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Alternator bad?

MINERALGSVT

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'99 Saleen XP8
Ok, so my wife's '99 Eddie 5.0 has 218k and original alternator (we are original owners), battery is 6 months old.

Last night, she calls and says - ABS light just came on, radio and heater just turned off, headlights are dimming. It was also starting to run rough. She was 6 or 7 miles away and managed to make it home. I put the volt meter on it after she shut it off - 11.3 volts or so.

So, I put my trickle charger on it and went to bed. This morning, the battery voltage is 12.4 or so, and the truck fired right up with no issues. I checked on the terminals with truck running - 13.2 or so volts. Also, there is some noise ( not squealing) coming from the alternator area.

About 6 months ago, we replaced the battery with a nice Motorcraft part. At that time, the alternator was putting out 14.2 or so.

So, my guess is alternator.

Any other things I should look at? It's been a great truck with really no issues out of the ordinary.

Thanks
 
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brezzy

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Start it up and remove the negative wire on the battery if it shuts off its the alternator
 
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SoNic67

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Output at 13.2V means that you have a diode shorted inside the alternator. As is, it outputs DC with an AC component.
Using it like that will damage the battery. Also it can mess up the sensors (like Camshaft Position Sensor).
 
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imp

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Output at 13.2V means that you have a diode shorted inside the alternator. As is, it outputs DC with an AC component.
Using it like that will damage the battery. Also it can mess up the sensors (like Camshaft Position Sensor).

Good point! Wonder though, do diodes often short out, rather than open? imp
 
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MINERALGSVT

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Swapped the alternator and voila - charge is now 14.4, 14.2 with all accessories on, and no noise from the new alternator.
 
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SoNic67

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Good point! Wonder though, do diodes often short out, rather than open? imp
In my experience they always short out. But then, because of the excessive current flowing trough them and their solder points, some of those solder points melt and, with luck, the shorted diode gets disconnected.

At that point the alternator it would still output some 14V on a charged battery (when demand of current is not high), but driving with lights on will result in discharging the battery to some 13V...

Swapped the alternator and voila - charge is now 14.4, 14.2 with all accessories on, and no noise from the new alternator.
:thumbsup:
 
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