tachometer wire how to measure , verify
Printed From: the12volt.com
Forum Name: Car Security and Convenience
Forum Discription: Car Alarms, Keyless Entries, Remote Starters, Immobilizer Bypasses, Sensors, Door Locks, Window Modules, Heated Mirrors, Heated Seats, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=136268
Printed Date: May 13, 2025 at 11:32 AM
Topic: tachometer wire how to measure , verify
Posted By: hlone
Subject: tachometer wire how to measure , verify
Date Posted: March 12, 2014 at 9:44 PM
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.
Replies:
Posted By: tedmond
Date Posted: March 12, 2014 at 9:58 PM
meter AC volts, black to ground, read to tach wire. voltage fluctuates with rpm.
correct tach wire is PCM blue connector BROWN / white
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Ted
2nd Year Tier 1 Medical School
Still installing as a hobby...pays for groceries
Compustar Expert
Posted By: i am an idiot
Date Posted: March 12, 2014 at 10:43 PM
Depending on your meter, some read on AC volts, some on DC volts.
Posted By: hlone
Date Posted: March 13, 2014 at 6:05 AM
Thanks for the input. The Brown and White wire is the one that I believed to be the tach wire based on the readings.
Posted By: hlone
Date Posted: March 13, 2014 at 6:07 AM
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!
Posted By: harryharris
Date Posted: March 13, 2014 at 11:45 AM
That's because what you're meter is seeing is the + half of an AC sine wave.
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Test before boxing up.
Posted By: oldspark
Date Posted: March 13, 2014 at 2:27 PM
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.
Posted By: harryharris
Date Posted: March 13, 2014 at 3:12 PM
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.
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Test before boxing up.
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