Finding Negative Voltage
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=67856
Printed Date: May 12, 2025 at 2:03 PM
Topic: Finding Negative Voltage
Posted By: offroadzj
Subject: Finding Negative Voltage
Date Posted: December 07, 2005 at 12:08 AM
What is the best way to test for something listed on a wire diagram as negative. I usually use an ohm meter, but it doesnt always work for me. Is there a "right" way to check a wire that is listed as negative? (ie factory security, etc).
Kenny
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Kenny
Owner / Technician
KKD Garage LLC
Albany, NY 12205
Replies:
Posted By: 05gt
Date Posted: December 07, 2005 at 2:28 AM
just use a digital multimeter or a test light that does negative and positive, like the snap on one I have
Posted By: Powermyster
Date Posted: December 07, 2005 at 3:18 AM
use a DMM put the live probe to a constant live and use the earth probe to test.
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Why oh Why didn't i take the blue pill
Darren Power
Posted By: Mike M2
Date Posted: December 07, 2005 at 6:18 AM
Put your meter on Ohms, it should read "1". Ground the black lead. When you touch the red lead to ground it should read "0" if it's ground...
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Mike M2
Tech Manager
CS Dealer Services
Posted By: dualsport
Date Posted: December 07, 2005 at 9:12 AM
Assume you're looking for negative trigger, rather than negative voltages, which isn't used much on cars-
The way Powermyster said would be the safest way to go, if you're probing unknown circuits.
Use the DMM voltage measurement mode and put the red lead on a 12V source, then use the black test lead to check the wire. If it's at ground, it should read 12V.
Usually want to avoid using the resistance mode unless you know for sure the wire is unpowered, because some DMMs may not like being fed power when trying to read resistance.
If you don't have a convenient point to get 12V, you can use a two step process, where you just check the voltage from the wire to ground; if it reads 0V, then you can switch to resistance mode and check for continuity to ground.
Test lights could overdraw some circuits, so there's a bit more risk using those instead of the DMM, which won't put much of a load on the circuit in voltage mode. Test lights are handy for monitoring the higher powered stuff though, like lights and sirens, when you're testing or setting up.
Posted By: Powermyster
Date Posted: December 07, 2005 at 10:36 AM
You can use a power probe but only if you know your stuff and use it wisely don't wanna apply any unwanted voltages
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Why oh Why didn't i take the blue pill
Darren Power
Posted By: Tight_dbz
Date Posted: December 08, 2005 at 1:19 PM
Correct me if my advice is wrong. I like to use a DMM with continuity beep. Ground one probe and probe the other suspect wire. if it beeps its negative. handy when testing negative door triggers when you are on the opposite side of the vehicle and opening the door, just listen.
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