I have a Mustang that the battery light stays on.
I changed the battery and the alternator was checked by Autozone, they said it was good.
I need help
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First off, you may want a 2nd opinon on the alternator. If both battery and alternator are good, then its either a wiring or ECU problem. Check the contacts on the battery terminals/connectors to make sure they're not loose, dirty, or corroded. Then follow the ground wire (the black one) back to the chassis of the car to make sure it has a solid ground..
The battery light works by connecting the battery's plus side (through the ignition switch!) to the alternator's positive output. It's probably the oddest piece of circuitry in a car, but that's why it glows when either isn't working. And why it has to be an incandescant globe (but you could use two LEDs to indicate which has failed).
It sounds like a wiring problem, but not necessarily with the light itself. I'd check the engine, alternator and battery are properly grounded (though that should create ignition problems) and that the charging cable is whole. If the alternator is failing to charge the battery, you will probably notice the engine note drop sharply when you turn on something high-current, such as the headlights.
In most cases, the red battery light in the instrument panel is fed a voltage sense through a yellow wire that is part of the alternator connector. Seems a lot of aftermarket and remanufactured alternators fail to provide a charge sense signal (i.e., alternator output) and the result is the instrument cluster illuminating the warning. The regulator in the alt is responsible for this and a factory replacement is the only sure fix.
I have a 2007 Ford Mustang. The red battery symbol appeared along with the check charging system and soon after my battery was completely drained. I charged the battery and then had the alternator tested. I replaced the alternator and the car has dtiven like new ever since.
I'm surprised nobody has mentioned checking the fuses in an answer or a comment.
I had the charging system warning light on, on an old Opel Vectra. Battery? Just fine. Alternator? Just fine. It was the fuse! And the fuse apparently burned because it couldn't withstand the high load I put on the electrical system of the car by connecting a Peltier element based cooler to the cigarette lighter socket. I'm somewhat surprised this caused a fuse that affected the charging system to burn instead of causing a fuse of the cigarette lighter socket to burn, but that's what happened.
So, if you know it's not the alternator and not the battery, I wouldn't start from wiring or ECU. I would start from the fuses!