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Battery light comes on during hard acceleration

2000 Ford Crown Victoria, 133K miles (new engine at 96K)

When I’m accelerating onto the highway or some other time when I need to lay into it, the battery light will briefly come on for perhaps a second at high RPMs; it goes out immediately upon shifting to the next gear. There is a voltage gauge in the cluster, and I do notice a very slight drop in the gauge when the light comes on…though it’s not getting anywhere near the red area. I should mention that, when the car upshifts to the next gear and the light goes out, at that moment I sometimes hear a brief chirp from the belt.

The battery connections are clean and tight; battery voltage tests normal when the engine is off (~12.5 volts), and the charging voltage tests normal at idle (~14-14.5 volts). The serpentine belt is in good shape and feels tight. Anyone have an idea what might be happening?

Sounds like your belt is slipping. Even with a good belt at the proper tension, it will slip if the pulley is glazed. You may have to rough up the pulley a little with some fine sandpaper. If this has been happening for a while, I would replace the belt too.

It sounds to me like the belt is slipping. How old is the belt? I’d install a new belt, regardless of how the old one looks.

Once they start slipping they become glazed and the slipping will only become more frequent.

A belt’s not too expensive, so I’ll probably start with that. I’ll also have a look at the alternator pulley while it’s off and see if the pulley might be a bit glazed.

One other thought; is it possible the belt tensioner is the problem here? I’m not really leaning that way since, in that case, I’d guess the belt would be slipping all the time.

Yes, a belt or belt tensioner can cause this kind of problem. With the engine off, you might try placing a socket on the alternator pulley nut and attempting to rotate the pulley by hand.
This should be very difficult to do. If the pulley rotates somewhat freely then the belt is the fault more than likely.

Another more obscure cause could be a worn brush or weak brush spring which is allowing the brush to hop off of the commutator on the alternator armature.

I am going to suggest something different.

You say the serpentine belt feels tight. V-belts can slip though they feel tight, if they are worn out, but when a serpentine belt is tight, it grips, regardless of age, unless it is really crusty and hard on the surface.

I suspect that you are loosing alternator field signal under acceleration due to a loose connection or a chaffed wire. That causes the alternator to ‘free wheel’ while you are accelerating, and then you hear the chirp when the alternator takes its load up suddenly at high RPM.

I think that your alternator has an integral voltage regulator, so if you don’t see any chaffed or loose wires around it, I suspect that the diagnostic procedure for this hard-to-reproduce-in-the-shop problem will consist of trying a different alternator.

Along the same lines is the possibility that the brushes are worn out on the alternator.

Add my vote for worn, loose brushes. It’s at the right mileage for that.

Having read all of the suggestions, I’ll say that I tried ok4450’s idea of trying to rotate the alternator by hand with the belt in place. It turned very easily with little force, so I’m thinking the belt might not be getting the job done. The tensioner is pretty tight (rotating it out of the way took quite a bit of force) so I don’t think that’s the issue.

I picked up a new serpentine belt (a Gatorback, the most highly recommended by the guys at and installed it. Unfortunately, the light still comes on under hard acceleration. The chirping at upshifts is gone, however.

From what I’ve read, the brushes on these things are not separately replaceable, and I can’t find anyplace that has separate brushes. That said, everything works just fine as long as I don’t get the engine RPM up too high, so for the time being I’ll just avoid hard acceleration. Even when the light comes on, nothing is affected. Lights don’t dim, the engine doesn’t slow, the voltage gauge stays within the normal operating range, etc.

Alternator replacement is probably the only real fix for this, and I’m not in the market to spend $220+ on a new one right now.