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"Good" Alternator not Charging Battery

Discussion in '1994-2004 V6 Mustang Tech' started by blainestang, May 10, 2009.

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  1. blainestang

    blainestang 3 Mustangs, 0 Spoilers

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    Electrical is not my forte, and I've exhausted the majority of ideas I've had or can find, so I'm looking for advice.

    Background:

    Car worked fine, but then started losing the ability to run A/C, Blinker, and Radio at the same time. The radio would shut off. This happened once and then the same day, it shut off while I was pushing the brakes w/ the clutch in, then it wouldn't start for a few minutes. Started up after a few, and then on the way home was so low on electrical power that (presumably), the injectors stopped firing correctly and it stalled.

    I got a jump and let it charge off the other battery for a few minutes, and it worked for a few miles until (presumably) the battery ran out of juice again. Rinse and repeat until I got it home.

    So, I think, "bad alternator... it's not recharging the battery."

    I stick the multimeter on the battery with the car off: 11.8V (not fully charged), and then with the car running: 11.5V... not charging.

    I take it out and take it to Discount Auto and they test it... "It's good". They say test the battery, too. They test it, it's "good", too, and it charges to full power. So, they test the Alternator again and compare it to how a NEW alternator tests... they're identical.




    Cliff Notes:

    - Battery dies while driving
    - Less voltage drop across battery when car is running... instead of more
    - Alternator is "good" according to Discount Auto (same test results as a new one)
    - Battery is "good" according to Discount Auto


    Now, the car does have some aftermarket audio (been there since I bought it 30k ago), and there are 4 wires that have been cut that look like they were once maybe just 2 uncut wires. No one has messed with ANY of this stuff in the 30k I've owned it, but I thought it was worth mentioning.

    So, that's it... I've looked at a couple other things, but let's hear some ideas for what could be going on here.
  2. Kuplex

    Kuplex Unconventional Off Topic Moderator

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    IIRC correctly, the battery is used to start the car and run accessories when the car is not running, ie: if the car is running the alternator is powering everything. I'll check my wiring schematics and see if there is any kind of fusable link between the alternator output wire and the fuse block.
  3. blainestang

    blainestang 3 Mustangs, 0 Spoilers

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    I knew I forgot something... this is on my '93, so I don't know what differences there may be.

    I do believe there may be fusible links in there somewhere. Maybe that's the next thing to look for. Not really sure what I'm looking for, though. :eek:
  4. Kuplex

    Kuplex Unconventional Off Topic Moderator

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    Okay, I don't know how much the schematics vary between years (wire color, etc) but it looks like there is a fusable link before the power reaches the fuse block under the hood.

    Check to see if fuse "R" (http://3.8mustang.com/wiki/Fuse_and_Wiring_Diagrams#99-04_Engine_Compartment_Fuses) is blown. If not, check to see if it is getting any power with the engine on. If so, we'll go from there. If not, we'll need to trace back from the block to the alternator to see if the fusable link and wire are intact.
  5. 99mustang3.8

    99mustang3.8 J-Money

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    Had mine tested last week said both the battery and alternator were great. Before my battery light would come on whenever I got above 3000 rpms. Now last night it came on and stayed on. My battery gauge kept getting lower. Didnt drive it today. But went into the garage started it up and in about 2 min my battery was dead. So what do I do I checked my fuse and its fine. Ordered a new alternator but do you think thats whats wrong?
  6. Texas501

    Texas501 Texas501

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    Clean your battery terminals with a wire brush. Clean the battery posts as well. I had all sorts of electrical problems, and even replaced the starter (the alternator and battery checked ok). It ended up being the negative battery terminal that I had replaced a few years back (The factory terminal was cracked), it had residue in around the terminal. I scrubed it with a wire brush adn the terminal. That was the end of my electrical problems. I was so upset tha the problem was so simple and yet caused all sorts of electrical problems. Just give it a try, won't cost you a dime either.
  7. DEstang

    DEstang Why So Serious?

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    So wait, does the car otherwise run okay or will it die after 5-10 minutes of running?

    If the alternator isn't putting out 13.8v the car will not run correctly & charge the battery.
  8. blainestang

    blainestang 3 Mustangs, 0 Spoilers

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    If you're talking to me...

    The car runs, assumedly off the battery, for a few minutes, then dies. Then, I can Jump/Recharge the battery, and it will run for a few minutes again.

    Supposedly the alternator plugs are notoriously bad on these cars, so I think I'm going to replace that and see what happens. I'm going to check fuses and such, too.
  9. 99mustang3.8

    99mustang3.8 J-Money

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    Replaced my alternator today and everything seems to be working fine now.
  10. BLK96

    BLK96 New Member

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    Change the negative battery cable or both, but normally if you see corrosion at the terminal its too late at that point and the corrosion has run all the way thru the wire even if you see no wire swelling. Also get a good corrosion cleaner like Permatex and spray it in the battery box and around the radiator and fan shroud as it has probably spread there as well even if there are no visual signs.
  11. BlueSHO

    BlueSHO New Member

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    That really sounds like an alternator not charging. Since the alt tests OK, I would be looking at wiring, which seems to be the direction you are going. Make sure they are clean, tight, and non-corroded. In addition to the alternator terminals, check your main ground cable carefully, they can cause all sorts of electrical funny business.

    I agree it's worth checking fuses, especially since it is simple and free to do, but I would be extremely surprised if that was part of your problem. All a fuse does is breaks a circuit that is overloaded or shorting out. Your problem sounds more like wiring issues, especially considering your car is getting older....
  12. kgmx856

    kgmx856 Dirtbike Aficionado

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    I just had this same problem on a forklift I repaired today. It would run, but not start on its own, and stay at 12.6v, even at full throttle. Turned out the ground connection on the motor was coming undone. Check to see if your ground is connected properly.
  13. blainestang

    blainestang 3 Mustangs, 0 Spoilers

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    Did that... I don't think that's the problem.

    Thanks for the suggestion, though.


    I actually think that the problem is the fusible links... which is related to the 4 wires above.

    I think what happened was the original fusible links burned up. Then, instead of buying NEW fusible links, they just cut the "melted" part off and then spliced them back together. So, then, when they burnt up again, they didn't just bubble up to show they were burnt, they literally burned apart... resulting in the 4 wires that look like they should be two, like I mentioned above.

    So, that's my theory. Been too busy to try it out, but that's my guess.
  14. Surgin

    Surgin New Member

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    Could it be that the alternator is overheating? I have seen that before, the alternator will test fine, but run it for a while and heat it up, and it will stop charging.

    Justin
  15. blainestang

    blainestang 3 Mustangs, 0 Spoilers

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    When I hook it all up, get the battery charged up, and start the car, the battery outputs less voltage than without it hooked up to the car.
  16. Surgin

    Surgin New Member

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    Well i guess that blows the overheating problem out...


    Justin