'99 Ford Expedition Battery light when over 4000 RPM

I just noticed last night that when i go over 4000 the battery light comes on on the dashboard. I just changed the altenator and that did not fix the problem. I am now thinking it may be the battery since it is over 4 years
old. Does anyone know what this can possibly be?
Thanks Kenny
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Maybe your alternator drive belt is slipping at high revs. Tighten it up a bit and give it another try or replace the belt altogether. Alsi, I'd put a voltmeter on the battery with the car running (at idle) and make sure the alternator is charging the battery adequately. It should show around 14 volts or so. Good luck. jor

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Consider the following also; If the belt is slipping, then the belt tensioner is bad..serpentine belts tend not to slip that easily. Even with a slipping belt(unless it is severe) the battery light ought not to come on at 4000 rpm. As you suggest, check the alternator throughly.

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I have replaced the alternator with a brand new one and i also sprayed the belt with some belt cleaner which should also make it stick better. This did not fix the situation. Would this mean the belt tensioner is most likely bad?
THanks

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The belt's not the problem. Think about it. If it turns fast enough at idle to turn the light out, then anything faster at all won't make it come on. OH, BTW, don't spray ANYTHING ('cept water for noise diagnosis) on a belt. It ruins them.
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ok.
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Steve (here to have fun) Barker

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No, no, V belts do slip and devol glazing when the tension is low. A serpentine belt with a tensioner has very little slippage.

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Ok............. I didn't know so much about the sepertine belt, but at the V belt in most Chevy's we are agree. I thought maybe the same ting happend with Ford's sepertinebelt. I have had and seen this problem lots of times in Chevy's. When you rew. up the voltmeter goes down. Sorry about not been to clear about it whas the Chevy's V belt I talked about, and wondered about if the same happened on Ford's Sepertinebelt. Tor E.....

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wrote:

Have your rectifier diodes checked.

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