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Shut off while driving, is the car shorting?

Tiny
STEVE-PATTERNMAKER
  • MEMBER
  • 2007 HONDA ACCORD
  • 2.2L
  • 4 CYL
  • 2WD
  • MANUAL
  • 168,000 MILES
My car shut off while I was driving It, the charging light came on about a mile before the car shut off. Now I have no power at all. It also drained a 6-month-old battery, I replaced this and initially, the lights came on, but now nothing works. The battery is still good at 12.6 volts, I am getting a new 100 amp fuse(when I tested it there was continuity between one of the tabs and the central blade, but not the other tab to blade) When I tested the circuits, I disconnected the negative battery terminal, connected a multimeter between the negative pole of the battery and the car body, I had a reading of 12.6 volts. Then I disconnected every fuse and relay in turn and the only response was the voltage dropped to about 1.6v when I removed the 100-amp fuse. For about 5 months before this I had what I think is called a parasitic drain on the battery. If you can help, that would be great. T.I.A.
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Friday, December 22nd, 2023 AT 7:23 AM

10 Replies

Tiny
AL514
  • MECHANIC
  • 4,692 POSTS
Hello, it sounds like you have a bad alternator, the 100-amp fuse is in series with the alternator. Not sure why you disconnected the negative to check battery voltage. You were reading 12.6 with the negative cable disconnected because you were reading voltage drop through the battery. Your alternator probably shorted out and blew the 100amp fuse. See if the Black wire on the alternator has continuity to ground (alternator case) with the 100 amp fuse out. Set your meter on Ohms, (resistance). It will probably read 0 ohms, meaning the alternator shorted out, and stopped charging, then the vehicle stalled out.
Here is a charging system wiring diagram, it shows the 100amp fuse in series with the Alternator:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

https://www.2carpros.com/articles/how-to-use-a-voltmeter
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Friday, December 22nd, 2023 AT 11:29 AM
Tiny
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Hello, thanks for the quick response. I will try your suggestion tomorrow. I disconnected the negative clamp not to check battery voltage, but to try and find a possible short circuit (a google search suggested this diagnostic!) Electrics are not my best subject. Will let you know how I get on.
Steve
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Friday, December 22nd, 2023 AT 11:41 AM
Tiny
AL514
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Was the 100-amp fuse actually blown? You stated your battery voltage dropped off when you pulled that fuse, did that fuse have a small plastic clear cover over it and that's how you checked the middle of it (central blade)?
These are the components powered by that fuse.
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Friday, December 22nd, 2023 AT 12:12 PM
Tiny
STEVE-PATTERNMAKER
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I checked 100-amp fuse with multimeter, side tab to central had continuity, other side tab to central did not! Looking through the clear plastic it was not very obvious.
Steve
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Friday, December 22nd, 2023 AT 11:50 PM
Tiny
AL514
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Okay, that's a really large fuse, so I would suspect the alternator. Also, since you had a battery light and then it stalled out. You can unplug that connector from the alternator and check each pin for continuity to ground through the alternator to its case and see if either the regulator shorted out or the diode trio. You can see the 6 diodes on the lower left side there. There shouldn't be a direct continuity to ground.
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Saturday, December 23rd, 2023 AT 9:12 AM
Tiny
STEVE-PATTERNMAKER
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Oh dear, I did the previous test you suggested and have ordered an alternator, it did drop to zero on the Ohms test between black wire and ground! I am hoping this will arrive by the 29th of December. Services are not as good here in the UK. On another note, I have been slightly worried that my Euro spec. Accord is not the same as the US version (I know the Haynes manual was not the same). Time will tell.
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Saturday, December 23rd, 2023 AT 11:01 AM
Tiny
AL514
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Did you test the pin that the black wire plugs into the alternator with the connector unplugged from the alternator? So that you were only checking the alternator for a short to ground? That 100-amp fuse (black wire) feeds other components as well.
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Saturday, December 23rd, 2023 AT 2:18 PM
Tiny
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Just to verify what you tested was not the black harness wire that is in the Alternator connector?
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Sunday, December 24th, 2023 AT 1:12 PM
Tiny
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All sorted, it was the Alternator that was shorting out. New alternator fitted along with new 100 amp fuse, so far so good! Have run the engine for a few minutes, which is more than it has done for two weeks.
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Sunday, December 31st, 2023 AT 10:55 AM
Tiny
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That's good to hear, I'm glad the 100amp fuse popped first, so there was no damage to the engine computer. When those diodes inside the alternator go bad, it starts to put out AC voltage and that is really bad for computers designed for only DC voltage.
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Sunday, December 31st, 2023 AT 1:05 PM

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