Battery Light on but Alternator is good

[snatale42]

Hey guys, just did plugs and wires on my 02 ranger 3.0 flex and got in to start up and my battery light is on. At first assumed my alt had awesome timing, but checked voltage at the battery and it's 14.2 while running. Battery itself is good as well. Any ideas what else could cause the light? Checked fuses and nothing is blown. Only thing I could google up was fusible links at the starter, but the truck starts and my running voltage is good any ideas? Timing seems convenient but I can't find anything unplugged or loose.

 

[Rearanger]

The battery light operates from a voltage differential. If voltage is the same on both sides (alt side/batt side) the light goes out.

I would start by cleaning your battery connections (at battery & chassis ground) and cleaning the alternator connections to make sure that high resistance is not causing a voltage differential.

You can also double check the alt output by taking a voltage test from the alt "B" connection to chassis ground.

 

[RonD]

You have a volt meter so with Key Off:

Test battery voltage, 12.3volts or higher is what you need

Test for 12volts(battery voltage) at B+, larger terminal on the back of the alternator, Black/orange strip wire.

Unplug 3 wire connector on alternator
Test Yellow/white stripe wire on that connector, should have 12volts

If either of these(B+ or Yellow) doesn't have 12volts then yes a fusible link is bad, there are 3 of them used.

If above tests are good then turn on the key
Test Light Green/red stripe wire on 3 wire connector, it should have 12volts(with Key On), this is the Battery Light Wire.

If it tests good then check the White/black stripe "jumper" wire on 3 wire connector, unplug it from alternator and make sure wire is not frayed or damaged.

 

[snatale42]

The battery light operates from a voltage differential. If voltage is the same on both sides (alt side/batt side) the light goes out.

I would start by cleaning your battery connections (at battery & chassis ground) and cleaning the alternator connections to make sure that high resistance is not causing a voltage differential.

You can also double check the alt output by taking a voltage test from the alt "B" connection to chassis ground.

You have a volt meter so with Key Off:

Test battery voltage, 12.3volts or higher is what you need

Test for 12volts(battery voltage) at B+, larger terminal on the back of the alternator, Black/orange strip wire.

Unplug 3 wire connector on alternator
Test Yellow/white stripe wire on that connector, should have 12volts

If either of these(B+ or Yellow) doesn't have 12volts then yes a fusible link is bad, there are 3 of them used.

If above tests are good then turn on the key
Test Light Green/red stripe wire on 3 wire connector, it should have 12volts(with Key On), this is the Battery Light Wire.

If it tests good then check the White/black stripe "jumper" wire on 3 wire connector, unplug it from alternator and make sure wire is not frayed or damaged.

Thanks guys, as of now here's what I got

Voltage at Battery 11.9v
Voltage at Alt B+ 11.9v
Voltage at Yel/Wht 11.9v
Voltage at Green/Red 11.7v I'm assuming this is my light cause?

 

[RonD]

Check volt meter on another battery if possible.
Hopefully it is off

11.9v means battery is bad.

A Car battery has 6 separate cells inside, each will have 2.15volts when new
6 x 2.15 = 12.9volts

So new battery will show 12.8 to 13volts

at 5 years old it will be down to 12.3 to 12.4volts, cells lose volt and amp storage as they get older

At 12.2 volts and under battery will still start engine but is fading fast, if engine doesn't start first time cranking you probably won't get a second chance, and on a cold morning no chance at all.

No, the 11.7v on green wire is OK if battery voltage is 11.9v, there will be some loss on that circuit.

With key on, Ground the green wire, battery light should come on.
Then put a jumper from yellow wire to green wire, Battery light should go off.

If it does then voltage regulator in alternator is the most likely problem.


Battery light works this way:
A light bulb lights up if it has 12volts on one contact and Ground on the other contact.
If you put 12volts on both contacts the bulb will not light up.

When you turn on the key the Battery light gets 12volts on one contact, dash power.
The other contact is the green wire at the alternator.
Alternator is a Ground when it is not spinning, so battery light has 12volts and a Ground, it lights up.

When alternator starts spinning it is now 12volts(14volts) not a Ground, so battery light goes off.

There is a little more to it in that, the green wire supplies the initial voltage the alternator needs to start generating its own voltage, that's part of the voltage regulator circuit, which is most likely your problem.