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Discussion Starter #1
I noticed when I disconnected the neg side of the battery - it energized the frame. I turn my key to run and then I attached my circuit tester to the unconnected negative post and it lit up my tester when I touched the other side of the tester anywhere on the body. Also when I leave open the door it will light up the tester with the key in the off position. Is this normal - some how is the power side leaking to the ground side? A dead short somehere? When I attach the neg post to cable it doesn't light up the circuit. I have get my mulitmeter out and see if there is any volts going to ground (but not enough to light the bulb).


If anyone has an explanation of any kind I would love to hear it.


Thanks,
Dean
 

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Doesn't sound weird to me, if I understand correctly you are grounding the battery through the test light. Try to run your headlights set up like this and you will likely burn out your test light.
 

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Discussion Starter #3
Doesn't sound weird to me, if I understand correctly you are grounding the battery through the test light. Try to run your headlights set up like this and you will likely burn out your test light.

Ok that makes sense I'm just completing the circuit I guess. It just caught my attention I attached a temporary tach and I had the negative side terminal off and when I had the switch in the run position the tach light lit up when I brushed the white light wire against the power steering reservoir?
 
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Discussion Starter #5
I had the negative wire from the tach attached to the post but the cable was not. Maybe that's normal for it to light up?
 

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Any wire connected to the - side of the battery will potentially complete the ground circuit, thus lighting your test lamp. Had you removed all wires from the - battery post, then you'd have some weird issues...
 

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Before B+ powered computer systems came on the scene using a test light in series with the negative terminal of a battery and cable was standard testing protocol for a parasitic drain . No light meant no battery drain.
 

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Where are you connecting the other side of the tester, that's confusing me. But basically your putting the tester in series in the circuit making it part of the circuit. As such everything has the same amount of amperage going through it. What item has the most resistance drops the most voltage which means you'll get the highest voltage reading.
 

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The majority of systems in your mustang run the ground through the frame. That’s why it is so important for your ground connections to be solid. Power runs for: your battery + post to systems like your radio, ignition, lights, etc. Then the systems ground to the frame. The power flows through the frame then up the negative battery cable to the battery - post. If you disconnect the negative cable not thing will work. By connecting your test light between the frame and negative battery post you essentially gave the system an alternative path to close the circuit. The test light won’t allow too much power t9 flow though before you burn it out.
 

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Discussion Starter #10
Sorry for the confusion I understand the series connections. - I'll hook up everything and see if there is any voltage going into the frame (checking for shorts). What I can't understand is why when I disconnect the negative post only keep the positive hooked up and have the tach negative wire attached to the post (in effect completing the circuit) and then turn the key to 'run'. Why would the tach light work when I touched white wire to the frame? - somehow the power is being relayed to light circuit.


Thanks for previous responses,
Dean
 

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"The test light won’t allow too much power t9 flow though before you burn it out." Your first thought it could be that a bulb would blow as if it was a fuse but it wont. The voltage (pressure) with amperage behind it would have to be substantially higher than the bulb is designed for before failure. As another use for a test light that one would think would pop a bulb is determining a possible bad plug wire on a running engine or checking for cranking spark. Though spark is thousands of volts there is little current coupled with a duty cycle. It survives no problem.
 

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It simply means something is "on" in the car that is directly connected to the battery. If you have a dash clock then there you are. Or a newer style radio that has a clock and can electronically recall radio stations. If you disconnected those type things then the light probably wouldn't light. On newer model cars people use an ammeter to check how much power is being drawn by the car with everything turned off. There is ALWAYS some draw on those (alarms, body control modules, keyless entry). Some people allow up to 50 milliamps rule of thumb before considering it a enough to be an issue, that's enough that a small test light connected in series will light.
 

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Sorry for the confusion I understand the series connections. - I'll hook up everything and see if there is any voltage going into the frame (checking for shorts). What I can't understand is why when I disconnect the negative post only keep the positive hooked up and have the tach negative wire attached to the post (in effect completing the circuit) and then turn the key to 'run'. Why would the tach light work when I touched white wire to the frame? - somehow the power is being relayed to light circuit.


Thanks for previous responses,
Dean

You may not get an answer to your question as you are making a circuit thru a component that was never intended to work that way.
 

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Discussion Starter #14
Thanks guys for the possible explanations and there may be a draw somewhere possibly the aftermarket stereo.
 

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Why are you performing some sort of diagnosis on your electrical system? Is there a problem you are attempting to isolate?
 

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Ok, if you have the negative cable off the battery post you've effectively removed the battery from the circuit in theory. If it is still providing voltage with the cable off, there is only one possiblity. The battery must have some sort of contamination on the exterior conducting to ground completing the current path. On electronics with high internal resistance it doesn't take a whole lot of current to operate.
 
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