308 alternator warning light is dim | FerrariChat

308 alternator warning light is dim

Discussion in 'Technical Q&A' started by 78-308gt4, Jan 18, 2006.

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  1. 78-308gt4

    78-308gt4 Formula Junior

    May 22, 2005
    735
    Memphis, TN
    This problem started last month. Previously the light came on when I put the key into position III, went out upon starting and a quick blip of the throttle. One cold night in December we drove it to a party. Everything was fine on the way. Starting the car to come home, however, the alternator light came on in position III but it was very dim (perhaps 1/4 normal) and didn't go out regardless of RPM. We made it home OK but it did take some charge off the battery. So... I've since searched here, studied the threads that came up, and tried some of the solutions. No luck thus far.

    I used a multimeter to take some measurements: battery disconnected = 12.5+V, battery connected = 12.5+V, voltage at the "H" connector on relay panel = 12+V, battery connected engine running (regardless of RPM) = 11.5V, alternator warning light lead engine running = 11.5V. Now here is where it gets curious - the other lead to the alternator warning light measured 7V. This struck me as odd so I measured the current through the light = 90mA. I was quite surprised to see the light go out when I measured the current! Unfortunately I know just enough to be dangerous so my next idea as to what current to measure next blew the fuse on my multimeter.

    I removed and cleaned the braided copper ground wire and contacts. I cleaned the contacts to the alternator warning light. I switched lamp assemblies with the choke warning light with no effect.

    To summarize, running my car drains the battery, the alternator warning light stays on (albeit dim) from when I put the key in position III before starting and when the engine is running at any RPM with no change in brightness. Any ideas?
     
  2. greg328

    greg328 F1 Rookie

    Nov 17, 2003
    4,209
    Austin, TX USA
    Full Name:
    Greg
    Time to rebuild the alternator. Could be a bad diode...

    Greg
     
  3. Ken

    Ken F1 World Champ

    Oct 19, 2001
    16,078
    Arlington Heights IL
    Full Name:
    Kenneth
    Could be a loose belt, but the diodes or VR in the alternator are more likely. 11.5 V running is way too low; should be about 13-14v. The 12v readings are a touch low too. Don't go driving far from home at night until you track it down.

    Ken
     
  4. ham308

    ham308 Formula Junior

    Nov 3, 2003
    358
    NE Switzerland
    Full Name:
    Richard Ham
    When mine did this it was the voltage regulator in the alternator and it was relatively easy and not expensive to replace the whole alternator.

    Unfortunately the regulator failed because it was getting covered in oil from somewhere else, which turned out to be a bigger can of worms altogether :)
     
  5. Ken

    Ken F1 World Champ

    Oct 19, 2001
    16,078
    Arlington Heights IL
    Full Name:
    Kenneth
    If your meter is resurrected, check for AC voltage when running. If you get any it points to the diodes.

    Ken
     
  6. 78-308gt4

    78-308gt4 Formula Junior

    May 22, 2005
    735
    Memphis, TN
    Ken,
    Where would I place the probes to measure the AC voltage?

    Also, my initial post has an error... every reference to key position III should be position II... HA well I tried to be accurate and succinct.

    The alternator looks clean though I noticed some drops of oil on the bottom edge of my A/C compressor.
     
  7. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,932
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    Measure the AC voltage between the small GV wire at the alternator or at the alt warning light and chassis ground.

    (When things are working as they should, the diode network rectifies 3 out-of-phase AC signals into a quasi-DC signal. If something in the diode network fails, you'll get a big AC component in that signal which is the basis for Ken's suggestion -- i.e., big AC = bad)
     

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