f355 tach signal

I'm thinking it comes from the alternator somehow. Exactly how I'm not sure? The reason I say is because the tach on my car was acting wacky when my alternator (nippondenso unit, same as in a 355) went bad. After the rebuild the problem went away. I never really dove into it because the problem never resurfaced after the alt rebuild.

 

The tach signal is definitively generated by the ECU. It is a square wave signal obtained by a frequency division of the signal from the flywheel sensor. The WSM is not very clear about its frequency (2 or 4 pulses per revolution ?) nor about the amplitude (0-5V or 0-12V?). I have not yet looked at this signal with a scope so I cannot tell more about it ....

Ernie, this signal is sent to the tach wich contains some electronics to convert the frequency to an analog signal able to drive the needle. If the car 12V is bad, because of bad grounding, bad voltage regulator, bad battery, this may affect the tach indication.

 

Ah, that makes sense.