F355 alternator - Where does the "S" wire go from the alternator? | FerrariChat
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F355 alternator - Where does the "S" wire go from the alternator?

Discussion in 'Technical Q&A' started by AZLambo, Aug 13, 2006.

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  1. AZLambo

    AZLambo Formula Junior

    Nov 4, 2003
    511
    Phoenix, AZ
    Full Name:
    Jeff
    #1 AZLambo, Aug 13, 2006
    Does anyone have a wiring diagram showing where the "S" (voltage sensing) wire goes from the alternator plug connection? It senses battery voltage. I am wondering where Ferrari decided to do this from? What would connecting straight to the Battery + output stud on the alternator do? That would definitely be a battery voltage sensing spot, but it is also the battery output of the alternator. Wouldn't both necessarily be the same?
     
  2. tbakowsky

    tbakowsky F1 World Champ
    Consultant Professional Ferrari Technician

    Sep 18, 2002
    19,827
    The Cold North
    Full Name:
    Tom
    #2 tbakowsky, Aug 13, 2006
    I have the diagram in front of me right now.

    According to this diagram, the colors on the 3 wire plug should be as follows:

    Yellow- heads on up to the fuse panel

    Red-splits off to the battery through a connection behined the fuse box in the passenger footwell. This also appears to be where the sensing wire hooks up to.

    yellow/green-is indicator light

    Then of course you have the large main batt connection at the alternator.
     
  3. wolftalk

    wolftalk Formula Junior

    Jan 27, 2004
    367
    san franciso area
    Full Name:
    phil
    #3 wolftalk, Aug 15, 2006
    the purpose of the sense wire is to ensure that the voltage/current at/near the battery is sufficient to charge the battery.

    when there is no current draw in the system, then the voltage at the b+ cable at the alternator is the same as the battery positive post. When you start loading the system with high current draw(fans, headlights, ignition, etc), small resistances in the connections and the inherent resistance of the wire itself will cause a voltage drop between the alternator output and the battery.

    You can see this by sticking a voltmeter between b+ on the alternator and the positive battery terminal with and without the car running. With the car running and as many power consuming devices as possible turned on, you'll see a bigger number. That number is the difference in voltage between the alternator output and the battery, and it should be low, but won't be zero. Note that you aren't referenced to ground here...just the difference in voltage between two ends of the same cable.

    The S wire only has a tiny amount of current flowing in it at all times, so voltage drops are not a problem unless there is a big resistance in the wire...like at a connector or the S wire attach points.

    The voltage regulator has a more accurate picture of what is happening in the system by looking at wherever S is connected to, and it can vary the alternator output to keep the voltage at the other end of the S wire at the value it wants.

    I think this is the 3rd time someone has mentioned a connection in the passenger footwell, and I don't recall you've ever said you checked/cleaned it.
     
  4. AZLambo

    AZLambo Formula Junior

    Nov 4, 2003
    511
    Phoenix, AZ
    Full Name:
    Jeff
    #4 AZLambo, Aug 16, 2006
    I took off the cover in the passenger footwell. All I can see are the fuses/relays. I cannot get to the "big battery cables" or even see them for that matter. I was told that they are "up and to the left" if I stick my ear on the floorboard and look up through the opening. Also, God knows where the sensing wire connections are and how it is routed. I am not mentally up to the task of hassling with this anymore. I have changed alternators myself three times now and am sick of "dinking" with this car anymore. The dealer is doing the "major", sense it has been 5 years since the last one. Also, they are replacing the right exhaust manifold which had a slight leak. (I don't want to hear about how the replacing it with a Ferrari manifold will do the same thing over time, and how I should go to a Tubi, or QV in London, or Napaformance. I know about all of that) I am also having the water pump rebuilt and all of the charging-related wiring replaced. Hey, it's only money, and I don't have to stress out mentally anymore! Of course, I will now keep the car for another 5 years and enjoy it! F355's are at the bottom of their value now. They will hold at $75,000 or close to it for another 5 years in my opinion. 360's are still well over $100,000, which makes the F355's a real bargain now.
     

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