Check for 12V at the monovalve wires with the AC on and set for max cold. If you get voltage, the solenoid is shot and you need or replace it (or the valve it binding). If no voltage, the PBCU is bad, replace (or take apart and look for a bad solder connection).
This happened to a friend of mine last year. Had to replace the PBCU.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!