher RPMs.

Electrical problems involving the monovalve are usually the result of poor/corroded connections. This will cause lower than required voltage to be sent to the monovalve, which will make the air warmer than it should be, not cooler. Like another poster said, 12 volts to the monovalve and it should be all the way closed; 0 volts and it should be all the way open. If you have more than 0 volts with the heater on maximum, you have ACC issues (pushbutton unit, maybe?).

More than likely you have a torn monovalve diaphragm, though. This part is a frequent failer, and some people in colder climates than mine go so far as to rebuild the monovalve as regular maintenance. When mine failed I had heat at idle but the air got cooler at higher RPMs.

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1982 300D, black w/ palomino leather, 203K
2001 E320, champagne, 105K (Mom's)
1999 Dodge Dakota, squeaky & battered, 142K