Anoter no start cause difficult to diagnose

Just as the Automatic Transmission Kick Down relay can cause a no start condition due to grounding the RPM signal from the spark module to LH module pin 1, there is another no-start issue that has puzzled many mechanics.  The same circuit providing the RPM signal to the fuel module also connect to tachometer on all years of Porsche 928.  If there is a short circuit to ground anywhere in this circuit or if the tachometer has failed with a short to ground the engine will not start.  

After module testing and discussion with a mechanic diagnosing a 1985 Porsche 928, the mechanic cut the wire to the tachometer and the engine started right up.  He has since found this issue in a few other 928s.

For this case and the Automatic Transmission Kick Down relay problems, the cause of no-start is no RPM signal to the fuel system.  For 1978 to 1984 US 928s, the RPM signal connects to the fuel pump relay (special relay, do not attempt to use a standard relay) that will only turn on when the RPM is 50 (starter cranking speed) or higher. For 1984 Euro and 1985 - 1995 928s, the RPM signal is the basis for the digital fuel injection operation.  If there is no RPM signal, there is no fuel injection.  A failed transistor ignition unit (1978 - 1983 Euro and 1984 US),  EZF (1984 Euro & 1985 - 1986) and EZK (1987 - 1995) also can be the cause of no RPM signal, yet it can provide the signals to create spark.  So spark module testing is necessary to rule out a failed module.  

The RPM signal is a rather short duty cycle square wave, so an oscilloscope is needed to test for it.  A standard volt meter will not provide any meaningful information.