Hello,
This is my first post and I tried searching but it's kind of a specific question.

My question is this: what kind of signal output is at the terminal IG-?

I want to make my own tach using an Arduino. If it was an analog voltage porportional to rpm, this would be easy, as Arduinos have analog inputs. I started my truck this morning and used a digital multimeter to measure the output voltage at the IG- terminal and it did NOT change with RPM, so I'm assuming it is a digital pulse or digital signal.

The 93 FSM shows a picture of a tachometer that you hook up to batt +, batt - and the actual sensor lead connects to IG-.

Does anyone know anything about this?
Thanks!