Battery charging issues. Alternator or wiring?

Canary94GT

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Jun 27, 2009
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I am having some issues with my battery charging in my 1994 GT and need it fixed ASAP because it's my daily driver.

My 5-year old battery died slowly while driving about a week ago (eventually wouldn't start), and I didn't bother getting it recharged and just bought a new battery.

Well now I notice my battery isn't getting charged all the time(which explains the dead old battery). When I start the car, the battery gauge is usually about 3/4th full, and the battery reads 15V. After I drive for a while, the gauge sometimes goes down to about 1/3rd full and the battery reads only around 11.88-12V with the car on. It appears to me the battery is not getting charged at that point.

Usually this transition from 3/4th to 1/3rd on the gauge happens rapidly when driving (one second 3/4th, next second 1/3rd). I have noticed it happen a couple times while at a stop light when the electric fan kicked on, but sometimes it happens when driving or starts out at 1/3rd when I start the car.

I just had the alternator tested at both O'Reilly's and AutoZone and it passed at both. But I realize they're not testing it for long.. only about 15-30 seconds. The AutoZone guy told me it might be the alternator giving out when it's hot.

Any ideas on what this could be? The alternator, the volt. regulator, or the wiring? Also, I'm a noob when it comes to electrical - are there any easy resistance/voltage tests I can do on the wiring?

Thank you
 
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jrichker

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This may help, but if you have problems understanding electrical circuits, you may get lost. The best test is the in car test when the voltage starts to drop off. If you can get AutoZone or O'Riley to test it then you stand a better chance of finding the problem.


Alternator troubleshooting for 86-93 5.0 Mustangs:


Never, never disconnect an alternator from the battery with the engine running. The resulting voltage spike can damage the car's electronics including the alternator.

Do all of these tests in sequence. Do not skip around. The results of each test depend on the results of the previous tests for correct interpretation.

Engine off, ignition off, battery fully charged.
1.) Look for 12 volts at the alternator output. No 12 volts and the dark green fuse link between the orange/black wires and the battery side of the starter solenoid has open circuited.
3G alternator: Look for 12 volts at the stud on the back of the alternator where the 4 gauge power feed wire is bolted.
No voltage and the fuse for the 4 gauge power feed wire is open or there are some loose connections.

2.) Look for 12 volts on the yellow/white wire that is the power feed to the regulator. No 12 volts, and the fuse link for the yellow/white wire has open circuited.

Engine off, ignition on, battery fully charged.
1.) Alternator warning light should glow. No glow, bulb has burned out or there is a break in the wiring between the regulator plug and the dash. The warning light supplies an exciter voltage that tells the regulator to turn on. There is a 500 ohm resistor in parallel with the warning light so that if the bulb burns out, the regulator still gets the exciter voltage.
Disconnect the D connector with the 3 wires (yellow/white, white/black and green/red) from the voltage regulator.
Measure the voltage on the Lt green/red wire. It should be 12 volts. No 12 volts and the wire is broken, or the 500 ohm resistor and dash indicator lamp are bad. If the 12 volts is missing, replace the warning lamp. If after replacing the warning lamp, the test fails again, the wiring between the warning lamp and the alternator is faulty. The warning lamp circuit is part of the instrument panel and contains some connectors that may cause problems.

2.) Reconnect the D plug to the alternator
Probe the green/red wire from the rear of the connector and use the battery negative post as a ground. You should see 2.4-2.6 volts. No voltage and the previous tests passed, you have a failed regulator. This is an actual measurement taken from a car with a working electrical system.

Engine on, Ignition on, battery fully charged:
Probe the green/red wire from the rear of the connector and use the battery negative post as a ground. You should see battery voltage minus .25 to 1.0 volt. If the battery measured across the battery is 15.25 volts, you should see 14.50 volts

Familiarize yourself with the following application note from Fluke: See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. .


You will need to do some voltage drop testing of several of the wires.

Start looking for these things:
1.) Bad diode(s) in the alternator - one or more diodes have open circuited and are causing the voltage to drop off as load increases. Remove the alternator and bench test it to confirm or deny this as being the problem.

2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Do the voltage drop test as shown in the Fluke tech note link. Measure the voltage drop between the alternator frame and the battery negative post. Watch for an increase in drop as the load increases. Use the Fluke voltage drop figures as guidelines for your decisions.

3.) Bad regulator that does not increase field current as load increases. Remove the alternator and bench test it to confirm or deny this as being the problem.

4.) Bad sense wire - open circuit in sense wiring or high resistance. The yellow/white wire is the voltage sense and power for the field. There is a fuse link embedded in the wiring where it connects to the black/orange wiring that can open up and cause problems. Disconnect the battery negative cable from the battery: this will keep you from making sparks when you do the next step. Then disconnect the yellow/white wire at the alternator and the green fuse link at the starter solenoid/starter relay. Measure the resistance between the alternator end of the yellow/white wire and the green fuse link: you should see less than 1 ohm. Reconnect all the wires when you have completed this step.

5.) Bad power feed wiring from the alternator. Use caution in the next step, since you will need to do it with everything powered up and the engine running. You are going to do the Fluke voltage drop tests on the power feed wiring, fuse links and associated parts. Connect one DMM lead to the battery side of the starter solenoid/starter relay. Carefully probe the backside of the black/orange wire connector where it plugs into the alternator. With the engine off, you should see very little voltage. Start the engine and increase the load on the electrical system. Watch for an increase in drop as the load increases. Use the Fluke voltage drop figures as guidelines for your decisions.





