Question on alternator charging voltage.

lethal6

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Left my lights on today @ work. 9 hours later, I go to leave and it is dead as a door nail. I jumped it and let it run while I locked up the shop and then drove home (12-14 minute drive home).

I noticed on the drive home my volt gauge was sitting at a hair above 12v while at a stop light and then it would go up to 14v when I was accelerating but then would drop back to around 12 when cruising at 35. It bounced back and forth the whole way home. It normal sits at 14v the whole time.

I also noticed that there was terrible alternator whine through the speakers (normally only does it when the lights are on). I have a bad ground loop that I haven't completely located yet but it hasn't done it during the day until today.

When I got home I left it idling and put the multimeter to it [battery] and it read 13.3v, pretty steadily.

It's supposed to be 14.4v @ idle isn't it?
 

lethal6

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Forgot to add the info. Will put it here even though it is in the link under the avatar.

1993 LX 4 door. D15B7. Current alternator says mitsubishi so it is a OEM equivalent replacement right? Stock was Denso was it not?
 


lethal6

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It's on the charger now. Will test again in the morning and report back.

Thanks Ron!
 


lethal6

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Update:

Battery is back to full charge and the alternator is now putting out 14.5 volts steady at idle.

I never knew that the alternator charging voltage drops when the battery charge is low. I would have figured that it would have been up higher because it is trying to charge the battery back up to spec. It is true that you learn something new everyday.
 

RonJ

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Think of it this way. If the alternator is outputting max voltage but the battery is sucking up the voltage for charging, the system voltage will drop lower than normal until the battery is fully charged.
 

lethal6

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Makes sense thinking about it that way. I always figured when the battery was low the system increased voltage until it was fully charged and that is why a bad battery can kill an alternator because of it overworking. Thinking about it, it can kill it just as bad this way because it overworks with the voltage drain.
 

Aldundra

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Welcome to the wonderful world of Honds's ELD (electronic load detector)

If your driving your car and the computer doesn't sense a demanding electrical load when you're slowing (coming up to a stop light for instance) it cuts the alternator off to drop the revs and (supposedly) save fuel. It reengages the alternator at 40mph+, that's why is kept dropping when you were crusizing at 30-35mph

One way to stop it from happening is to run with your headlights constantly on (which is annoying). i do know that there is a write-up somewhere to bypass it, google it and you'll find it.
 


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