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How do you know when your alternator is bad?

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  • How do you know when your alternator is bad?

    Today I noticed a light on my dash of a 'little battery.' I also noticed the the normal voltage of ~14V dropped to ~12.5V. I have a 1998 Chevy Blazer 4.3L 4X4 with 105000 miles on it.

    I went to a friend's house and he put some water in the battery resevoir, and said to wait a day or two to see if that improved my voltage reading.

    I have a feeling that the alternator is dead, or getting there. Fortunately the alternator on my car is easy to get to. Should I just replace it, or is there any sure fire way to rule out any other potential problam.

    I took my multimeter out and tried to get a reading off of the alternator while the car was running. The wierd thing is, I see the red wire (+) coming off of it, but the black wire (-) is grounded to the engine block where the negative side of the battery is also grounded. Basically the (+) sides are tied together, and the (-) sides are also tied together. How can I get a reading off of the alternator alone without the battery tainting the reading?

    Forgive my mechanical ignorance. I can change the oil, brake pads and flush my radiator. Anything more than that I leave to the 'pro's.'

    Also, my local parts place wants to charge me $149 for a new alternator. Does that sound about right? The refurbs are only $10 less, so I'll take a new one over the refurbed one for a $10 hit.

    Sorry for the long post, but I'd hope some of you who are more savy to this may have some insight. Thanks in advance.................

  • #2
    I dno what it's called in english but the relay that controls the charging may me dead
    -
    My cars
    -

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    • #3
      It's call the regulator and it's built inside the alternator. SinnerG, your alternator is shot. There's no other possible problems. Adding water to the battery aint gonna help. Alternator puts out 13.5+ volts when the engine is running. If your battery light is on and the voltage drops to 12v while the engine is running, it means that the alternator is charging and it's dead. replace it. Also while you're at it, see if they have a high output alternator for your blazer since you're gonna be putting a system with DD5.1 in it.
      NEW complete and updated My project with 100+ pics on 7-4-03
      If you have a Shuttle FV24 motherboard in perfect working condition for sale, please PM me.

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      • #4
        No high output alternators aren't availible at the local AutoZone, Kragen, or Pep Boyz. I want to plug and chug. I know of places that have HO alternators, but you usually have to accomodate them to stock. They usually don't fit like the OEM's. 105A should be plenty for my setup.

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        • #5
          If your battery light is on and the voltage drops to 12v while the engine is running, it means that the alternator is charging
          I'm assumming you meant that the alternator is not charging.

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          • #6
            my bad, its not charging.
            NEW complete and updated My project with 100+ pics on 7-4-03
            If you have a Shuttle FV24 motherboard in perfect working condition for sale, please PM me.

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            • #7
              sure fire way to test:

              start car
              remove positive battery cable.
              (if car dies, alternator definately bad)
              If car doesn't die:
              check voltage between positive from alternator and ground.

              If too low, then alternator bad.
              [H]4 Life
              My next generation Front End is right on schedule.
              It will be done sometime in the next generation.
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              • #8
                Advanced Auto? or one of those places will test your electrical system for free. Maybe they can help diagnose the problem.
                -TJ
                Smart Playlists Plugin for RoadRunner
                CarPC - 05 Acura TL Navi (pics soon)
                Retired CarPC - 03 Infiniti Qx4

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                • #9
                  if you dont want to pay $150 for a new alternator you can probably get it rewound or something
                  92 Taurus SHO - Daily driver - Epia M10000 80GB 512mb 7" touchscreen, Opus PSU etc

                  93 Black SHO - 30mpg cruiser

                  90 Silver SHO - 419whp fun/project car, CarPC to come (for tuning)

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                  • #10
                    I'd try to get a used instead
                    -
                    My cars
                    -

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                    • #11
                      Get one from a junkyard. Save yourself a bunch of money.
                      A digital mind lost in an analog world.

                      Learn to ask smart questions!
                      http://www.catb.org/~esr/faqs/smart-questions.html

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                      • #12
                        Originally posted by frodobaggins
                        sure fire way to test:

                        start car
                        remove positive battery cable.
                        (if car dies, alternator definately bad)
                        If car doesn't die:
                        check voltage between positive from alternator and ground.

                        If too low, then alternator bad.
                        You dont ever want to do that!!!! Alternator puts out unregulated DC Current. sometimes it could spike up to 20V. Battery acts as a filter and smooths out the ripple kind of like a surge protector. If you disconnect the battery terminal while the engine is running, there's nothing to smooth out the ripple. When the spike hits the 20V, you gonna be replacing more than just the alternator. We had to replace alarms, head units, and amps b/c the owner did exactly what you suggested. Bottom line is if your meter reads less then 13 volts while the engine is running, the alternator is shot. It needs to be replaced.
                        NEW complete and updated My project with 100+ pics on 7-4-03
                        If you have a Shuttle FV24 motherboard in perfect working condition for sale, please PM me.

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                        • #13
                          Originally posted by cproaudio
                          You dont ever want to do that!!!! Alternator puts out unregulated DC Current. sometimes it could spike up to 20V. Battery acts as a filter and smooths out the ripple kind of like a surge protector. If you disconnect the battery terminal while the engine is running, there's nothing to smooth out the ripple. When the spike hits the 20V, you gonna be replacing more than just the alternator. We had to replace alarms, head units, and amps b/c the owner did exactly what you suggested. Bottom line is if your meter reads less then 13 volts while the engine is running, the alternator is shot. It needs to be replaced.

                          Your alternator does not put out unregulated current. If it did, you would be blowing up your battery as well. And if your current is running below 13 volts, it's not going to be enough to fry anything.
                          [H]4 Life
                          My next generation Front End is right on schedule.
                          It will be done sometime in the next generation.
                          I'm a lesbian too.
                          I am for hire!

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                          • #14
                            frodo is right your alter. has a regulator in it....and yours is shot so go take yours to a alter. shop and have them rebuild it for less than a 100 bucks.

                            just had mine done and now it puts out more juice for my second battery went from 85 to 135.

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                            • #15
                              Originally posted by cproaudio
                              We had to replace alarms, head units, and amps b/c the owner did exactly what you suggested.


                              Ummm, you should ALWAYS disconnect those kinds of items before playing with your car's electrical system.


                              .

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