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Today the battery light decided to rear its head on my Celica. I know this normally points to the alternator, but given that I just replaced a bad alternator in my Chrystler, I can tell what a bad one sounds like when running. As a side note, it's still the original.

The battery was replaced over the winter, so I doubt there's anything wrong there. One thing I can definitely note is that the spark plug wires are loose on the distributor cap and tend to pop off sometimes. Wondering if that was related.

The car died after driving it a few miles today, but had no battery light on the way home after charging it up and made the full trip. I didn't have any problems starting it either. One last note, the car hadn't been driven in about a month or two until today due to a bad radiator (new radiator was put in today).

Other details... Year is 91 Toyota Celica ST 1.6L engine approx 70,000 miles

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    This would be a really really good time to buy a cheap multimeter and learn how to use it to track down automotive charging / power issues... also a good time to make a cigar/cigarette lighter adapter for it so you can monitor voltages while you drive. – TDHofstetter Aug 8 '14 at 3:14
  • I actually do have one, but I wouldn't know how to use it to check the voltage from the alternator. – MGZero Aug 8 '14 at 12:11
  • So just a followup, but the car has been fine since and hasn't had any problems. – MGZero Nov 6 '14 at 22:08
  • Cool! Your most likely culprit, then, was a slipping serpentine belt. Spilled motor oil during a top-off? – TDHofstetter Sep 5 '15 at 22:34
  • Hah, I forgot I opened this question. Ironically, the alternator was just replaced today. It's been working on and off lately. One week it would get you where you need to go and another week it barely makes it down the block. So hopefully that'll be the end of that! – MGZero Sep 8 '15 at 16:01
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I'd still suspect the alternator - they don't always get noisy when they fail, as there are multiple things that can fail...

To check it with a multimeter, measure the voltage across the battery terminals. With the engine off this should be around or just over 12v, and with it running it should be around 14.5v - if it is significantly higher (e.g. 18v) then the regulating circuits have failed, and if it's low (<13v) then the alternator isn't charging.

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Bad alternators don't always make noise, and sometimes good alternators can appear to be noisy. That reminds of those guys who check tire pressure by just looking at the tires...

Like others have said, don't play the guessing game, go to your local hardware store, grab a multimeter and figure out what's going on.

It's not difficult to test your charging system:

Spark plug wires popping of your cap is not good, but I wouldn't suspect it to turn your battery light on or cause your car to die, unless they are all popping off simultaneously. I would suspect they would cause your vehicle to misfire and throw a trouble code (check engine light).

That being said, I would suspect a bad alternator. The light and your car dying on the road usually indicates you have a charging problem and the first things I would check are the alternator and battery in that case.

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Set your multimeter to the "20 VDC" or similar setting. NOT "AC", be sure it's on "DC".

With the car off, measure between the two battery posts. NOT the clamps, but the posts themselves that are part of the battery. Note the reading - it should be above about 12.5V if the battery is fully charged.

Now start the car and measure again. With the engine idling, you should now read something between about 13.4V and 13.8V. If your helper gives it a little throttle to speed up the engine, it should never rise above about 13.8 unless the weather outside is very cold and everything in the engine compartment is cold (a good voltage regulator will raise the charging voltage in cold weather because car batteries don't charge as readily in the cold).

If you read lower than about 13.4V or higher than about 13.8V with the car running, you probably have a voltage regulator problem. If you read lower than about 13.4V with the engine running FAST, you may have a diode trio (inside the alternator) problem. If you read below about 12.5V with the engine running, you have a serious charging issue with any of a number of possible causes - brushes or alternator windings or diode trio or voltage regulator or poor connections.

If you read higher than about 13.8V in warm weather, then you definitely have a voltage regulator problem and it needs to be tended to soon, before you boil your battery dry.

If the problem turns out to be intermittent, make up a simple adapter that fits your cigar/cigarette lighter socket and has two wires - one positive and one negative - extending out of it. The positive wire should be shorter than the negative one, so short that it CANNOT accidentally touch any part of the metal inside the passenger compartment. You can leave the wire ends stripped back about 1/2" and wrap the stripped wire around your multimeter probe tips, even securing the connections with any sort of tape... then you can read voltages while you drive the car. If the battery light comes on again, immediately (as immediately as is safe, of course) glance at the meter to see what the voltage is. If you want to make it a little fancier, you could put an alligator clip on each of the wires to make connection & disconnection simpler & faster.

  • Where are you getting your figure of 13.8v as the maximum charging-state voltage? It depends on the car, but that's near the minimum of what any functioning alternator should read -- at least a volt above the battery's nominal 12.6. My car's alternator is specified by the manufacturer to run in the 14.1 - 14.8 range, depending on temperature. Going over 13.8v while charging does not necessarily indicate a problem. – Josh Caswell Nov 6 '14 at 20:30
  • I used the language "higher than about 13.8V" because 13.8V is the optimum charging voltage for a 12V lead-acid battery at 70*F. At colder temperatures, that voltage should rise because the optimum charging voltage rises with falling temperature. At a certain temperature (I think it's around 0*F, but I'm not sure), the battery simply will not take a charge at any voltage until its temperature rises (that's why the battery is in the warm engine compartment). – TDHofstetter Sep 5 '15 at 22:30

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