New Battery, Good Alternator. but battery still drains. - Ford Mustang Forum
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post #1 of 17 (permalink) Old 03-24-2009 Thread Starter
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New Battery, Good Alternator. but battery still drains.

2002 ford mustang. 3.8. serpentine belt still good, connections to the battery new, all fuses seemed to work. i took the car to a mechanic, he said that the cables from the alt to the fuse box werent making good contact with each other, now he supposedly fixed the problem. but 2 days later the battery started draining again. we checked the cables again and they seemed fined, took the car to the mech again and he said that we should change the alternator anyway. the only thing is that the alternator is working fine, there's nothing wrong with it, i've taking it to have it checked 4 times and all 4 times it has come out good. now 4 days ago i've found out that if i hit the pedal to go to above 2500 rpm the battery light comes off and the gauge that measures the battery goes back up to high. so i've been doing that for the past 4 days and it has been working fine until today that when i go above 2500 rpm it doesn't seem to work anymore. the only dif. is that the battery is not draining as fast as before.
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post #2 of 17 (permalink) Old 03-24-2009
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sounds like it could be the alt. Let your engine get cold and see if the alt feels warm, that could be a sign of it draining the battery. also if the alt is bad, the atl isn't charging back to the battery and will probably die soon. you can always take it off and take it to a autozone or whatever you have there and they can do a free check on it.

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post #3 of 17 (permalink) Old 03-24-2009
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Do you have an aftermarket stereo or a sound system?? IF you do try pulling the wires from the stereo and leave them like that till tomorrow and see that happens... I already notice that this was causing me the same prob.It was my stereo maybe some wires weren't connected correctly...
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post #4 of 17 (permalink) Old 03-24-2009 Thread Starter
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Originally Posted by Big Daddy's Stang> View Post
Do you have an aftermarket stereo or a sound system?? IF you do try pulling the wires from the stereo and leave them like that till tomorrow and see that happens... I already notice that this was causing me the same prob.It was my stereo maybe some wires weren't connected correctly...
I FOUND THIS. could this be my problem?
Quote:
So, how do you know when your alternator is going bad? Most of the time the alternator fails in stages. A little techie talk here. The alternator gets its name from the fact that it generates alternating current (AC). The old generators I mentioned before generated direct current (DC). Well the battery can't use alternating current so the alternator output is fed into what are called diodes, which convert the AC into DC. The alternator has a unique feature in that it is able to generate a relatively high voltage while the engine is at idle. The old generators needed to be running at a fast pace before they got up to 13 or 14 volts. The alternator can do this since it is really three alternators in one body. Each of the three sections of the alternator generates its voltage out of phase with the other two sections. Since the complete cycle (one revolution) of the alternator is 360 degrees, each phase is shifted by 120 degrees from the next phase. So in one revolution of the alternator it puts out three separate voltages.
OK, back to the failure mode. Each of the three phases has its own windings in the alternator and each of the windings has its own pair of diodes. Each of these windings and/or diodes can fail, one set at a time. If this happens the alternator can still charge the battery, but only with a limited current, approximately 2/3 of its original capacity if one system fails. If two systems fail, then it puts out only 1/3 of its rated capacity. What that means to you is that you can go a long time on a limping alternator. Chances are if you don't need headlights or air conditioning or other high current using accessories, you would never know that the alternator was in the process of failing! The time you will find out is when it is 10 below zero and you wear down the battery by cranking the [COLOR=blue ! important][COLOR=blue ! important]starter[/COLOR][/COLOR], then put the fan on high for heat, and then drive in the dark.
So, how can you tell if the alternator is failing without taking it apart and doing some measuring inside the alternator? It's really pretty simple. You will need a simple voltmeter. You can get one at Radio Shack for under ten dollars. Here's what you do - start the car, make sure all the accessories are off and rev up the motor to a fast idle. Set the Voltmeter to the DC scale (not AC or Ohms). Measure the voltage across the battery terminals - red lead of the voltmeter on the positive terminal, black on the negative (ground in most cars). The voltage should, and probably will, read around 14 volts. If it reads less than 12 volts you may indeed have a failed alternator and you can skip the next step. Next, turn on the heater, the rear window de-fogger, the radio, the headlights and anything else that draws power. Now rev up the motor and watch the voltmeter. It should still be reading around 14 volts. If it reads lower than 13 volts the chances are that your alternator is not up to snuff.
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post #5 of 17 (permalink) Old 03-24-2009
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Quote:
Originally Posted by SAINTSEIYA12 View Post
2002 ford mustang. 3.8. serpentine belt still good, connections to the battery new, all fuses seemed to work. i took the car to a mechanic, he said that the cables from the alt to the fuse box werent making good contact with each other, now he supposedly fixed the problem. but 2 days later the battery started draining again. we checked the cables again and they seemed fined, took the car to the mech again and he said that we should change the alternator anyway. the only thing is that the alternator is working fine, there's nothing wrong with it, i've taking it to have it checked 4 times and all 4 times it has come out good. now 4 days ago i've found out that if i hit the pedal to go to above 2500 rpm the battery light comes off and the gauge that measures the battery goes back up to high. so i've been doing that for the past 4 days and it has been working fine until today that when i go above 2500 rpm it doesn't seem to work anymore. the only dif. is that the battery is not draining as fast as before.
sounds like the alternator to me.
you should also check your continuity/current draw.
do you have a dvom?
you dont have udp's do you?

