I'll presume you're asking about the 81 SD in your sig line.

I don't know that it's a bad switch. I was just guessing that something was loose in the switch. I retract that guess.

The switch at the door does two things at the pump. First is to get it running (it only goes one way), the other is to connect the hose that goes to the lock actuators to the pressure side or vacuum side of the pump as needed. The act of locking or unlocking triggers actuators in the pump assembly to set the pump and hose to expect the next signal; lock if it just unlocked, unlock if it just locked.

I'll bet the 85mph speedo that shouldn't be in my 83 SD (reads only 110Kmiles) that the problem is a sequence pressure switch actuator stuck sending +12V to the pump when the door switch is in the lock position. That would send a constant +12V to the pump triggering a loop.

Could there be a keyless entry or aftermarket alarm system in the mix?

Sixto
91 300SE
87 300SDL
83 300SD