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.
Peter
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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!
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