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Old 03-12-2007, 10:02 PM   #1
williaty
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Default How Do I Test a Ground Switched Car?

So the wackos at Subaru decided to make a ground switched system. Now I have to tear into it and figure out which wires are doing what. On a positive switched system, I could just use a standard test light and poke around to I found a hot one. Not so easy with a ground switched system.

OK, so here's what I'm thinking. If I poke around under the dash, with the test light connected to ground, when I hit the load side of a switch, the light will turn on because the only path to ground for the circuit is through the test light. Then when I flip the switch to the on position, the light will go out because the path to ground through the switch is lower resistance than the path to ground through the test light.

Am I right?
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Old 03-19-2007, 12:59 PM   #2
cMags
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You could also connect the light to +12V, and it will light anytime you probe a ground... Lamps do not have polarity. This way you don't have to test only at a switch - you can probe as you normally would.
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Old 03-19-2007, 01:07 PM   #3
williaty
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/me facepalms

Good god, why didn't I think of that!
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Old 03-19-2007, 01:24 PM   #4
rkramer
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keep in mind playing around with an actual light bulb based tester can and will fry certain things. for a light bulb to light, you need to pass current THROUGH the circuit. IC's don't take kindly to that sort of thing. Use a multimeter instead. Then if you really wanted to you could still do ground-> ground testing through resistance.
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Old 03-19-2007, 01:33 PM   #5
Mechie3
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+1 on the multi meter. You can either drop some good cash on a nice one, or get a cheapie pocket one at radioshack. Plus, you get the bonus's of resistance testing (as rkramer said) and can see if wires are hooked to other things (such as dimmer switches) or what the actual voltage you are getting is.
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