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My voltmeter shows 12.46 volts across the terminals of the battery when ignition is off and on.

As soon as the car starts, the voltmeter reads 1. This reading would normally be seen when measuring resistance, not voltage. The 1 would indicate an open circuit.

After the car is shut off, the reading returns to around 12.46. (Actually, a little less than that due to the voltage drop.) I am assuming this would indicate the alternator circuit is in series.

What could potentially cause these readings? Open circuit, blown fuse, ground short?

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    Set the meter to the next higher voltage range to see if you can get a voltage reading. If the voltage is really over 20 volts, the voltage regulator in the alternator is faulty - you shouldn't see more than about 14.4 volts with the engine running. – Peter Bennett Oct 22 '16 at 3:49
  • also verify on Vac scales that you do not have noise, it should be a low reading, otherwise indicates a problem in measurement or battery – Tony Stewart Sunnyskyguy EE75 Oct 22 '16 at 4:14
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    user127467 - Your misinterpretation is here: "As soon as the car starts voltmeter reads 1 . This reading would normally be associated to a resistance check not a volt check and would indicate and open circuit." - A reading of "1" in the left digit alone means "over-range" for any measuring function on non-autoranging DVMs. The likely issue is as explained by Peter Bennett. Flagged for transfer to Motor Vehicle Repair SE. – SamGibson Oct 22 '16 at 4:16
  • What made you put them multimeter on it in the first place? What is the problem with the car? – Zach Mierzejewski Oct 22 '16 at 15:00
  • I'm voting to close this question as being too broad. The reason being is, we need a more information about the vehicle and other things before we can answer this. The OP has not come over from Electronics.SE to try and give us any more information, thus the close vote. If the OP comes over with more info, I'll retract my close vote. – Pᴀᴜʟsᴛᴇʀ2 Oct 23 '16 at 17:52

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