[ksa8907]

Originally Posted by Piwoslaw

Here is the long answer.

The short answer is that a diesel will run as long as you supply fuel, no spark plug is needed, so in order to stop it you have to cut the fuel. You could just kill the injectors, but the newer TDs might now like that, so the ECU has its own stopping procedure. There are a few ways to invoke this safe procedure, for example when the ignition key is turned to OFF, the engine starts to stall, etc. If you cut the signal from the camshaft or crankshaft sensor, then the ECU might(*) not know the engine speed, so it'll go into shutdown mode.

*) Some ECUs calibrate the two sensors at startup, so if one is lost during operation the engine can still turn. In my case unplugging the camshaft position sensor wouldn't allow the engine to start, but wouldn't stop it once it got turning. The crankshaft sensor, on the other hand, is needed both for starting and operation.

what he said, and to elaborate, engine controllers will read the crank sensor for timing up to around 2500rpm then switch to the cam sensor above that, because it runs at half the engine speed.