Not charging - but battery light is off

[cosworth]

Key on, the dash lights up normal, the battery light works. Key on, engine on, the battery light goes out, but it’s not charging.

I only noticed on the drive home today once the radio and dash backlight started crapping out. Turned off all electrical and limped home. The charging gauge on the dash was dangerously close to L and I just made it home.

12.8v and slowly decreasing once I charged it up for two hours and ran it.

Am I replacing the regulator, the entire alternator or is this a 20a fuse issue? I can’t quite get my head around the battery light not coming on. That the point of it right? To tell you it’s running off battery? Lol.

 

[shucker1]

First thing I wold do would be to put a meter on the output terminal of the alternator (Engine Off) to see if it matches battery voltage.

It should. You could have a burned out connection between the alternator and battery.

Next step would be to check all battery connections (Positive \ Negative) to make sure they are clean and tight.

 

[410Fortune]

The battery light lets you know that the voltage regulator inside the alternator is powered up, if it goes off then power is being sent to the voltage regulator, that is all. It does not mean the alternator itself is charging
I would check alternator fuse (it is probably fine since the batt light comes on with the key then goes off) then I would check the alternator to battery wiring, and finally check to see that you have 14.0-14.4 volts at the battery during idle. My guess is your alternator has died and is not charging the battery for whatever reason, but the battery light in the dash is happy because the voltage regulator is getting powered up...at least until the battery gets below 12.0V

Let us know what you find! Likely time for a new alt or something went fishy with alternator charging wire

 

[koda2000]

In layman's term's the voltage regulator kind of floats with battery voltage on one side and alternator voltage on the other. When the alternator isn't producing the necessary voltage to charge the battery the BATT/CHG light turns on and you're running off the battery only.

As your BATT/CHG light functions and goes off when the engine is started the first check I would make is at the alternator's "B" terminal (the large terminal on the rear of the alternator). With the engine running you should be able to test the voltage at the "B" terminal and should see approx 14.5 volts. If this is the case the alternator is working fine and you should check wiring and connections from the "B" terminal to where ever it goes (starter or the rear of the power distribution box. My guess is you have a bad connection, corrosion or a blown fuse, which is causing the charge not to make it to the battery.

Tip: When testing voltage always test from the connector and from the actual post the connector is attached to. Corrosion can exists between the connector and the post and prevent current flow (been there done that).