CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Random Focus battery light

Need help from the automotive hive mind.
I have a a 2005 Focus, 199,000 miles.
The battery light has been come on randomly. It doesn’t seen to be related to any other event in the car.
Things I’ve done to diagnose it.
I cleaned the battery cable connections before I started any diagnostics
I have two code reader and no codes are being thrown.
I connected a multimeter to the battery while the car was idling in the driveway. I got 13.8-14.1 volts as the car sat and ran. When the light came on no change on the meter.
I have a Scan Gauge that allows me to watch the voltage in real time. Four days and no change when the light came on. ~13.8-14.1 at idle and ~14.1-14.4 underway.
I have the Car Chip. I plugged it in drove for about 10 runs and the voltage trace was pretty flat on all runs, battery light on or off.
The only thing that is consistent is if the light is on and I rev the engine it goes out BUT they rpm when the light goes is inconsistent.
So before I take it to a shop that wants to sell me a battery AND and alternator I hoping others have seen this and can point me to a fix that won’t break teh bank.

Thanks

I troubleshot a battery light that was coming on for a 2000 focus. It was random at first, then eventually stayed on. I found a blue wire coming off the alternator, broken about 18 inches from the alternator where the alternator wire bundle went into a main, larger wire bundle. I had to tear open the entire wire bundles, unwrap the tape, open the looms and expose the wires. It wasn’t too big a job, just time consuming and when i tugged on the blue wire, it came off into my hand, a pretty lucky find for how deep it was buried. Something to try. The time consuming part is getting it all back together and making it look like the bundles were not disturbed.

How is the alternator drive belt? If it slips then the battery light will come on briefly. I would have it checked. Have the belt tensioner checked as well.

Benny. Thank’s I will check that harness to see if it cuasses the light to flucuate

Missleman,
I should have added to my post the like is random but it does not come on momentarily. It comes on for minutes at a time with no change to the voltage. The belt and tensioner are less than a year old plus there’s no belt squeal. If the belt drive system were hanging the belt would squeal.

I’m going with what benny described. There is a dedicated signal wire for the battery light. I’d start by just pulling the alternator connections, cleaning them up and reinstalling. I had something similar once on my Escort, and it was just a loose connection of that signal wire at the alternator.

Cigroller,
Yeah that makes a lot of sense. It sure acts like a connection issue.