Emergency overboost conditions are generally handled by the emergency boost cut-off valve (name is something like that). In my 1985 745t the unit is located just above and in front/right of the gas pedal (inside the dash).

You can see it from the bottom if you remove the plastic panel, and you can generally find it by tracing teh vacuum tube from the engine compartment. That tube splits inside teh dash, with one hose leading off to the turbo boost gauge, and the other leading to the turbo kill switch.

Its actually a good idea to check all these hoses from time to time since they can get oily. I've sometimes pulled the console and found the kill switch hose has slipped off the splitter or off teh kill switch itself...

As for your 1990, I'd be surprised if it doesn't have a kill switch of some sort since that would be a SERIOUS safety problem. I've had the waste gate actuator vacuum hose break on me, overboosted the turbo, and been miraculously saved by the presence of the kill switch. Take a look in the location I indicated and see if you can't spot yours.

good luck,
rt