Engine speed / tach
The crank sensor measures engine RPM and timing using a magnetic pickup on the flywheel. The sensor produces electrical pulses as the flywheel teeth pass by and sends them to the DME for engine control. The DME converts the pulses to a square wave with a frequency proportional to RPM. This signal, designated "Tn" is distributed as shown below.
You can access Tn at the OBD port. On OBD II 993's it's on pin 9(+) and 4(-). On OBD I 993's it's on pin 14(+) and 10(-). The waveforms below are what you should expect to see on an oscilloscope - a 50% duty-cycle square wave switching between 0 and 12 v. RPMs = frequency x 20. If you don't have an oscilloscope, you can get a rough idea if the system is working by measuring Tn with a multimeter set for AC volts. Tn should read ~ 6-7 VAC when the car is running and ~5 VAC when just cranking. The voltage will track battery/alternator voltage but should be independent of RPM and steady under steady RPM.
OBD port pinouts here.
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