Trying to hook up a Moon tach and it's come down to two wires which are both black. My ECU is located in the trunk and I extended the tach wire with a black wire. I would like to identify the tach wire by the output when the car is running. What kind of a signal should I expect when the engine is idling?

 

this help so far this is all I have found

 

Solved! Turns out the tach signal is a square wave which starts out at 12 volts and goes DOWN with RPM. I was probing it with the key on and thought it was gauge power.

 

When in a hurry and just cut wires, label them even if it's just A-A, B-B, C-C etc. Saves time.