How to test negative circuits with a DMM?

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=61209
Printed Date: May 19, 2025 at 8:18 AM

Topic: How to test negative circuits with a DMM?

Posted By: kosoku
Subject: How to test negative circuits with a DMM?
Date Posted: August 12, 2005 at 10:29 AM

I installed an audiovox alarm / RS in my truck, and everything works as it should, except my driver's side door lock switch. I can lock the doors with it, but not unlock. This is weird, since i didn't use the priority door lock feature, and only tapped in to the wires that unlock both at the same time. The pass. side switch still works fine.

I suspect the problem could be traced by checking if the switch is grounding properly or not, so how would I test this with my digital multimeter? I'm handy with it at testing positive wires, but never have learned how to test negative switches.

Thanks...

Replies:

Posted By: 5150azn
Date Posted: August 12, 2005 at 11:14 AM

Put the positive lead to a constant +12v wire. Use the negative lead to probe your trigger wire. When you hit the switch, it should supply the wire with a -12v and it should show 12v on your meter.

In short just put the positive lead to positive. And probe with the other test lead.

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Tell the Snap-On guy I'm not here!

Posted By: kosoku
Date Posted: August 12, 2005 at 12:32 PM

D'oh.

That makes sense. Thanks!

Posted By: sienna
Date Posted: August 12, 2005 at 12:48 PM

Posted By: sienna
Date Posted: August 12, 2005 at 12:57 PM

Use the resistance setting on your digital meter. Touch the two leads together, this should show you what a short circuit (low resistance) reading should look like (1 ohm or less, the better the meter the lower the resistance it can measure - e.g. 0.001 ohms). Now connect the black lead to a good ground on the car, then connect the red lead to the wire you are testing. If there's a good ground, you should get a reading that is close to what you got when you touched the black and red leads together. If the meter doesn't read anything or flashes 1000, then this means that you don't have a ground (-ve).

Posted By: OhioMike1101
Date Posted: August 12, 2005 at 6:40 PM