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Alternator/electrical system question

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  • Alternator/electrical system question

    While chasing my little green turn signal indicator light problem, I find that when the car is at hot idle, turning on the headlights will shut down the radiator fan. As soon as I cut the headlights off (parking lights still on), the fan turns back on. This was replicable 100% of the time. Is it part of the car's design to do this, or is this a sign of a problem? It would seem to me that the car should be able to run both lights and fan simultaneously while running, especially at a hot idle when the car needs the fan the most.

    I am wondering if my alternator is having a problem. I have heard that as Banner batteries die, they can take the alternator with them, perhaps as the result of constantly trying to charge the battery. What test might I perform to check the alternator, and what other issues might cause the car to not be able to power all of it's electrical systems while the engine is running?
    2013 Boss 302 Mustang
    2005 SV Roadsport (gone but not forgotten)

  • #2
    When the fan turns off completely it is probably not related to alternator weakness. I would rather suspect you have power fluctuation in the wiring somewhere else (bad connector crimps or bad ground connection). Such voltage change in the system can screw up the temperature sensor reading and the ECU thinks temperature is lower.

    Check the voltage at the alternator output, battery and 12V line inside the dash under different operating conditions (idle, higher revs, with and without light/fan. You can also check if the various ground lines from alternator, lamps, fan, ecu etc. are really ground (i.e. <<1 Volt or better lower) or if there is a bad connection.


    • #3
      In addition to checking Slomove's suggestions you might look to see if something is just plain wired wrong.

      The first test of the alternator is to see if it's putting out about 13.5 volts or a bit more when running at 2000 rpm or so. Test it at the battery. Otherwise most auto parts stores can test the alternator. You may need to know what car it's from.

      Do you have the wiring diagram for your car?


      • #4
        As an aside, may I intercede with a comment/question? I notice Caterhams don't come with a voltmeter and/or an amp meter (at least mine didn't)........why is that? I've also wanted to know, at a glance of the dash gauges, if my alternator was charging the battery or not. After a drive I always put the battery on a maintainer, and depending on the # of starts, low idle, etc. I can pretty much tell how long it'll be before it's up to full charge. One thing's for sure, my new Odyssey PC680 comes up quickly compared to the Banner I've since replaced. I'm thinking of installing just an ammeter as a reference gauge.......OK to do so? I'll appreciate your inputs.
        With Best Regards,
        Jim F.


        • #5
          A voltmeter is usually a better choice.

          You don't need to wire the fat battery charging wire through the dash (which may become a hazard if ever chafed). You just tap a 12V line somewhere in the dash.

          From the voltage you can assess your charging condition just as well or better. It will also give you an indication if your electrical system is overloaded or if the alternator crapped out.

          If you ever see the voltage drop below 12.7 V you know you ain't charging anymore. Last year I noticed to battery voltage going up over 16 Volt and sure enough the regulator was broken.