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post #6 of 17 (permalink) Old 03-24-2009 Thread Starter
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do you have a dvom?
you dont have udp's do you?
what are those?
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Quote:
Originally Posted by SAINTSEIYA12 View Post
what are those?
A dvom is a digital volts ohms meter, that check continuity.
udp are under drive pullies but I don't think that's your problem cuz I was just watching a thread about that it only makes your interior light dim a little that you won't notice..:smoke:
I think autozone also sells those voltmeters. but I can't understand if they told you that the alt is good why it could be failing..
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Originally Posted by Big Daddy's Stang> View Post
A dvom is a digital volts ohms meter, that check continuity.
udp are under drive pullies but I don't think that's your problem cuz I was just watching a thread about that it only makes your interior light dim a little that you won't notice..:smoke:
I think autozone also sells those voltmeters. but I can't understand if they told you that the alt is good why it could be failing..
could it be failing at stages/phases? look at quote.
Quote:
OK, back to the failure mode. Each of the three phases has its own windings in the alternator and each of the windings has its own pair of diodes. Each of these windings and/or diodes can fail, one set at a time. If this happens the alternator can still charge the battery, but only with a limited current, approximately 2/3 of its original capacity if one system fails. If two systems fail, then it puts out only 1/3 of its rated capacity. What that means to you is that you can go a long time on a limping alternator. Chances are if you don't need headlights or air conditioning or other high current using accessories, you would never know that the alternator was in the process of failing! The time you will find out is when it is 10 below zero and you wear down the battery by cranking the [COLOR=blue ! important][COLOR=blue ! important]starter[/color][/color], then put the fan on high for heat, and then drive in the dark.
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Originally Posted by Big Daddy's Stang> View Post
udp are under drive pullies but I don't think that's your problem cuz I was just watching a thread about that it only makes your interior light dim a little that you won't notice..:smoke:
I think autozone also sells those voltmeters.
the reason "the lights sometimes dim" is because udp's can cause the battery not to charge fully because the alternator spins slower. so if he does have udp's that would explain why he's getting a good charge when he revs the engine. but it doesnt matter im willing to bet he doesnt have udp's.

also i said dvom, not just a voltmeter. you cant measure current draw with a voltmeter.

to the OP: if you have a dvom or even just a voltmeter you can test your alternator yourself.

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Quote:
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the reason "the lights sometimes dim" is because udp's can cause the battery not to charge fully because the alternator spins slower. so if he does have udp's that would explain why he's getting a good charge when he revs the engine. but it doesnt matter im willing to bet he doesnt have udp's.

also i said dvom, not just a voltmeter. you cant measure current draw with a voltmeter.

to the OP: if you have a dvom or even just a voltmeter you can test your alternator yourself.
I know I forgot to quote that answer too I was talking about the alt on that part...(voltmeter.)
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the only thing is he said he had the alternator checked 4 times?.. still think its the alternator thats the problem?

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the only thing is he said he had the alternator checked 4 times?.. still think its the alternator thats the problem?
depends who checked it, and how.
if all they did to test it was measuring battery voltage with the engine off, and then measure again with engine idling, thats not enough.
you also have to measure voltage with the engine at idle with accessory loads on/off, and then you have to measure voltage with the engine at a higher rpm (2k-4k rpms) with the accessory loads on/off.

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son's car had a problem like this a couple of months ago. If car sat a couple of days, would crank but just caught to start. Found out by accident that his glove box light stayed on all the time.
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son's car had a problem like this a couple of months ago. If car sat a couple of days, would crank but just caught to start. Found out by accident that his glove box light stayed on all the time.
I don't think our models have a light on the glove box cuz one time I was looking for one and couldn't see it...
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I don't think our models have a light on the glove box cuz one time I was looking for one and couldn't see it...
i know i dont have one; maybe its an option, some have it and some dont. theres plenty of things that can cause an excess current draw. thats why you need to check your continuity. if you have or can borrow a dvom i'll tell you how to check it.

but if your Battery lamp is on at an idle and goes away when you rev the engine, that is a charging issue, usually indicative of the alternator.

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