If blocking the flow through the heater core fixed a no AC condition, either the monovalve or the temp controller is bad.
Monovalve can be checked by applying 12V across it from the battery -- must have no flow.
Or you can pull it out -- if the bellows are wrinkled or punctured, replace.
It it is good, and works from the battery, either the PBCU or the electronics for the temp are bad.
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!