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Old 12-02-2013, 03:14 PM
 
Location: Louisville KY
4,584 posts, read 4,479,062 times
Reputation: 3951

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Quote:
Originally Posted by NewHavensFinest View Post
Once the alternator goes bad, the car runs off the battery until it goes dead.

With my car, the voltage difference between a good, OK, and a bad battery is very small. Its within 1 volt Im pretty sure.

Also on most modern cars, when the alternator goes bad, your battery light on the dash should turn on right away. That will start your countdown and you have minutes.

If the battery was completely drained, like no lights and absolutely no power, time for new battery.
If the alt is dead the car will not run at all. The battery is only there for the starter, to turn on the alternator and keep radio memory. That's why it's connected to the starter directly and ground, from the starter the alternator is connected from there. Everything runs off the alt and it even slightly charges the battery- which a car does not need to run only start.
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Old 12-02-2013, 04:55 PM
 
8,402 posts, read 21,562,236 times
Reputation: 6791
Quote:
Originally Posted by JaxRhapsody View Post
If the alt is dead the car will not run at all. The battery is only there for the starter, to turn on the alternator and keep radio memory. That's why it's connected to the starter directly and ground, from the starter the alternator is connected from there. Everything runs off the alt and it even slightly charges the battery- which a car does not need to run only start.


I thought I knew the answer to OP's question, but there is so much wild misinformation that I'm confused.

My '95 F150 has killed the battery to the point where it won't charge, and the volt meter was showing lower than usual voltage and dropped when I turned on any electrical accessories, which hasn't been the case until the last couple months. I don't drive the truck often, so that's an issue, but usually it will start after a few weeks. A DVOM showed a slight drop from running to non-running (I forget the volts numbers) at the battery, Walmart has replaced the battery 3 times under warranty in the last 5 years. I assume this is an alternator problem, so I will replace it and the battery. Am I on the right track here?
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Old 12-02-2013, 05:18 PM
 
Location: Louisville KY
4,584 posts, read 4,479,062 times
Reputation: 3951
if the battery is still good, just replace the alt or at least have it checked. With the alt a car will start and run and die, I had a 93 mustang I was trying to get and no matter how much we jumped the car died. It was the alt. Ultimately the alt runs all electrics when the car is running, it relies very little on the battery which is why cars usually continue to run with it unhooked. because some cars don't actually have one strong enough, after a while they die before they should, manufactures only use bare minimum of what works.

This means- even if the car is stock can still suffer from it's electronics. IF you replace it, get a heavy duty alt. another thing you can do to insure it runs is use larger guage wire for your ignition system; batt cables, starter to alt wires, battery with more cca's, yours is probably 650, go 7-850. On a related note, a relay kit for your headlights, can take load off your headlight switch. You should join FordTrucks forums. Most people don't care about all that but performance is more than just going fast and handling. Increasing performance is anything to make a car run better, not be a race car. Do that and you can be sure it'll start everytime as soon as you turn the key- not turn the key and the engine turns over 3-5 times. All that extra/freed juice will hit the coil packs a lot faster than stock.
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Old 12-02-2013, 05:34 PM
 
428 posts, read 494,963 times
Reputation: 618
This is how I learned the difference (the hard way). My F150 truck would'n't start without a jump. Then, when I got the jump, I got to my destination ok, and shut it off. When I tried to turn it back on again, I needed another jump, then, I got back home ok. It was the battery. When it was the alternator, the truck would completely die out, while moving, on the road, in traffic, without turning it off. We had to keep jumping it at least a dozen times on the road, in order to get to my mechanic. It was the alternator.

Last edited by glenninindy; 12-02-2013 at 05:45 PM..
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Old 12-04-2013, 12:17 PM
 
1,728 posts, read 3,224,808 times
Reputation: 1053
I lost the drive belt the runs the alternator once and of course the car still runs. so its really alt and batt thats powers the car.
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Old 12-04-2013, 01:42 PM
 
22 posts, read 274,839 times
Reputation: 24
Some sort of dash lighting coming on for just a simple moment, perhaps dimmed headlights and a few flickering gauges. Probably there may be possibly a good unusual scent, or even a growling noise caused by beneath the cover.
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Old 12-04-2013, 02:24 PM
 
11,364 posts, read 46,953,267 times
Reputation: 15435
Quote:
Originally Posted by vmaxnc View Post


I thought I knew the answer to OP's question, but there is so much wild misinformation that I'm confused.

