It's too early to dismiss the key switch. A leak in the switch can mean insufficient vacuum to the acutator. It pulls on the shut-off mechanism enough to reduce but not cut fuel.

You need a MityVac with a gauge or at least a vacuum gauge. I don't know what spec is but I recall something like 14 inHg. If you aren't getting 14" to the actuator, the key switch is bad or there's a leak in the lines. If 14" doesn't shut-off the engine, the actuator is bad. A really basic test is to press down the shut-off lever and cap the fitting with your finger. If the lever springs up however slowly, the diaphragm in the actuator is probably shot. I wouldn't gamble the cost of a new actuator on that test.

If the lines haven't been replaced, you're looking for brown lines. IIRC it's plain brown from a fitting along the big vacuum line to the brake booster to the key switch and a brown line with red stripe to the shut-off actuator.

My 93 SD had a bad fitting along the brake booster line. Brakes worked fine but the there was a blockage in the nipple for the shut-off actuator. I don't know how that happened. I tapped into the the smaller feeder off the engine vacuum pump and there was enough vacuum, so to speak, to shut the engine.

The shut-off actuator sits atop the IP. See where the shut-off lever pivots on the IP. Directly above is a metal or plastic canister with a vacuum fitting. This is the shut-off actuator. Just aft and higher than the shut-off actuator is the ALDA. It's a square device sitting somewhat diagonally on the IP with a pressure fitting pointing towards the back of the car.

It's possible that your car doesn't have an ALDA. It'll run without one. I'm 99.999% sure it came from the factory with one.

Sixto
87 300D