2.5L ('98-'01) battery light on.... alternator?

[Mightyfordranger]

battery light comes on and off on my 99 2.5 but i checked the output and its putting out 14.2 volts anyone know what the number is that it trips the light?

 

[RonD]

It doesn't have a set voltage

It works like this
For a 12volt light bulb to light up it needs 12v and a ground(0volt)
If you hook it up with 12v and 12v the bulb stays off
Or 0v and 0v no light

The battery light bulb/circuit gets 12v from dash with key on, and its "ground" wire runs to the voltage regulator in alternator, the Light Green wire on the 2 or 3 wire connector
When key is on and engine is off the voltage regulator is 0volt, so battery light is ON

When you start the engine, and alternator is working, the bulb gets 14volts from dash, and 14volts from voltage regulator, so bulb is OFF

If the alternator/voltage regulator voltage drops below "system" voltage then battery light will start to glow, the bigger the difference the brighter it glows

The system voltage can't drop below battery voltage, say 12.5volts, so for battery light to come on the voltage regulator(green wire) would have to drop below about 8volts for you to see battery light glow

If battery light comes on when RPMs are low then one of the three fields in the alternator is bad, or if head lights dim at idle
If battery light comes on at any RPM just randomly it would be hard to say if its alternator or voltage regulator issue


But in any case it reads like an alternator issue, could just be voltage regulator and you can swap just the regulator, but it depends on the age of the alternator as well because their bearings don't last forever, so if its high miles you may want to swap whole unit out

 

[Mightyfordranger]

ok im following you a bit. i had a spare one in my parts pile it looked much newer than the one i pulled. fired it up and it still came on.
so you say its the green wire off the alt right? that would be the ground to that battery light? wonder if its got a fray in it somewhere shorting to ground and causing the light?
Edit; the output voltage on the (new) alt i installed is 14.6v
More Edit: so if i check the output on the batt it would drop to 8volts to light the bulb right? or are there two seperate circuits within the alt?

 

[Mightyfordranger]

Also thanks ron your the best.

 

[RonD]

If you have a volt meter or test light

Unplug the 2 or 3 wire connector and with key off test green wire to battery positive, should be no volts, unless that green wire has a short to ground somewhere

Turn on key, test green wire to alternators case(ground), should be battery voltage, say 12.5v

Also test Yellow wire next to the green the same way, should also see battery voltage, but key doesn't need to be on, can be but the yellow wire is a live 12v wire 24/7

Check the white wire, if you have one, its a short jumper wire, make sure it is not frayed at a connector

The "alternator" is a separate part from voltage regulator, i.e. they were separate parts from 1984 and back, 1G, using external voltage regulators that use mechanical relays, the advent of "reliable" integrated circuits made internal voltage regulators possible

But they are connected by wires internally or externally
I can't find a diagram for a solid state regulator

And I am not sure why your battery light is doing that on two different alternator

Not needed but you can read it if you want

The way it works is............
An alternator can not generate electricity just by spinning it
It has to have "startup" voltage, that's what the green wire provides, as seen with the above test, green wire has 12v with key on, traveling thru the battery light bulb down the green wire to the voltage regulator

The voltage regulator uses that 12volts to power the brushes on the armature(part that spins)
The armature is a "ground" at that point because its not spinning, so battery light comes on
The 12v causes a magnetic field in the armature coils
When engine is started these magnetic fields spin passed the 3 field coils in the case of the alternator, this generates AC voltage in the field coils
When the alternator starts to generate it own voltage the voltage regulator switches over to internal power, so green wire is not needed, but is now a monitor circuit, i.e. battery light, so has same voltage as alternator is producing

The voltage regulator only sends 9volts to armature to get out 14volts, or 8v to get out 13volts thats how it regulates the AMP/voltage output
The yellow wire is how the voltage regulator "knows" how much voltage it should send to armature
Electricity has whats called an electromotive force, we'll call it Pushback
After engine is started battery was drain so needs a recharge, low Pushback, so voltage regulator turns up AMPs/voltage to 14.5v by sending 9.5v to armature
As the battery gets recharged its Pushback goes up, so voltage regulator reduces voltage at armature to 8.5v so alternator output is now 13.5v

When you turn on Blower to High or head lights on Pushback drops so voltage regulator sends more volts to armature to compensate, so voltage may remain the same, 13.5v, but AMP output has increased because blower and lights are on

When you raise RPMs pushback stays the same but alternator output goes up, so voltage regulator lowers volts going to armature to keep volts and AMPs the same

If your voltage at battery stays above 14volts after 10 minutes or so of running that means battery is close to end of life, it can't be charged fully so low pushback all the time
It could also be a bad voltage regulator, they can "stick" at one voltage to armature, so voltage will go up and down with RPMs, so easy to test for that

And on top of everything else, they now use GROUND to vary armature voltage, but its much easier to understand using 12volts, and its virtually the same

 

[Mightyfordranger]

I got it now. Thanks Ron. Also I'm not sure if can change my post title. But I'd like to make it something more relevant for a google search. So anyone in need can find this info.