The overboost sensor sends a ground signal at ~1.1 bar. It's open below the setpoint. Easy to test.

When you swapped transducers, you might have sent a vacuum signal to the mixing pipe flap and starved the engine of air.

The sentiment is precisely that it's not worth most folks' time to understand and sort the system. There just isn't the knowledge base available to diagnose problems even with error codes. If cash is tight, a trip to a wrecking yard or pick-n-pull might get you a pressure actuator you can modify to fit this car.

Sixto
87 300D