How to properly read a tach signal

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=20670
Printed Date: August 19, 2025 at 4:24 PM

Topic: How to properly read a tach signal

Posted By: Illuminatrix
Subject: How to properly read a tach signal
Date Posted: October 27, 2003 at 11:43 PM

Can someone tell me how to read the tach signal properlly. My Tester has 200mv and 500mv in AC mode. What should my tach wire read ?? and does voltage go up or down with a rev of engine ??

Thanks again. I am destined not to use voltage mode.

Cheers

Travis

Replies:

Posted By: NowYaKnow
Date Posted: October 28, 2003 at 5:00 AM

Here's how I read tach wires..Take your meter. Put one side to ground and the other side to your tach wire. Set your meter to AC Volts and start the car. The wire shouldn't read anything with the car off, and then just up to like 1 - 1.5 volts with the car running. When you rev up the car it should jump to around 2 - 3 volts. That's how I read all my tach wires. The exact voltage numbers aren't too important you just want to see some AC voltage that rises when you rev it. Depending on the car they all read a little differently so yours may sit at like .5 volt and rise to 1 volt or may sit at around 2.5 volts and rise to 3 there's really no set numbers. As for finding the tach wire normally you would follow the spark plug wires back to the coil packs and it will be one of the wires coming off of that module. Sometimes accessing that isn't possible though so you may have to look elsewhere..Good luck,

Mike

Posted By: Illuminatrix
Date Posted: October 28, 2003 at 8:10 AM

So if my tester has 200mv and 500mv settings for AC what would it read. Sorry for such a dumb question I just want to be sure.

Thanks

Travis

Posted By: sid2k
Date Posted: October 28, 2003 at 8:15 AM

200mv

Posted By: Illuminatrix
Date Posted: October 28, 2003 at 9:03 AM

Ok I am dumb so at 200mv what should my tach wire read ????

Thanks, I want to get this dang thing set up properly.

Cheers

Travis

Posted By: derek123
Date Posted: October 28, 2003 at 11:21 AM

You may end up blowing that meter.

some tach signals are higher than 12 volts.

are you sure its 500 mV ?

from what your saying even a poor tach siganl ( less than one volt) is going to be too much for your meter.

also, NowYaKnow pretty much nailed it on how to test with a DMM, one thing missing is that some true tach references will keep the same voltage ( as RPM increases) but the frequency of the siganl increases instead. you need a pretty spiffy meter to read that though.

Posted By: Illuminatrix
Date Posted: October 28, 2003 at 1:03 PM

Will a DC 12 Volt battery test on my meter work to test this signal.