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Question about 12 volts in car #4439621
06/23/17 03:46 PM
06/23/17 03:46 PM
Joined: Apr 2004
Posts: 3,662
los angeles
daves66nova Offline OP
daves66nova  Offline OP
Joined: Apr 2004
Posts: 3,662
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

Last edited by daves66nova; 06/23/17 03:46 PM. Reason: edited

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Re: Question about 12 volts in car [Re: daves66nova] #4439625
06/23/17 03:52 PM
06/23/17 03:52 PM
Joined: Aug 2005
Posts: 2,252
Lexington, KY
BearZDefect Offline
BearZDefect  Offline
Joined: Aug 2005
Posts: 2,252
Lexington, KY
If it is an open circuit then the voltmeter should read zero.

Re: Question about 12 volts in car [Re: daves66nova] #4439626
06/23/17 03:53 PM
06/23/17 03:53 PM
Joined: Aug 2003
Posts: 12,905
NE,Ohio
Rand Offline
Rand  Offline
Joined: Aug 2003
Posts: 12,905
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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Re: Question about 12 volts in car [Re: daves66nova] #4439637
06/23/17 04:01 PM
06/23/17 04:01 PM
Joined: Jan 2008
Posts: 519
Richmond, BC, Canada
mechjames Offline
mechjames  Offline
Joined: Jan 2008
Posts: 519
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.

Last edited by mechjames; 06/23/17 04:01 PM.

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Re: Question about 12 volts in car [Re: daves66nova] #4439674
06/23/17 04:40 PM
06/23/17 04:40 PM
Joined: Aug 2011
Posts: 9,560
Boston, MA
HerrStig Offline
HerrStig  Offline
Joined: Aug 2011
Posts: 9,560
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.

Re: Question about 12 volts in car [Re: daves66nova] #4439791
06/23/17 07:03 PM
06/23/17 07:03 PM
Joined: Aug 2009
Posts: 3,903
Pittsburgh,PA U.S.A.
JimPghPA Offline
JimPghPA  Offline
Joined: Aug 2009
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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.

Last edited by JimPghPA; 06/23/17 07:07 PM.

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Re: Question about 12 volts in car [Re: daves66nova] #4439822
06/23/17 07:23 PM
06/23/17 07:23 PM
Joined: Jan 2008
Posts: 3,972
Guilford, CT
exranger06 Offline
exranger06  Offline
Joined: Jan 2008
Posts: 3,972
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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Re: Question about 12 volts in car [Re: daves66nova] #4439833
06/23/17 07:31 PM
06/23/17 07:31 PM
Joined: May 2016
Posts: 717
Canada
i_hate_autofraud Offline
i_hate_autofraud  Offline
Joined: May 2016
Posts: 717
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.

Re: Question about 12 volts in car [Re: JimPghPA] #4439927
06/23/17 10:01 PM
06/23/17 10:01 PM
Joined: Oct 2014
Posts: 11,857
Idaho
CT8 Offline
CT8  Offline
Joined: Oct 2014
Posts: 11,857
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.
charge light !!!!


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Re: Question about 12 volts in car [Re: HerrStig] #4439958
06/23/17 10:49 PM
06/23/17 10:49 PM
Joined: Dec 2002
Posts: 42,088
New Jersey
JHZR2 Offline
JHZR2  Offline
Joined: Dec 2002
Posts: 42,088
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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