tachometer wire how to measure , verify

I am wiring up my Chevrolet Express Van (2007) with an Alarm and Bypass and would like to know how to verify the Tach Signal wire and how to use a Multi-Meter to read it. I have been searching through forum and YouTube videos and am getting mixed messages on how to verify that I have the correct wire for Tach reading. According to my Alarm Install Manual the tach wire should be 1-4 v at idle and then increase in voltage as the engine revs up. I have measured many wires and have found ones with 12 - 14v constant; .02v constant, and then another one (Which I think is the tach wire) that jumps all around from 5 - 12 v. Thanks! P.S. I have read no less that 4 different posts that give me various locations and wire colors for the tach wire on my PCU.

meter AC volts, black to ground, read to tach wire. voltage fluctuates with rpm. correct tach wire is PCM blue connector BROWN / whiteTed

2nd Year Tier 1 Medical School

Still installing as a hobby...pays for groceries

Compustar Expert

Depending on your meter, some read on AC volts, some on DC volts.

Thanks for the input. The Brown and White wire is the one that I believed to be the tach wire based on the readings.

The interesting thing with the Brown and White Wire is that I could get AC and DC readings from it. Like I mentioned in the post the reading were from 5 - 8+ volts at idle. Thanks for your help!

That's because what you're meter is seeing is the + half of an AC sine wave.Test before boxing up.

Of a full AC wave with a DC offset... (Ha ha. Maybe more now agree with that than they used to. It's certainly the easiest way of solving circuit behaviour...) PS - I said "wave" in case it's square or whatever.

What it really is :- engine management sending timed signals (process control) to the injector as to when to squirt in the juice. I know Mr. Pierson, Mr. I and yourself Oldie could give more urbane answers and BTW screw DMMs I use an LED tester, again much faster, I'm just looking for a flashing LED, don't have to worry about the exact "AC" voltage.Test before boxing up.