Question about 1998 Ford Expedition

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98 expedition battery light comes on with increase of rpm

Battery light comes on @1700 + rpm then goes out at idle, no lost of voltage on dash gauge

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There may be no loss of voltage, but is there a surge of voltage?

As for the battery itself when tested, it should read 12-12.5Volts across the
leads when the car is off. When the car is running, the battery should be reading 13-14Volts across the leads, anything lower means the alternator is not charging your battery, anything higher means the alternator is over charging and damaging your battery.

Another cause for the battery light could be your ECU has detected an error

Posted on Aug 20, 2009

Testimonial: "thanks, haven't seen a alternator discharge light go on and off, normally it's either worked or didn't"

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Here I'll try to give you a clear answer. A while ago, my friend's 98 Expedition had a very similiar problem. Battery light came on at 2200 rpms and went away at idle.

The voltage never fluctuated however the battery light kept coming on. Took a voltmeter and it read 14.2v at idle and 12.1v with the car turned off (both ideal readings). And here is where most people make a mistake, they don't check the amp readings. Took an ammeter and the alternator produced MAX output just at idle with no accessories on. Did some testing on the battery and it turned out to be a short in the battery (and this was a new battery, only 3 months old). Basically the alternator was being overworked caused by the faulty battery. As the engine rpms increased the alternator could not keep up with the demand and started to draw the battery leading to a battery light. So just test and replace your battery.

If you are experiencing a voltage surge (14.5v or higher), there is a voltage regulator in the alternator thats faulty.

Hope this helps :)

Posted on May 01, 2010


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This sounds more like a faulty alternator. Sometimes an alternator can still produce adequate voltage and amperage to keep everything running, but they are governed by a voltage regulator. If the regulator goes bad, the alternator may have little of no output, or it will not limit itself properly causing gauge problems, and various light on the dash to not work or sporadically come on.
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Don't settle for one of the part store tests, they won't give you all the information you will need. A properly working alternator should charge anywhere between 13-14.6 volts and depending on the vehicle 90-150 amps.
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