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Old 08-26-2016, 08:57 PM   #1
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88 GT, battery won't keep charge

The other day I got in my car to leave, car wouldn't start. I figured a dead battery, i jumped it off and drove it for at least 30 minutes before I shut it off. it sat for about 45 minutes and it wouldn't start when I tried again. Not sure if it's the battery or alternator. Just wanna see what you guys think before I go and buy the wrong part.


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Old 08-26-2016, 09:52 PM   #2
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Just test both before you buy but i bet its the alternator

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Old 08-26-2016, 09:54 PM   #3
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Have your local auto parts store check both. It takes 5 minutes.

My first guess would be the battery. The alternator will be the next victim to go if it keeps charging a bad battery.

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Old 08-27-2016, 10:40 PM   #4
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The first thing to check is the dash light.When you first turn the key on,you should see the charge indicater light come on briefly then shut off.If you dont see the light come on,trouble shoot that circuit.If the bulb is blown or theres an open in the wire between this light & the alternator,that will cause the alternator to not charge.
Something else to check are the two alternator wiring harnesses.Both of them have a fusible link inline between the alternator & battery.If either fusible link is melted(open) the alternator wont be able to charge the battery.
You can also check the harnesses to make sure they dont have a short.Just disconnect the neg battery cable,attach the clip of a test light to the neg battery cable then touch the test light probe to the neg battery post itself.If the test light is lit,disconnect both alternator harnesses individually.Did the test light go out?? If yes,the alternator harness is shorted or the alternator is bad.

If the battery is completely dead,it'll be impossible for the alternator to charge it.The reason is simple.Jumping the battery will give it 12 volts again, but since the battery is empty,it has no amps.The alternator needs battery power in order to function,so having volts without amps wont create the magnetic field thats needed for the alternator to charge.The battery will need to be placed on a trickle charger for 12-24 hours.
Most of the time,if a battery is completely dead & a jump start gets the car running,then you take it for a drive,shut if off for a few minutes then try to start it and it wont crank,thats a sign the battery has a dead cell or the alternator isnt charging.The battery wont hold a charge if its got a dead cell.
The main component that goes bad on an alternator is the brush assembly.The brushes are located under the voltage regulator,which is attached by 4 torx screws & has the larger(of the two)wiring harness connected to it.If the brushes are really worn down,that could be your problem.Theyre easy to replace & available at most part stores for $5 or less.Simply remove the alternator,insert a skinny nail/paperclip into that little hole you can see on the voltage regulator.Remove the regulator torx screws then pull the regulator off the alternator while pulling the brushes out of their holder.Replace the brushes then install everything in reverse order.
Make sure the secondary ground strap is tight & corrosion free too.Its located between the cylinder head (rear side) and the firewall.Its the ground for the alternator and a bad ground here will cause the alternator to max out.
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Old 08-27-2016, 11:17 PM   #5
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Nice post. I have never rebuilt an alternator, but now I want to give it a try. $5 and a few twists of bolts sounds worth the attempt.

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Old 08-28-2016, 03:24 AM   #6
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While you have it out take it apart and clean out the inside. Bunch of dust. Should look like all copper on the wall and center that spins to create the charge


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Old 08-31-2016, 09:39 PM   #7
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The brushes are $3.69 @ Autozone.

An alternator also has diodes that can go bad.A diode converts alternator output from AC volts to DC volts.A bad diode can cause the alternator charging output to drop,which will cause the
battery to run down over time,depending on how many diodes are bad.A leaky diode can allow current to drain out of the battery through the alternator,while the vehicle is not being driven.

1st pic= brushes
2nd pic= voltage regulator (holds the brushes)
3rd pic= rectifier (contains the diodes)
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Old 08-31-2016, 09:42 PM   #8
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Quote:
Originally Posted by wbrockstar View Post
The brushes are $3.69 @ Autozone.

An alternator also has diodes that can go bad.A diode converts alternator output from AC volts to DC volts.A bad diode can cause the alternator charging output to drop,which will cause the
battery to run down over time,depending on how many diodes are bad.A leaky diode can allow current to drain out of the battery through the alternator,while the vehicle is not being driven.

1st pic= brushes
2nd pic= voltage regulator (holds the brushes)
3rd pic= rectifier (contains the diodes)
I don't see any pics.

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Old 11-19-2016, 09:17 AM   #9
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My friend has the same problem with his 1988 fox body mustang battery is brand new


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