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Old 01-19-2010, 07:40 PM
GMR GMR is offline
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help with basic question

I made a test board to try to understand parallel and series circuits better. While fooling around I disconnected the bat neg cable and attached the black DVM lead to bat neg terminal and red lead to bat cable - got 12.57v. Then I replaced the DVM leads with a test light and the light did not light. I understand that the light isn't supposed to light unless you have a parasitic draw. But, why doesn't it light? I can't get past why the bulb isn't acting like a load. So I obviously don't understand basic electricty. Please help.

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Old 01-19-2010, 07:54 PM
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Does your test light work at all?


Quote:
Originally Posted by GMR
I made a test board to try to understand parallel and series circuits better. While fooling around I disconnected the bat neg cable and attached the black DVM lead to bat neg terminal and red lead to bat cable - got 12.57v. Then I replaced the DVM leads with a test light and the light did not light. I understand that the light isn't supposed to light unless you have a parasitic draw. But, why doesn't it light? I can't get past why the bulb isn't acting like a load. So I obviously don't understand basic electricty. Please help.
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Old 01-19-2010, 08:05 PM
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Based on your description, the bulb in the test light should have worked.

Bruce
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Old 01-19-2010, 09:21 PM
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Yes, the test light is functional.

The bat neg cable is disconnected from the bat. I have one lead of the test light connected to the bat neg cable and the other lead connected to the neg bat terminal.

It was my understanding this was a method of checking for parasitic drain. If the light goes on, even faintly, you have a parasitic drain. I don't have that problem so the light, correctly I believe, does not go on. Am I right on this much?
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Old 01-19-2010, 09:57 PM
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i have used that test light theory many times. first i disconect the batt neg cable and see if i have a drain with a test light. if i do i then remove fuses one at a time till the test light that is connected to the batt and the ground cable turns off. that way i know where and what circuit has the drain....

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Old 01-20-2010, 07:33 AM
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Yes, I thought I understood that concept correctly - that is - the test light should not light unless there is a parasitic drain.

I see a lot of people have read this so I guess I'll just drop it. Thanks to those who replied.
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Old 01-20-2010, 09:19 AM
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I don't think your question got answered, so here is my shot at it--The reason that you would see a "draw" with a meter and not a test light would be due to a clock, fancy radio, or the vehicle's computer "keep alive" circuit. They will draw voltage, but not enough amperage to light up a test light. If the light stays out, you don't have enough of a "draw" to drain your battery.
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Old 01-20-2010, 10:13 AM
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I will try to explain a different way.
The DVM has an extremely high resistance in the voltage scales. It was designed thay way, because the idea with a volt meter, is that hooking one up should not cause any changes in the circuit you are testing, by drawing any current in it.

A DVM only "uses" a few millionths of an amp of current to read a voltage.
The test light needs maybe a third of an amp to light.

The cars electronics are designed similar to the volt meter. They do not use much current to "keep alive", and remember stuff. (radio settings, the time, engine computer programming, any trouble codes) But all these electronics will still use more current than a volt meter. So, the volt meter will show a voltage, but the test light will not light.

If you have an older car, with no electronics, and everything off, it may be possible to hook up a volt meter, and get no reading, but even then it will still show something, because the leads of the volt meter will actually pick up radio signals.

I hope this helps you to understand.
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Old 01-20-2010, 08:22 PM
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Fellas, thanks much for your replies. I do appreciate it and you did help.
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