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Old 10-01-2009, 08:18 AM
Snickers Snickers is offline
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Diagnose my car problem: Battery won't hold charge

So, I've got a 1985 Corvette as my fun car for summer, and it's been performing admirably. Until now: the battery won't hold a charge. I came out one day and it was dead, dead, deadsky.

We replaced the battery Sunday - the old one was 5+ years old according to anyone's recollection - and it worked like a champ for the beginning of this week. (Although I did notice Monday that it seemed to take a while to start.) Unfortunately, I didn't drive it yesterday as I drove my workaday car instead. (I tend to drive the workaday Mazda once per week in summer, just to keep its battery charged too. Probably unnecessary, but I skipped a week last year to, guess what, a dead battery. In that case, replacement was all that was needed.)

Came out this morning to a dead 'Vette. Absolutely nothing in the battery, no click, nothing.

At this point, I'm guessing it's my alternator, but are there any other causes I should consider? Sadly, I'm not very mechanical, nor is my husband, so we'll be taking it in to get it fixed. How much might this run me to get it fixed?

Thanks!

Last edited by Snickers; 10-01-2009 at 08:18 AM..
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  #2  
Old 10-01-2009, 08:35 AM
Bijou Drains Bijou Drains is offline
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A good shop will be able to find the problem , they probably will start by testing the alternator. In most cases those are not hard to replace so it won't cost too much beyond the cost of the part. The voltage regulator could be bad too.
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Old 10-01-2009, 09:04 AM
Gary T Gary T is online now
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From your description, it sounds like something is draining the battery's charge, rather than the battery not getting charged. While the latter situation is almost always a faulty alternator, the former can be caused by a number of things, including but not limited to the alternator.

The first step in resolving the problem will be to charge the battery. The next steps will be to test the alternator's operation (is it charging properly?) and to test for an electrical drain on the battery. If there is a drain, then it will be necessary to test various circuits to isolate and identify the cause of the drain. There are too many different possibilities for us to realisitically figure what it might cost to fix.
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Old 10-01-2009, 09:09 AM
CalMeacham CalMeacham is online now
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When i had this problem with my first car, I spent a lot of time looking for unexpected drain paths -- were there lights on all the time? Was there a worn-through cable shorting to ground? I checked everything, and found nothing.

The problem ultimately turned out to be my battery. It was getting old. Replacing it solved the problem.

From what you say, you've already replaced the battery. But it still might be a faulty one. If you can't find any other cause for the drain, I'd look at that.
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Old 10-01-2009, 09:13 AM
billfish678 billfish678 is offline
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Quote:
Originally Posted by CalMeacham View Post
When i had this problem with my first car, I spent a lot of time looking for unexpected drain paths -- were there lights on all the time? Was there a worn-through cable shorting to ground? I checked everything, and found nothing.

The problem ultimately turned out to be my battery. It was getting old. Replacing it solved the problem.

From what you say, you've already replaced the battery. But it still might be a faulty one. If you can't find any other cause for the drain, I'd look at that.
I'd second that. Back in the day, batteries often seemed to die a slow death, slowly getting worse and worse.

These days, they often seem to be fine, then bam! Its deader than a door nail in one fell swoop.

And yeah, its not rare (I am looking at you China) these days to get a replacement part that doesnt work from the get go either.
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Old 10-01-2009, 09:22 AM
Snickers Snickers is offline
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Bummer - I was hoping that this would be "It's your alternator." Bingo bango, done.

I agree with you, GaryT - I don't think it's holding charge, instead of not getting charged up. But maybe not - since replacing the battery Sunday, I drove to work Monday and Tuesday, a distance of 100ish miles. Let's call it 200, for safety - would driving 200 miles with a bad alternator be enough to discharge the battery?
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Old 10-01-2009, 09:40 AM
pan1 pan1 is offline
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Its quite possible that both the alternator and the battery were bad.

You need a voltmeter to hook up to the battery terminals.

Check it with the car off. Turn the car on, check it again. It should go up to about 12+ volts with the car running. (Check your manual for exact numbers)

It will drop again when you turn the car off. If it drops to zero, the battery is dead.
If it doesn't go up when you turn the car on, the alternator is not working. (Or if it doesn't go up to the right number) TO be sure, reconnect to the terminals on the alternator - just to eliminate the chance of a bad cable.

Its very likely you alternator from what you've said.
yes, a car will run without an alternator on just battery power for a while, especially an older car that isn't computer dependant.

One other thing to check - the belts. Make sure there is one going to the alternator.
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Old 10-01-2009, 10:21 AM
Gary T Gary T is online now
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Quote:
Originally Posted by pan1 View Post
Check it with the car off. Turn the car on, check it again. It should go up to about 12+ volts with the car running. (Check your manual for exact numbers)
Rest voltage (engine not running, nothing turned on) spec is 12.6. Charging voltage (engine idling, headlights on) should be around 14-14.5. This applies to any car.

Quote:
It will drop again when you turn the car off. If it drops to zero, the battery is dead.
?? It's not going to drop to zero unless it was at or near zero before starting the engine -- and the engine wouldn't start if it was that low.
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Old 10-01-2009, 12:16 PM
pan1 pan1 is offline
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Originally Posted by Gary T View Post
It's not going to drop to zero unless it was at or near zero before starting the engine -- and the engine wouldn't start if it was that low.
The above process assumes you have access to a starter pack or jumper cables to assist in starting the vehicle with the battery dead.

I often forget: nothing is obvious.
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Old 10-01-2009, 12:40 PM
CookingWithGas CookingWithGas is offline
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Car diagnosis threads are like medical diagnosis threads, except they shut down medical diagnosis threads around here. Only a mechanic will be able to tell for sure what is wrong and how much it'll cost to fix it.

Quote:
Originally Posted by pan1 View Post
Its quite possible that both the alternator and the battery were bad.

You need a voltmeter to hook up to the battery terminals.
The first time I read this I thought you were suggesting a volunteer to hook up to the battery and I was wondering what bizarre procedure you had in mind....

Quote:
Check it with the car off. Turn the car on, check it again. It should go up to about 12+ volts with the car running. (Check your manual for exact numbers)

It will drop again when you turn the car off. If it drops to zero, the battery is dead.
If it doesn't go up when you turn the car on, the alternator is not working. (Or if it doesn't go up to the right number) TO be sure, reconnect to the terminals on the alternator - just to eliminate the chance of a bad cable.

Its very likely you alternator from what you've said.
yes, a car will run without an alternator on just battery power for a while, especially an older car that isn't computer dependant.
IANAM but I believe you can get a reading of 12V from a 12V battery that doesn't have much capacity left. I think they measure an in-line measurement of how many amps the battery can supply under load rather than a parallel measure of voltage (or maybe both at the same time).

Also, I have read on this board and elsewhere many times that in the old days you could disconnect the battery from a running car and it will still run but in modern vehicles you should never do this. I think it has something to do with voltage spikes, possibly frying the electronics.
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