Does anyone know signs of a bad alternator. - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 12-24-2007, 02:32 PM   #1
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Default Does anyone know signs of a bad alternator.

Well i think that mine is going to die soon because when i start my car it will stay around 12 or 13 but then i will drive for maybe 10 mins and it will slowly start to drop. Everything affects the battery the heater,cd player,speakers, even just sitting at an idle it will get lower and lower. Then it will drop to the red and move back up. Anyone know why it does this?
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Old 12-24-2007, 03:23 PM   #2
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A real good bet would be that'* there'* a fault in the windings. As they heat up, and electrical resistance builds, the output drops.

Might also be the internal regulator, but that'* a chip and generally falls into the works/don't work category.

At any rate, it'* easy to get to and install. Go to the parts puppies and buy a rebuilt one and install it before the car strands you in BFE.
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Old 12-25-2007, 12:12 AM   #3
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Start the car, disconnect the positive wire from the batt VERY CAREFULLY not to touch any other steel with the wrench, see if still runs for a minute or two. If it slowly dies, you need a new alternator. I'm tired, and on cold medicine, but if I screwed that up, somebody correct me.

Either way, most auto parts places (autozone like) does free alternator tests. Do it, more accurate.
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Old 12-25-2007, 12:25 AM   #4
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Quote:
Originally Posted by putertopia
Start the car, disconnect the positive wire from the batt VERY CAREFULLY not to touch any other steel with the wrench, see if still runs for a minute or two. If it slowly dies, you need a new alternator. I'm tired, and on cold medicine, but if I screwed that up, somebody correct me.

Either way, most auto parts places (autozone like) does free alternator tests. Do it, more accurate.

DO NOT do this with any Bonneville......

This may have been fine back in the day with cars that were not Computer controled... But this is not a good thing to do to any modern car..

Please do not do this...

Have the system and battery load tested

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Old 12-25-2007, 01:36 AM   #5
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Quote:
Originally Posted by putertopia
Start the car, disconnect the positive wire from the batt VERY CAREFULLY not to touch any other steel with the wrench, see if still runs for a minute or two. If it slowly dies, you need a new alternator. I'm tired, and on cold medicine, but if I screwed that up, somebody correct me.
Bad idea. The old way to test an alternator (the SI style) was to disconnect the field at the plug so the car is running on bat only, reconnect the field, then use a screwdriver in the appropriate hole to activate the field at full load (all while watching the system voltage.) This doesn't work on the more modern Denso style alternators, so don't try it.

You can take the alternator off and have one of the parts stores bench test it (they plug it in, spin it up, and see what it'* output is.) Have the battery checked as well...most of the alternator failures I've encountered were caused by, or at least accompanied by, a bad battery. They do it for free and it'* the easiest way to know if it is bad or not. And don't be surprised if it comes up marginal to bad...GM alternators die..it just happens (I've done the alternators on every GM car I've ever owned, it just about a given that it will give up the ghost sooner or later.)
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Old 12-25-2007, 10:27 AM   #6
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Every car here will die instantly with the battery disconnected, regardless of the condition of the alternator. As mentioned above, not only will it not work, but you could damage your car in doing so. I suggest removing the alternator (very easy to do) and have it load-tested. Make sure they run it long enough to get heated up well. Do NOT buy a rebuild from Autozone. They are very cheaply built and crappy alternators that do not last long.

NAPA and Advanced Auto seem to have the best rebuilders under contract, and back their stuff up if you have problems.
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Old 12-25-2007, 10:59 AM   #7
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Quote:
Originally Posted by willwren
NAPA and Advanced Auto seem to have the best rebuilders under contract, and back their stuff up if you have problems.
IIRC Advanced Auto has DELCO remanufactured alternators. That would be my pick.

I've had one alternator go out on GM cars in the last 25 years, a 1993 LeSabre. Replaced with GM remanufactured and was still great at 150K. Have 98 LeSabre with original at 160K miles and 03 at 60K miles on original. My opinion is it'* battery deterioration and failure that kills the alternator in most cases. Replacing a battery as it becomes less than optimum is good money spent early that pays off in the end; don't wait for a battery failure.
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Old 12-25-2007, 08:47 PM   #8
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Quote:
Originally Posted by willwren
Do NOT buy a rebuild from Autozone. They are very cheaply built and crappy alternators that do not last long.

I'll second that. I bought one for my Ranger and 6 months later it went south. If it dies again soon I'll just buy a different brand rather than taking all my time swapping these out!
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Old 12-25-2007, 10:11 PM   #9
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Well.. my bad. Apologies lol, but at least I suggested to wait for somebody else to chime in on that one.
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Old 12-26-2007, 12:17 AM   #10
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thanks for all the thelp im going to try and get that done in the next few days will im on break.
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