Bad alternator???

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WhyMe

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started my sub a couple of days ago and the battery light came on. i checked the output at the battery and it read low. i took the natural assumption it was the alternator. i went out and bought a used alternator and installed it. started it up and the lights went out and all is good. all the wiring in the car is in good condition

So today i took the old alternator to Orileys to have it checked out. To my surprise the machine said it was good. So could an alternator have a one time problem or fluke??
How accurate are the machines in determining the condition of an alternator?
 
The machine at O'reilly said my alternator was good when the alternator was bad. I had a 1979 Mercury Monarch and my battery light was on. I disconnected my negative cable with the car running; the engine immediately died. I brought the alternator to O'reilly Auto Parts and they said it tested fine. I didn't believe them and replaced the alternator; the problem went away. I don't know if you should disconnect the negative cable with the car running on newer vehicles but on older cars without computers it was an easy test to see if the alternator was charging.
 
That is why before changing an alternator it is best to charge the battery, start the car, and then turn on ALL your accessories.

Even the windshield wipers, fans, stereo, and rear defrost.

If the Voltage starts to drop steadily, you know the alternator is bad. But you need a cheap multimeter to check this.

If the voltage doesn't drop, may be a bad battery or you have a parasitic draw.
 
There are many different sleeper problems that could be going on. The alternator could be intermittent, the drive belt might of been slipping due to a bad tensioner and or bad belt, the battery might be going bad.

I would be taking a close look ant the belt, tensioner, and battery, if it were mine.
 
I had the battery light problem recently with my F150. It ended up being the pigtail that connects to the alternator. 10.00 for a new connector and no problems since so it is good to check your cable connections and grounds as well.
 
Last edited:
Originally Posted By: Chris142
Sounds like the brushes are worn. They are not always rubing on the commutator. Taking it in to be tested jiggled them enough to make contact.


+1 Brushes.
 
I had autozone test a lifetime warranty alternator for my Taurus and they said it tested fine. Luckily the other guy heard the bearings squealing and said it should be replaced. The new alternator fixed the problem. I can't believe how in accurate those machines seem to be.
 
Originally Posted By: 901Memphis
I had autozone test a lifetime warranty alternator for my Taurus and they said it tested fine. Luckily the other guy heard the bearings squealing and said it should be replaced. The new alternator fixed the problem. I can't believe how in accurate those machines seem to be.


A machine is not required to listen to possible bearing noises.
 
Originally Posted By: Chris142
Sounds like the brushes are worn. They are not always rubing on the commutator. Taking it in to be tested jiggled them enough to make contact.


thats what i thought it might be. i want to change them and then recoup some of the cost by selling the alternator, but i have had no luck finding brushes for the mitsu alt
 
I would always if possible take it to a rebuild shop and have them determine what the story is.

Could be brushes or bad ground, connector.

The brushes only carry the "excite" current, so nothing close to the alternator output.
 
Originally Posted By: Donald
I would always if possible take it to a rebuild shop and have them determine what the story is.

Could be brushes or bad ground, connector.

The brushes only carry the "excite" current, so nothing close to the alternator output.


True. But according to the alternator machine, which supposedly checks the rectifier and asst other items, they were all good.

maybe i should pull the new used one and throw the old one back in and see what happens. Good thing about my sub, it about a 15 min job to do so
 
Originally Posted By: GSCJR
Originally Posted By: 901Memphis
I had autozone test a lifetime warranty alternator for my Taurus and they said it tested fine. Luckily the other guy heard the bearings squealing and said it should be replaced. The new alternator fixed the problem. I can't believe how in accurate those machines seem to be.


A machine is not required to listen to possible bearing noises.


It wasn't making noise on the vehicle or testing machine, it wasn't until after they tested it and told me it was okay thanks I was standing by the counter with my alternator discussing what else could be the problem that I was playing with the alternator spinning it by hand and we both heard it start to squeak.
 
I agree with the guys above that are talking about the brushes. But there are also many other possibilities that come into play when you have an audio amplifier added in. I've known people to kill multiple alternators before my stock one went with my system installed. On a side note, the load that the machines put on the alternators to test them is not nearly the load of an improperly tuned amp (not saying that is the case, but no one knows for sure).
 
I recommend getting one of these:

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Equus 3721 Battery and Charging System Monitor

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Also available at some Walmarts.
 
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