How to test negative circuits with a DMM?

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kosoku

Copper - Posts: 76

Joined: December 16, 2004

Location: United States

Posted: August 12, 2005 at 10:29 AM / IP Logged

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...

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5150azn

Silver - Posts: 584

Joined: June 21, 2004

Location: United States

Posted: August 12, 2005 at 11:14 AM / IP Logged

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.

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

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kosoku

Copper - Posts: 76

Joined: December 16, 2004

Location: United States

Posted: August 12, 2005 at 12:32 PM / IP Logged

D'oh.

How to test negative circuits with a DMM? - Last Post -- posted image.

That makes sense. Thanks!

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sienna

Member - Posts: 12

Joined: May 10, 2004

Location: Canada

Posted: August 12, 2005 at 12:48 PM / IP Logged

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sienna

Member - Posts: 12

Joined: May 10, 2004

Location: Canada

Posted: August 12, 2005 at 12:57 PM / IP Logged

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).

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OhioMike1101

Silver - Posts: 343

Joined: August 22, 2004

Location: United States

Posted: August 12, 2005 at 6:40 PM / IP Logged

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How to test negative circuits with a DMM?

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