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#4439621 - 06/23/17 03:46 PM Question about 12 volts in car
daves66nova Offline


Registered: 04/11/04
Posts: 3652
Loc: los angeles
Question. If a car has all fuses removed, no radio, no accessories that are using any power whatsoever, and you disconnect the pos battery cable then connect a voltmeter inline between the pos battery post and the pos battery cable, should the meter show 12 volts, plus or minus or so? I was telling someone no, because there's no circuit closed,they said it should show 12 plus or minus . Which is it? Car in question is a '66 ChevyII


Edited by daves66nova (06/23/17 03:46 PM)
Edit Reason: edited
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#4439625 - 06/23/17 03:52 PM Re: Question about 12 volts in car [Re: daves66nova]
BearZDefect Offline


Registered: 08/15/05
Posts: 2233
Loc: Lexington, KY
If it is an open circuit then the voltmeter should read zero.

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#4439626 - 06/23/17 03:53 PM Re: Question about 12 volts in car [Re: daves66nova]
Rand Offline


Registered: 08/20/03
Posts: 12643
Loc: NE,Ohio
Theoretically if you had nothing unfused and every fuse was pulled it should be an open circuit so no voltage.

In real life this would never happen.
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#4439637 - 06/23/17 04:01 PM Re: Question about 12 volts in car [Re: daves66nova]
mechjames Offline


Registered: 01/16/08
Posts: 516
Loc: Richmond, BC, Canada
Yes the meter should show 12 volts, if you are checking two ends of a circuit, you are now reading voltage drop not system voltage. Since your cable is disconnected, it's showing a 12V voltage drop between the post and the cable, because it's not connected to anything.


Edited by mechjames (06/23/17 04:01 PM)
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#4439674 - 06/23/17 04:40 PM Re: Question about 12 volts in car [Re: daves66nova]
HerrStig Offline


Registered: 08/24/11
Posts: 9560
Loc: Boston, MA
Across the two battery terminals you
r voltmeter should see 12V. In SERIES with the plus terminal and nothing connected you should be looking at current rather than voltage because there is no voltage drop to measure unless a current, however slight, flows.

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#4439791 - 06/23/17 07:03 PM Re: Question about 12 volts in car [Re: daves66nova]
JimPghPA Offline


Registered: 08/22/09
Posts: 3859
Loc: Pittsburgh,PA U.S.A.
The reverse current through the diodes in the alternator will provide a path for the positive cable to connect to ground, but the maximum current will be very small and there will be some voltage drop across those reversed diodes. So it will depend if the current draw of the volt meter is low enough to register a voltage with the very limited amount of current that the reversed current on the alternator diodes allows.

The diodes of the alternator are wired in a three phase full wave bridge, which is essentially 3 sets in parallel, of two diodes in series, with the wingdings of the armature between each of the two diodes that are in series.


Edited by JimPghPA (06/23/17 07:07 PM)
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#4439822 - 06/23/17 07:23 PM Re: Question about 12 volts in car [Re: daves66nova]
exranger06 Offline


Registered: 01/25/08
Posts: 3907
Loc: Guilford, CT
Even if you did have fuses installed and circuits live, you should have 0 volts between the positive battery post and cable. Fuses pulled, still 0V. You never check for voltage by disconnecting wires and putting the multimeter between them; that's how you measure current. If you have all fuses pulled, you should have 0 amps of current. If you have fuses installed, you'll have a certain amount of current over 0.
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#4439833 - 06/23/17 07:31 PM Re: Question about 12 volts in car [Re: daves66nova]
i_hate_autofraud Offline


Registered: 05/19/16
Posts: 705
Loc: Canada


As mentioned earlier, an alternator is hard wired into the harness and will have a permenent fusable link
in the harness, the leakage current thru the diodes in the alternator may allow a meter to
read a 12V circuit.

A moving needle meter (old school) may indicate 0V or very low, Digital meters can read a full 12V
because their "input impedance" is at least 1 M ohms and will be driven by the leakage current from the
alternator diodes.

However, the test you tried has little merit, if you want to know there is no current
draw, put the meter in amps mode and drop the range to see if it's zero or a few milli-amps.

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#4439927 - 06/23/17 10:01 PM Re: Question about 12 volts in car [Re: JimPghPA]
CT8 Offline


Registered: 10/09/14
Posts: 10979
Loc: Idaho
Originally Posted By: JimPghPA
The reverse current through the diodes in the alternator will provide a path for the positive cable to connect to ground, but the maximum current will be very small and there will be some voltage drop across those reversed diodes. So it will depend if the current draw of the volt meter is low enough to register a voltage with the very limited amount of current that the reversed current on the alternator diodes allows.

The diodes of the alternator are wired in a three phase full wave bridge, which is essentially 3 sets in parallel, of two diodes in series, with the wingdings of the armature between each of the two diodes that are in series.
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#4439958 - 06/23/17 10:49 PM Re: Question about 12 volts in car [Re: HerrStig]
JHZR2 Offline



Registered: 12/14/02
Posts: 41786
Loc: New Jersey
Originally Posted By: HerrStig
Across the two battery terminals your voltmeter should see 12V. In SERIES with the plus terminal and nothing connected you should be looking at current rather than voltage because there is no voltage drop to measure unless a current, however slight, flows.


Correct.

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