My '95 F150 has killed the battery to the point where it won't charge, and the volt meter was showing lower than usual voltage and dropped when I turned on any electrical accessories, which hasn't been the case until the last couple months. I don't drive the truck often, so that's an issue, but usually it will start after a few weeks. A DVOM showed a slight drop from running to non-running (I forget the volts numbers) at the battery, Walmart has replaced the battery 3 times under warranty in the last 5 years. I assume this is an alternator problem, so I will replace it and the battery. Am I on the right track here?
very likely a failed alternator diode, causing a parasitic draw on the battery ... and a floating A/C component which damages the battery when the alternator is charging.

Unfortunately, most shops don't test for this failure, they only look to see a max amp and voltage output test to declare an alternator "good". Few will ever look at the output waveform on a 'scope after checking the amp output under load.

The alternators in this series of Ford (among other series and other car lines) are well known for this failure. I've seen a lot of off-the-shelf "remans" come through from other shops where they didn't cure the battery problem because the replacement alternator is as defective as the one they removed from service.

FWIW ... Jax has the electrical system totally wrong. You can remove the alternator from the vehicle and a serviceable battery will start the engine and run the accessories until such time as the stored energy is depleted to a voltage level lower than required to run the vehicle. The alternator recharges the battery in service.
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Old 12-05-2013, 11:11 AM
 
1,176 posts, read 2,244,758 times
Reputation: 1412
Quote:
Originally Posted by Caligula1 View Post
Seems a very long time ago I remember someone showing me to remove one of the Battery cables while the car was running to test if the Alternator is indeed dead or the Battery..

Was I dreaming or this just a figment of my imagination..?

I need to try a home test of some sorts to figure it out..
If your alternator gauge isn't approximately in the middle of the gauge, it's probably the alternator.

If your battery is more than about two years old, it's probably the battery.
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Old 12-05-2013, 12:20 PM
 
2,341 posts, read 10,209,776 times
Reputation: 2040
Quote:
Originally Posted by Cleonidas View Post
If your alternator gauge isn't approximately in the middle of the gauge, it's probably the alternator.

If your battery is more than about two years old, it's probably the battery.
Ummmmmmmm.......... These things are not necessarily true.

If a gauge has numbers, it should read in the 12-14 volt range. If the gauge doesn't have numbers, it usually has a "green zone" where the needle should fall.

Also, unless there's some factory defect, most automotive batteries make it a long time past 2 years. And when they die, there's no question.
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Old 12-06-2013, 12:56 AM
 
Location: Not far from Fairbanks, AK
17,446 posts, read 31,090,857 times
Reputation: 13536
Quote:
Originally Posted by GTRdad View Post
I lost the drive belt the runs the alternator once and of course the car still runs. so its really alt and batt thats powers the car.
The reason why it kept running is because your car's battery had sufficient power left. But turn the headlights and the heater blower on, and the battery dies pretty fast, at which point the motor stops running. The motor stops running because it loses power to the ECM (computer), which in turn can't power the spark plugs, fuel pump, and the injectors.
http://autorepair.about.com/od/gloss...alternator.htm

A couple of years ago on my way home from work, the Honda Civic I was driving stopped running while driving. It's dark almost all day and night in Alaska, so I had the heater and headlights on. All of the sudden the lights dimmed and the motor stopped running, so I immediately pulled to the road's shoulder. The battery was completely dead, and I kew that if the alternator would have been running properly, the motor should have continued running.

But I had a battery jump starter with me, so I connected the jump starter to the battery, tucked in there engine bay so I could close the hood, turned it on, and started the car. I made sure that only the headlights were on (turned the blower off), and drove home about two miles away.

My former job required me to assemble and test alternators, inverters, and aircraft generators. I did this for 20 years, and then retired.

Last edited by RayinAK; 12-06-2013 at 01:18 AM..
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