Question about 1999 Cadillac Seville

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Overcharging Alternator I have a 1999 Cadillac STS with an alternator I replaced. The alternator is a 140 amp and after being replaced, started the car right up. But now instead of my car NOT charging the battery, its charged all the way up to as high as 16.4 volts when idling. If I turn all the accessories on it still hovers around high 15v and low 16v causing my stovk stereo to clip and windows not to work. If the engine is off with the key in the ignigtion, the car will read at 12v. (keep in mind I tested the voltage with a meter as well as what the car said, and my battery is the same one that never drained past 11.8v with the previous alternator)

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Very short lesson on the charging system. Remember this, The higher the voltage the less the turn low voltage, high amps. When the Alternator is charging at or above 15v, It is virtually pure voltage and as a charging device is cooking your battery, not charging it. The alternator, though defective, is safe and your battery is in harms way. When an alt. is charging at or below13 volts, The charging amperage is approaching max output of the alternater, which, if maintained at that rate will overheat and cause alt, failure, The proper charging range is 13.6 to 14.2. Each end is the limit just short of maximum and eminent failure. Example: charging at 12v means the alternarer is at max output and is in danger of sling solder, litteraly. When you smell dulphur coming from the battery, The alt. is running wide open on the voltage and your batt ison death row. So, back to your problem. One more thing. All accessories that are designed to operate on 12v are art risk of being damaged from excess voltage. Some have a voltage limiter to protect against surges but not all. Oh, That is one heck of a hot alt at 140 amps. Is that original equipment? Well, I know I may have rambled a bit but I hope it helped you understand how it works so you can come to the conclusion I have. The Alternator has a bad regulator and is in whole or part defective. Good Luck, Ned

Posted on Nov 12, 2009


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