There is electrical switch in the door that runs the motor, and the vacuum/pressure switches in the pump tell it when to turn off. The two wires, blue and yellow, to the pump come from the driver's door and one of them should have voltage to ground. Whenever the door is locked/unlocked the voltage switches between the two wires. You could have a bad actuator in the pump unit that isn't switching properly which might account for your symptoms.

With the mityvac hooked into the vacuum circuit use the key in the door to cycle the pump (i.e., lock the door). Does the mityvac register vacuum or pressure? Now unlock the door. Does the mityvac register the opposite now? What happens when you try to measure the voltage from the door switch at pump connector?

Also, the current through the fuse also travels through the door switch. It's possible the excessive current has burned the contacts and now the motor isn't getting the current it needs to run.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)