Finding Negative Voltage

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). KennyKenny

Owner / Technician

KKD Garage LLC

Albany, NY 12205

use a DMM put the live probe to a constant live and use the earth probe to test. Why oh Why didn't i take the blue pill

Darren Power

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...Mike M2

Tech Manager

CS Dealer Services

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.

You can use a power probe but only if you know your stuff and use it wisely don't wanna apply any unwanted voltagesWhy oh Why didn't i take the blue pill

Darren Power

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.