Electrical Battery light comes on and goes off

mstang95

Member
May 6, 2018
22
1
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26
Pennsylvania
I've had this happen a couple times now. Not often, but it happened yesterday. Battery light came on as I was driving. I stopped, checked connections at battery posts, removed and re-connected terminals, tightened them up - nothing fixed it. BUT - multimeter showed 14.1 - 14.2 volts across the posts. So it seemed like nothing was wrong. And indeed, the car ran fine. It shut off and started again several times, no problem. It drove all the way to the auto parts store (to replace the negative terminal I broke while tightening), and then all the way home - no problems.

This morning, as I expected, the light was out when I started and didn't come back on when I drove to work. Battery tester on battery before start showed proper voltage. Load test showed operating within appropriate parameters. The only thing I did before the light came on was accelerate hard, shifting right before redline.

Last time this happened, I went under the hood and tightened the positive terminal's wire clasp - so where it grabs the positive cable, not where it grabs the battery post. Then it went out. I thought I had finally fixed it, but I guess not.

Any ideas what I should test? I mean, it seems to run fine, so I guess it's not exactly urgent - but I don't like warning lights in my car. It just looks bad, if nothing else.
 
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schelini

Member
Jul 14, 2018
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New York
I have this same issue and my car is running fine too. My battery appears to be good. The light goes off after revving the engine so it may be a bad alternator. I would test the alternator next.
 

jozsefsz

Mustang Master
Aug 11, 2013
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Cleveland OH Area
Ford alternators wear out at almost exactly 150k miles routinely. Usually they die with some sporadic warning-lights, and then it'll eventually completely die. If you're in that neighborhood mileage-wise, your alternator brushes are probably on their last legs. Time for a replacement.
 

mstang95

Member
May 6, 2018
22
1
13
26
Pennsylvania
Ford alternators wear out at almost exactly 150k miles routinely. Usually they die with some sporadic warning-lights, and then it'll eventually completely die. If you're in that neighborhood mileage-wise, your alternator brushes are probably on their last legs. Time for a replacement.
220k, actually. Is there a way to test the alternator first? To make sure that's really the problem, so I don't overlook something else, and just in case this is a replaced alternator already (I'm not original owner).
 

jozsefsz

Mustang Master
Aug 11, 2013
1,155
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Cleveland OH Area
You could take it to Autozone or similar and they'll test it for you. 50/50 whether that'll show the problem, may test just fine until it fully fails. You could take out the brush-pack and see how much life is actually left in it if you wanted to verify it's not a next-to-new alternator. At your mileage I'd change it, they're not that expensive.
 
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HISSIN50

"How long does it take to get help in here?
SN Certified Technician
Nov 29, 1999
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Jozsefsz said exactly what I was going to. To check the brushes on a 3G takes only a minute, just pull them and note how short they are (compared to the 'holes' in each brush). If they're worn down near the holes, swappie time.
 

96pushrod

Mustang Master
May 15, 2018
880
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Savannah
I had a similar issue on my car, but the voltage started dropping down to 13.7. Changed the alternator and now it’s all good.