The 81 SD has a central locking key switch in the driver door only. The 85 SD has a central locking key switch in both front doors and the trunk. Not sure what the 83 SD has. The switch sends a signal to the pump when you set the key in that position. I don't remember if it's a continuous signal or an impulse. If the pump responds in any way to the key switch I'd say the key switch is fine.

When you turn the driver key switch to lock, the pump should run for about 5 seconds to suck all the vacuum pods to locked position. When you turn the driver key switch to unlock, the pump should run for about 5 seconds to push the vacuum pods to unlocked position. All the doors should unlock when you open the driver door from inside.

If you can borrow a known working pump, try that. If not, apply a vacuum to the fitting at the pump and see if that gets the doors to lock. Also check that the system holds a vacuum for at least a minute. Then apply pressure and see if that gets the doors to unlock. This should tell you if the problem is inside the pump or a leak somewhere downstream.

Sixto