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Old 04-03-2015
Thread Starter
Join Date: Apr 2015
Location: Jefferson, WI
Posts: 12
Battery Light

Question, Yesterday when driving home I had the battery light come on. This happened as i was passing someone on the interstate (high RPM/3rd Gear). After I noticed I checked and all the voltages seamed normal (13.4V-14.1V) while driving.

Got home, vehicle still starts good, all voltages seem fine but the light is still on, Ignored it and went to work, Still have the light.

From what i read the light is for alternator issues. however as i have said all voltages look correct while driving.

Checked the alternator fuse and it is good.

What else can i check, as i dont like warning lights when i drive 500 miles a week

95 Ranger XLT
2.3L 5 Speed
259,000+ miles
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Old 04-03-2015
Join Date: Apr 2011
Location: Ontario , Canada
Posts: 48
get the alternator tested , the internal regulator could be on its way out
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Old 04-03-2015
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Join Date: Jan 2014
Location: Vancouver, BC
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First, how does a light bulb work, I know you know but do you really "know", lol.

So lets say we have a 12volt light bulb
12v battery + ---------Bulb------------12volt battery -

Bulb will be lit up since it has 12v on one terminal and 0v(-, ground) on the other.

What if we did this

Would the bulb light up?
it has 12volts

No, it won't because for a bulb to light up voltage needs to pass thru it, 12v on one terminal and 12v on the other means no voltage flows thru the bulb.

Now the Battery Light part.
When you turn on the key the battery light bulb gets 12v(battery voltage), the other terminal is connected to the alternator, which is 0volts with engine off, so voltage flows thru the bulb and light is on.
When engine starts alternator produces 13.5volts(approx.), so now the one terminal connected to the key has 13.5v and the other terminal connected to the alternator has 13.5v. so..........light bulb is off.

This circuit is also the alternators ON/OFF switch, an alternator is basically an electric motor used in reverse, so if left ON when engine is off it would drain the battery.

Ignition switch-------battery light----------alternator

The alternator connection is part of the voltage regulator circuit, so as said above by mazdaman96, the voltage regulator is probably out or close to going out.
To test voltage regulator make sure all lights and heater fan are off.
Test battery voltage, needs to be above 12.2volts
Start engine
Test voltage at battery, should be above 14v for a few minutes to recharge battery from starting engine.
The it should drop to 13.5volts, 1.1 volts above battery's voltage, so if battery was 12.5v then you would see 13.6v, that's the maintenance charge, it keeps battery charged but won't "boil off" the chemicals inside the battery.
With stable 13.5 volts turn on the lights, voltage should drop then come back up to 13.5v
Turn on fan to high, voltage should drop then come back up to 13.5v
If voltage drops and doesn't come back up then voltage regulator is going bad or 1 of the alternators fields are bad.

And on a longshot, check the wire from the alternator to the Battery Light it could be shorted to ground.
Unplug the 3 wire connector on the alternator, turn on the key, if battery light is on then you have a shorted wire.
Light green/red stripe wire on that 3 wire connector, runs to Battery Light.
Diagram here:

Last edited by RonD; 04-03-2015 at 11:47 PM.
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Old 04-04-2015
Thread Starter
Join Date: Apr 2015
Location: Jefferson, WI
Posts: 12
Thanks for that information, ill check this during lunch, this is probably the most detailed explanation i have ever got, THANKS!!!
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Old 04-04-2015
cheese_man's Avatar
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Join Date: Nov 2009
Location: arthur
Posts: 2,100
the alternator is no longer generating any amperage output

had the same problem on my old 86 toyota pickup alternator

it would generate voltage without issue , but amperage generation was NIL

hence the battery light is illuminated
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