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

  1. #11
    Maximum Bitrate fluffy2097's Avatar
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    Get one from a junkyard. Save yourself a bunch of money.
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  2. #12
    FLAC cproaudio's Avatar
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    Quote 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.
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  3. #13
    I'm sorry, and you are....? frodobaggins's Avatar
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    Quote 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.
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  4. #14
    Newbie blkpython's Avatar
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    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.

  5. #15
    Maximum Bitrate none's Avatar
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    Quote 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.


    .

  6. #16
    I'm sorry, and you are....? frodobaggins's Avatar
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  7. #17
    Maximum Bitrate fluffy2097's Avatar
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    Disconnecting wires from the alternator while the engine is running is a really stupid thing to do. Just replace it. One of my roommates has worked for major car companies on their electrical systems. He's said loosing the ground to your alternator while it's running will burn out every non essential electrical system in your car, (think stereo, alarm system, amp, computer, the schmuck disconnecting the wires). You end up sending a massive surge of energy through the system, and anything not designed specifically to handle it is toasted.

    The moral of this story? Don't **** with electronics if the thing is powered on.
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  8. #18
    I'm sorry, and you are....? frodobaggins's Avatar
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    Quote Originally Posted by fluffy2097
    Disconnecting wires from the alternator while the engine is running is a really stupid thing to do. Just replace it. One of my roommates has worked for major car companies on their electrical systems. He's said loosing the ground to your alternator while it's running will burn out every non essential electrical system in your car, (think stereo, alarm system, amp, computer, the schmuck disconnecting the wires). You end up sending a massive surge of energy through the system, and anything not designed specifically to handle it is toasted.

    The moral of this story? Don't **** with electronics if the thing is powered on.

    I said remove the positive cable.
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  9. #19
    FLAC cproaudio's Avatar
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    http://www.forparts.com/techbattery.htm
    http://www.cs.wisc.edu/~lists/archiv.../msg00079.html

    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.

    Alternator puts out AC current hence the name alternator In the alternator there are rectifiers that block all the neg current and let all the pos current pass through. THe battery smooths out the ripple and obsorbs the high voltage spikes.

    If the battery voltage is below 13 volts, it means that the alternator is not producing any power and is defective. So no you're not gonna fry anything when you're running off just the battery while the engine's running. top fuel drags dont have alternator. They use batteries.

    If you feel comfortable disconnecting the battery while the engine's running then go ahead and try it. But if you fried anything, dont blame me. I'm not just talking out of my ***. Over the years I've seen some people did it to their cars after we told them not to do it but they feel the need to prove us wrong much like you. 3 people did it and 2 of them fried the deck. The last one was a funny story. He disconnected the battery and we started talking for a while. All of the the sudden his car started to run like ****, sounded like it's running out of gas then started knocking and the engine died. He reconnected the battery and tries to crank over the engine. it just cranks and cranks and nothing happened. He had half tank of gas so its not like it ran out of gas. Finally he called the towtruck and had it towed. A month later he came in to have the amp swapped out cuz he also fried the amp too. We ask him what happened and he told us that his engine control computer was fried. The dealer replaced it under warranty. We were rolling our asses off after he left. Damn that was a funny story to tell after all these years.
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  10. #20
    FLAC cproaudio's Avatar
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    Quote Originally Posted by none
    Ummm, you should ALWAYS disconnect those kinds of items before playing with your car's electrical system.
    Re-read my post again. I never said that I did it. I said that Other people did it not thinking that it would damage the components. I said that we had to replace them because of they disconnected the battery while the engine's running.
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