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308: Generator Light comes on with brakes

Discussion in 'Technical Q&A' started by Mike328, Feb 26, 2004.

  1. Mike328

    Mike328 F1 Rookie
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    Oct 19, 2002
    2,655
    Boulder, CO
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    Mike
    I've had this problem for a while now, and am pretty much out of ideas and was looking for some insight.

    Whenever I apply my brakes--meaning, whenever the brake light circuit is activated--my red generator light comes on (the "alternator" light in other cars). I mostly notice the problem at night, when my headlamps are up (headlamps are aftermarked and relay-wired to the battery). It doesn't come on full, just slightly; maybe 30%.

    If I use my turn signals with my brakes, the generator light's intensity will increase, to maybe 50% intensity, and only when the actual turn signal circuit, as directed by the turn signal relay, gets juice. So, with brakes and turn signal, the generator light alternates between 30% and 50% on with the signals.

    I just replaced the fusebox last year with one I build with a NAPA block. I have a 2mo old Optima Red Top battery. I've got a voltmeter in the car, and it confirms a voltage drop with the brakes on (but also with headlights, windows going up and down, etc.).

    The alternator was also rebuilt last year. It's behaving properly, I believe; The battery on startup gets a healthy 14.1-ish charge, falling down when charged back up to 13.6 - 13.8-ish; the battery in the morning is strong right at 12.5.

    Any ideas? I feel pretty confident that I may have mucked up something with all the rewiring of the new fusebox (though I'm 100% everything went back to where it's supposed to).

    One last thing. When I *did* replace the fusebox, I was blowing one of the turn signal fuses (don't remember which one), which would happen whenever I activated the hazard light switch. I traced it for a whole day, where I isolated it to the brake circuit. Then, that night, right in the middle of it, the fuse blowing behavior just stopped, and I haven't messed with it since.

    Any ideas? Is this a short?


    Thanks,

    --Mike
     
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  3. Steve Magnusson

    Steve Magnusson F1 World Champ
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    Jan 11, 2001
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    Steve Magnusson
    Your "blowing fuses" is probably a different issue, but I think all carbed 308s suffer to some degree from the "glowing gen light with other high current draws (headlights, turn signals, etc.) are on" disease.

    The generator warning light is just a bulb filament with one side connected to the GV (yellow/green) wire coming from the alternator (which is a completely independent alternator output) and the GN (yellow/black) wire coming from the top of fuse #3. The way it's supposed to work is that if the alternator goes dead then the GV wire goes to near ground while the GN wire stays at the battery voltage of ~+12V so the warning light comes on strong. Unfortunately, the bulb isn't a smart device so if the alternator GV wire has good voltage output and the voltage on the GN wire droops a little (due to the slight I^2*R losses in the ignition switch, longer wiring, more connection, etc. -- which are greater when you've got a lot of other electrcal stuff on) then the bulb still glows a little.

    Measure the voltage on the GN wire coming from the top of fuse #3 under your worst conditions -- if it's significantly lower than the voltage at the "+" battery terminal under those same conditions then you could try to check/improve the various interconnections in-between, but might be something you have to live with.

    I believe with the addition of a (high power) diode you could prevent the false result (i.e., bulb lights when GN voltage < GV voltage) while preserving the desired result (i.e, bulb lights when GN voltage > GV voltage), but the problem wasn't bad enough on my ex-308 for me to pursue.
     
  4. Mike328

    Mike328 F1 Rookie
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    Oct 19, 2002
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    Mike
    Steve,

    Thanks very much for your detailed post. I understand clearly what the connections as you've described.

    I did a little additional research and found some useful threads in the old FerrariChat archives:

    308 generator light on, running hot
    http://www.ferrarichat.com/discus/messages/256120/2707.html

    308 Brake lights/turn signals not working?
    http://www.ferrarichat.com/discus/messages/256120/590.html

    308 Running off of battery?
    http://www.ferrarichat.com/discus/messages/256120/187460.html


    First of all, it's comforting to know that this isn't just my 308 having this problem. If we're all in the same boat, then we're more likely to find a fix (or just be content in ignoring it).

    Secondly, the first post mentioned above is of particular interest because Stacey, the poster, appears convinced that he had an direct fix for the generator light by replacing the power distribution terminal (which, in his case, was corroded). I suspect that this may be the case for this often-overlooked component in many of our 308s.

    I'm going to investigate it, see if I can't get a photo or two, and have a go.


    --Mike
     
  5. DN35

    DN35 Formula Junior
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    Nov 22, 2003
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    D. Norton
    Mine ('78) does the same thing. Never had an electrical issue.
     
  6. Steve Magnusson

    Steve Magnusson F1 World Champ
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    Mike -- I had forgotten about stacy's post, but that would be a likely candidate (that 4-way connection is one of the connections in the path from the +12V battery post to that GN wire of fuse #3). Could you please post the top of #3 fuse voltage and the +12V battery terminal voltage (under the worst conditions) before (and after) making any changes? -- might be a useful future reference for someone...
     
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  8. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,076
    Why not take the volt meter along with your next drive, and see what it says when the light comes on. If you have one, also hook up an ameter to see if the load is being carried by the alternator (good) or by the battery (not so good).

    If the battery is 13.5+ and stays there, the light is indicating that the generator is producing the electricity to run the heavy load. As the engine RPMs gose down, more field current is needed to produce the same output voltage and current to carry the load. THis field current illuminates the light.

    If it drops below 13.5 then the batter is being used to run the heavy load with some help from the generator. Once again the volt meter will help you isolate the cause.
     
  9. brownyjs

    brownyjs Rookie

    Nov 9, 2003
    9
    I had the same problem on my 1980 308. I found it to be the 4 way power connector next the the fuse boxes. The 4 way connector distributes power from the alternator into the ignition and the fuses. Have a close look at the slide connectors and change any that are annealed. The generator light is in circuit with the alternator, is connected to that 4 way and will only extinguish when it sees the same voltage as the alternator. High resistance connections will cause this problem. The 4 way block has a white plastic cover and is mounted alongside the fuses.
    Good luck.
    Browny
     
  10. thecarreaper

    thecarreaper F1 World Champ
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    Sep 30, 2003
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    I NEED A JOB
    bump............
     

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