Bad Alternator? or other? Voltage loss - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 07-10-2013, 08:50 PM   #1
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Default Bad Alternator? or other? Voltage loss

Hi all,

Any hep would be greatly appreciated.

I got an odd ball one here. When I am driving the voltage will be fine on the gauge and then flutter and fall to apx 10 volts. At that time the warning bell goes off. If I rev the motor a few times is will bounce back up. but not always.

When it last happened I was able to stick a volt meter in the cigarette lighter and saw 12.5 volts then when the problem stopped it went back to 13.8.

This car has a past history of eating the alternator.

special note:
battery, starter, alternator replaced 6 months ago. (not to say one didn't die)
new computer, coils, ignition module 2 years ago. new ignition switch last month.
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Old 07-11-2013, 06:01 AM   #2
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I assume this is on your 92 Bonneville SE?

Most cheap alternators have a hard time charging when at idle, especially when the car has a high electrical load on. This gets worse when the alternator is old and can be compounded by bad battery wires.

If you already replaced the alternator and battery, you might as well replace the cables too.
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Old 07-11-2013, 09:39 AM   #3
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I would take the alternator into an Autozone (or some place like that) and they will test it for free. Sounds to me like that is the problem. Never hurts to check/clean all the wires and connections.
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Old 07-11-2013, 04:49 PM   #4
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I don't think they check alternators at the speed of engine idle do they?
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Old 07-11-2013, 05:16 PM   #5
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Quote:
Originally Posted by Mike1995 View Post
I don't think they check alternators at the speed of engine idle do they?

I think they do, but all I know for sure is I hand it to the guy, he puts on the machine, it whirs, clicks and buzzes for a few minutes, and then he tells me whether or not I need a new alternator.
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Old 07-12-2013, 04:31 PM   #6
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Took the alternator to get tested and it is dead.

It may be just me but this is the fourth alt this car has gone through. Could something else be killing them?

Any crazy ideas would be helpful.

92 bonneville se 3.8
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Old 07-12-2013, 06:11 PM   #7
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where is the regulator, is it external or internal to the alternator....might be something like a parasitic draw draining the battery causing a big load on the alternator. this is a rough sketch , you want to disconnect both battery cables then connect a DVM set on amps , 20 is usually the highest, from the battery positive to the positive battery cable and then connect the negative battery cable to the battery. See if you get a reading,,, dont forget you are working with amps high enough to weld with and explosive hydrogen gas , wear eye protection and gloves.Ignition off, do not turn on the ignition,,,,of course if the doors are open the courtsy lights will show a draw.
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Old 07-12-2013, 06:21 PM   #8
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The alt is internally regulated, according to the online parts look up.

I understand how a parasitic draw would kill the battery, how would that kill the alt? Causing it to get over worked?

Thanks for all the help!!!
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Old 07-12-2013, 10:02 PM   #9
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if there was a parasitic draw it probably would run the battery down when parked. If you do the parasitic draw do it with the negative lead, less dangerous and don't open any doors you might overload the meter . start on the highest scale. could be the car computer since that commands the alt on and off.... will think some more on this.
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Old 07-12-2013, 11:09 PM   #10
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could be a high impedance. Maybe take the rpm up to 1500 rpm measure the voltage from the positive alternator lead to the case , then measure the battery voltage, they should both be the same. engine off disconnect the negative and measure the resistance to ground. I found my shock tower bolts a great ground in the engine compartment. How is the condition of the serpentine belt. You can also do a load test, start the car turn on everything, A/C , fan on high, radio, high beams and measure the battery voltage, should be about the same w/o the load 14volts.
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