The crank sensor monitors engine RPM and timing using a magnetic pickup on the flywheel. As the flywheel teeth pass by, the sensor generates electrical pulses and sends them to the DME (Digital Motor Electronics) for engine management. The DME converts these pulses into a square wave, with the frequency proportional to RPM. This signal, known as "Tn," is distributed as shown below.

You can access the Tn signal through the OBD port. On OBD II 993s, it’s available on pin 9(+) and pin 4(-), while on OBD I 993s, it’s on pin 14(+) and pin 10(-). When viewed on an oscilloscope, the expected waveform is a 50% duty-cycle square wave switching between 0 and 12V. RPM can be calculated as frequency × 20.  

If you don’t have an oscilloscope, you can get a rough idea of the system’s functionality by measuring Tn with a multimeter set to AC volts. Tn should read about 6-7 VAC when the car is running and around 5 VAC when cranking. The voltage will generally track battery/alternator voltage but should remain stable and independent of RPM at steady engine speeds.  

OBD port pinouts here.