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Thread: How do you know when your alternator is bad?

  1. #21
    Constant Bitrate SinnerG's Avatar
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    To make this disagreement go away, I tried Frodo's technique, but before I got the nut from the positive side totally disconnected, the car died. I just finished replacing the new alternator, and everything 'seems' to be working fine. Radio, amp, alarm are all working. The only thing I noticed is the remote start on my alarm cranks excessifully. I believe there is a learning of the starter that the remote start has to go through again. Besides that, everything is ok. I'll let you all know if everything goes well over the next few days.

    Thanks all for your insight.

    On a side note, I broke one of the mounting holes that the alternator. The screw is going through the alternator hole to the threads on the other side. It's in, but I hope it holds.
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  2. #22
    I'm sorry, and you are....? frodobaggins's Avatar
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    Quote Originally Posted by cproaudio
    Frodo, you cant fry a battery by putting 20V spikes to it. 40amp heavy duty battery chargers puts out as high as 18 volts to the battery while charging. I should know because, before my regulated power supply time, I fried my Xenarc screen with a battery charger. I measured the voltage right after the screen fried and it read 17.9 volts.
    Yes you can. High voltage chargers are made to be used for a limited time.
    Run them too long, and your battery will fry.


    If the battery voltage is below 13 volts, it means that the alternator is not producing any power and is defective.
    This is true, and I did not dispute it.


    As far as those articles are concerned, that is interesting reading. And thank you
    for that. But any car which runs that way has a poorly designed electrical system and voltage regulator. But that is a very good thing to
    know for those that are unlucky enough for that to happen.

    Unhooking the battery from any of my vehicles the voltage reads in the range
    of 13.8 - 14.6 volts with occasional small spikes above that, but nothing near 20+ volts.
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  3. #23
    FLAC cproaudio's Avatar
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    If you leave it for days then yes it'll fry the battery and possibly explode. I've left my drycell battery on a high power charger (18volts) for 9 hours straight and the battery's still good.

    Most meters are not fast enough to read the hi voltage spikes that alternators puts out. It'd take a lot of high voltage spikes to fry something. When you disconnect the battery while the engine's running for a sec or 2 might not damage anything. But if you left it off for a long period of time like that last guy I said, then you might fry something. I guess it's experience. If you ever seen someone fried anything b/c they disconnected the battery while the engine's running, you'd know what I'm mean. You'd get this feeling of "ha I told you so" and you're laughing on the inside while you see the guy's face turn white and his jaw just drop. It's a wonderful feeling. Then you tell it to your co workers and it becomes the joke of the week. it's great
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  4. #24
    Constant Bitrate SinnerG's Avatar
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    It's been a few days since my new alternator has been installed. It works fine, but I have noticed something strange. When I read voltage from my amp inputs, I used to get 13.9V consistently. Now I can only get 13V, at best.

    My question is, does an alternator need time to 'break in?' I know we're only talking about 0.9V (a little over the voltage drop of a diode).

    Maybe I should've just had my stock alternator rebuilt.

  5. #25
    Constant Bitrate Suprapc's Avatar
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    To those guys that think that your charger actually puts 18v in to the battery. Connect that charger to a battery and put your volt meter on the battery. I'll bet it's about 13~14 volts.
    What Frodo said is correct however it does continue running do not count out the possibility that the alternator only works half ***. If it dies you ether have a wiring problem or need an alternator.
    Best way is bring it to a (reputable) parts store and let them test it.
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  6. #26
    FLAC cproaudio's Avatar
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    Quote Originally Posted by Suprapc
    To those guys that think that your charger actually puts 18v in to the battery. Connect that charger to a battery and put your volt meter on the battery. I'll bet it's about 13~14 volts.
    We have a heavy duty battery charger at my work. When you put the charger on , it puts out only 12.5 volts as it charges. Eventually it will climb upwards of 17,18 volts after a few hours of charging.
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  7. #27
    I'm sorry, and you are....? frodobaggins's Avatar
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    Quote Originally Posted by Suprapc
    Best way is bring it to a (reputable) parts store and let them test it.

    True, because almost all of them will do it for FREE !
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  8. #28
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    Alternator test

    Very, very carefully use a screwdriver or other conductive metal rod to reach behind the alternator, and bring the the tip towards the center of the rear of the alternator "(opposite side of the pulley wheel, its the alternators axle for lack of better wording.). If the screwdriver/rod is pulled towards it like a magnet, the alternator is good. The reason behind this is while the alternator is generating current and working to recharge the battery, it generates a magnetic field, basically an electromagnet. Now you really do have to be carefull while doing this as 1. your near a running belt, quick way to have your hand or fingers ripped off. 2. There is a lot of high current there, it probally won't kill you like the coil can, but if your in the path of the short its gonna be a nasty shock, and mostly likely a bloody burn. 3. Not everyone has access to the rear of the alternator while its running with out being in a compromised position of sorts. Either way use a little commonsense while doing this, your screwdriver should be insulated, just in case you touch a postive lead to a ground, you lessne the impact on you. And, secondly, try this maneuver once as a dry run, as in with out the car running to see if you can even get in there before putting yourself in danger. Others have mentioned pulling off the negative battery lead, um yes it will tell you if your alternator is dead. If however it wasn't your alternator, that is when you get the voltage spike and various electronics through out your car go PoP Fizzle. Hope this helps someone.

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