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Bad alternator possibly?

Hello all! A little background information first. So last weekend my dad bought a used '98 Crown Vic off an older lady (original owner) who had only put 66,000 miles on it so it wasn’t driven a whole lot.

A day after he noticed the battery light came on for about 1-2 minutes and then turned off and stayed off and has continued to do so intermittently. That day we took it to an autozone and the battery&alterator checked out fine with the tool they use… for good measure we took it to another auto zone when it continued to trip the light and everything check out OK.

As far as I’m aware the alternator is the original so could it be possible that that could be the problem? she had installed a new battery so I’m leaning on thinking that isn’t the problem and thinking it could be the alternator seeing as it is about 19 years old.

Any ideas what could be tripping the Battery light? Can the machines these auto shops use get a false reading on a potentially bad alternator ? One more piece of information is that the light seems to stay off when the car is warmed up.

We haven’t had any time to take it to a professional mechanic because of a death in the family but I plan on to taking it in if it continues this

The car also starts right up and doesn’t show any symptoms of a weak battery or alternator problem.

I suppose you could stop and test the battery and alternator when this problem occurs. I had a similar issue some time ago and I purchased a gadget like the one shown here. It simply plugs into your cigarette lighter and allows you to monitor the battery charge live and display the output of the alternator when the engine runs. With the engine off you have the battery voltage displayed. You can get it at amazon, autozone or advance auto parts.


Yup, I agree with @kurtwm2010 s idea. If the alternator has a intermittent problem like sticking brushes, this will show you when that occurs - the voltage will drop from the 14.5 or so that the alternator supplies to the 12.5 or so the battery supplies and the light should match.

It may also be a slipping alternator belt that occurs at the high load point when the car is recharging the battery after a start. A new belt usually fixes this. If the pulleys are corroded, they may need a little cleaning with the new belt.

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I don’t think so. I assume they test at a high enough rpm to simulate a car in slow speed or idle.

Keep an eye out for the light and document under what conditions it comes on…idle, deceleration, high speed, during braking…etc. It might give some hints to what is happening.

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3 weeks ago, my sons 2002 Ford Taurus flashed the battery light when he was coming home from school. I checked it when I got home, and all checked fine. Everything was tight, clean, and had good amps and volts. Test drove it and no more batt light. I told him to start saving for an alternator.

2 weeks later, the batt light came back on, and the alternator had gone kaput. As most teenagers do, he hadn’t saved anything for a new alternator, so we ran to the local pull a part and got one off a junked car for $20. It’s a temp fix, but honestly, it’s working rather well so far. Again, though, I advised him to save up for when this one quits.

and, I made him do all the labor. :wink:

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From what I know it’ll light up when it’s a cold start… then after the engine warms it’ll stay off. I’ll have to buy that little gadget when I have time to keep an eye on what readings I get

I wager to say that it is your alternator, indeed. Probably on its way out with worn brushes. Running the engine and heating up everything under the hood, in addition to the internal heat the alternator produces, makes parts expand. As the alternator warms up and parts expand the brushes will make better and consistent contact.

The only other part I can think of, internal to the alternator, that might give you a problem is the voltage regulator.

If you like to tinker around, brushes are relatively easy to replace and dirt cheap.

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I’;ll chime in agreeing the kurt above, probably the alternator needs to be replaced. Monitoring the voltage at the battery as you drive around makes sense. It should be about 12.5 volts before the first start of the day, go up to 13.5-15.5 volts after starting the engine, then go back down to around 13-13.5 volts after driving it 10 or 15 minutes, with the engine still on. If it ever drops below 12 volts with the engine on, either the alternator is kaput or the belt is slipping.

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ill have my dad get the little gadget over the weekend and have him monitor it. Most likely he will just take it in and have it checked out & have the alternator changed out

Useless comments maybe but I used to overhaul my alternators about every 80,000 just as a preventive measure. Usually just brushes, the voltage regulator, diode trio, and rear bearing. Sometimes the front bearing too. Until they started crimping everything so you couldn’t get them apart. At any rate, on any of the alternators I had fail, I hardly ever had brushes so worn out and usually it was just the voltage regulator. Usually I’d get a flickering red light at low RPM or sometimes full on red light. The store testers should be able to see this though. However I did have one that would flicker the headlights and the store said it was fine, which it wasn’t. So I wouldn’t completely trust the testers for alternators that may be on the margin.