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456 Alternator Sensing Wire & Fuse?

Discussion in '456/550/575' started by DrivenHK, Nov 6, 2020.

  1. DrivenHK

    DrivenHK Rookie

    May 1, 2019
    28
    Hong Kong
    Alternator has been changed, it was not charging. Now the charge is at 16V+
    Charge Warning light intermittent - but charge is consistently above 16V.

    Would like to ask whether there is a sensing wire/fuse on this model and if so location?

    Thank you!
     
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  3. Qavion

    Qavion F1 Rookie
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    Feb 20, 2015
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    Ian Riddell
    If it's the same as the 456M, there is no fuse, but there is a small red wire going directly from the battery to the alternator plug "S" pin. If you can get access to the plug on the alternator, check for pin damage, corrosion, etc. Also, check for normal battery volts on the S terminal (car not running).

    Sorry, I've lost the ability to post links to my wiring diagrams (but I can mail the relevant diagram to you in PM if you give me your email address)
     
  4. Qavion

    Qavion F1 Rookie
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    Here's a sneak preview... the section of the diagram which includes the battery and alternator

    Image Unavailable, Please Login

    Not sure why I have two wire gauge values on the same wire :oops:
     
  5. DrivenHK

    DrivenHK Rookie

    May 1, 2019
    28
    Hong Kong
    Qavion, Really Appreciate this ... Thank you!

    Planning to test for continuity to batt and resistance between S and Connector @ Alternator.
    What kind of resistance should be seen on the S wire?
     
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  6. Qavion

    Qavion F1 Rookie
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    With both ends disconnected, I guess a few milliohms (assuming the test leads on the meter show around zero when touched together).

    (EDIT) If you're using the chassis as part of your test circuit, the values will go up to a few hundred milliohms... Even more if you use cheap crocodile clips.
     
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  8. DrivenHK

    DrivenHK Rookie

    May 1, 2019
    28
    Hong Kong
    Thank you for the advice. Are there any downsides to leaving the OE sense wire alone and to run a new sensing wire to rule out the cause?
     
  9. Qavion

    Qavion F1 Rookie
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    Sounds ok. Just inspect the plug on the alternator to make sure there is nothing shorting out inside the plug (which may negate the effect of the wire bypass).
     
  10. Qavion

    Qavion F1 Rookie
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    I'm just wondering why the 456 designers didn't put a short jumper wire from the big positive terminal on the alternator to the sense terminal on the alternator plug (instead of running a long small wire all the way to the battery), especially since there is no fuse in the line. Both small and large red wires from the battery should be at the same potential. Perhaps it's just hard to pigtail/crimp large and small wires together.
     
  11. Gizzi

    Gizzi F1 Rookie
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    It’s done on nearly all cars Ian to compensate for voltage drop from the alternator to the battery and for better voltage control. The alternator voltage and current output will generally be higher at the alternator than at the battery, by a small amount, because of this.


    Sent from my iPhone using Tapatalk
     
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  13. Qavion

    Qavion F1 Rookie
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    Thanks, Gizzi.
    Still doesn't make sense to me. There is is only one positive terminal on a battery and as far as I can see from that diagram above, the wiring is all joined up to that terminal. The only potential difference (on paper) seems to be the resistance in the wiring.

    Also wondering why Ferrari show multiple contacts on the positive side of the battery in their wiring diagrams... A, B and others.
     
  14. DrivenHK

    DrivenHK Rookie

    May 1, 2019
    28
    Hong Kong
    Wanted to post results. Thank you all for guidance and brainstorming.

    Bypassing the sensing wire to main B+ at the alternator brought charging back within expected range.
     
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