Alternator wiring circuit
Notice the green wire connects to a switched power source. The circuit contains a 500 ohm resistor in series between the switched power and the alternator. Connecting it to switched power keeps the regulator from drawing current when the engine is not running. The resistor limits the current flowing through the wire so that a fuse isn't needed if the wire shorts to ground.

Also notice the sense wire connects to the starter solenoid and it is fused. It connects to the starter solenoid so that it can "sense" the voltage drop across the output wiring from the alternator.
 
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Canary94GT

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Thank you very much! That looks quite useful. Hopefully even though it's for 86-93 5.0's, it will still apply to my 94.

Indeed, some of the terminology is unfamiliar but tomorrow I will see if I can follow any of it :)
 

Canary94GT

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Hmm. I went through these steps earlier. I did step 1 with the Engine off, ignition on, and my charge light was lit up. Disconnecting the D connector and probing the green/red wire gave me 12 volts.

When I got to step 2, the voltage on the red/green wire read the same as the battery (12V) for me, not 2.4-2.6 volts. But the instructions here do not say what to do in this scenario - only when voltage is not present.

After I came home today, I went and tested it again. This time I do not see the charge light lit up with the engine off, ignition on, which would indicate a problem in step 1 of engine off, ignition on.

Any ideas what is going on?
 

Canary94GT

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Well I pulled the instrument panel out and found the charging light bulb was burned out. Which is weird because it was working earlier today. I tried swapping a working turn signal bulb into its place and turning the key on, but still didn't see the light come on. I went to the store and got some replacement bulbs that I will put in tomorrow.

Blah, I really suck at electrical issues.
 

Canary94GT

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No I don't, the car's stock in terms of the engine compartment.

Also, just replaced the charging light bulb. I turned the key on, and the bulb lit up. I turned the engine on, and it still wasn't charging. I turned it off, then turned the key on, and the bulb did not light up :shrug:
 

Canary94GT

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Update:

Checked the resistance of the green/red wire from the voltage regulator to the dash charge bulb to the interior fuse box. It read fine, around 15 ohms.

I learned about another way to test the voltage regulator, which is to utilize the "ground here to test" bolt on the voltage regulator. Supposedly if you ground that bolt while the engine is running and the car starts charging, then the regulator is bad.

I grounded the bolt and it started charging. :o

Does anyone think I should replace the regulator ($70) or replace the alternator for a rebuilt one ($170)? I'm a jobless college student, so I need to conserve as much money as possible. But I'm wondering if I should rebuild the alternator at 83,000 miles?
 

Canary94GT

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Eureka!

I took the alternator off and took it to O'reillys to have them test it again. It FAILED this time! At that point, I decided to bite the bullet and buy the regulator + new brushes. Installed them using this awesome little guide, slapped the alternator back on and it was charging :D:D:D

Thanks for your help!
 

jrichker

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I am glad to hear that you got it fixed and it is charging now. Thanks for you input since it will help me update my alternator troubleshooter test path.
 

Canary94GT

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jrichker, I am really glad to hear that you will add it! Your guide is excellent and I really appreciate you both creating it and posting it. I needed this fixed asap and you helped me get to that point. Thank you.

That guide should really be a sticky.
 

Incon

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Jun 14, 2010
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Questions: Is the 500 ohm resistor on the card that pulls out on the backside of the gauge panel?

Could the Battery warning light work AND the 500 ohm resistor still be bad causing problems?

What would/could be the cause(s) of a frequently failing regulator?

Thanks for the help.
 

Canary94GT

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Jun 27, 2009
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Seattle, WA
Questions: Is the 500 ohm resistor on the card that pulls out on the backside of the gauge panel?

Could the Battery warning light work AND the 500 ohm resistor still be bad causing problems?

What would/could be the cause(s) of a frequently failing regulator?

Thanks for the help.

The 500 ohm resistor is located right near the "amp" (charging) warning light on the back of the gauge panel. It's connected to the copper flat wire sheet.

I am sorry, I do not know the exact answers to your other questions. Hopefully someone more experienced can answer them for you.
 

jrichker

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Questions: Is the 500 ohm resistor on the card that pulls out on the backside of the gauge panel?

Could the Battery warning light work AND the 500 ohm resistor still be bad causing problems?

What would/could be the cause(s) of a frequently failing regulator?

Thanks for the help.
Start your own thread and I will try to help you...
 

Blown88GT

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Eureka!

I took the alternator off and took it to O'reillys to have them test it again. It FAILED this time! At that point, I decided to bite the bullet and buy the regulator + new brushes. Installed them using this awesome little guide, slapped the alternator back on and it was charging :D:D:D

Thanks for your help!
I wonder why it passed the first time at O'Reilly's & AutoZone?
 

HISSIN50

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If not mentioned, that might have been a good scenario to have a dynamic test performed. Drive it till it acts up and take it to the parts store for the test.
 

Canary94GT

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If not mentioned, that might have been a good scenario to have a dynamic test performed. Drive it till it acts up and take it to the parts store for the test.
I think that's what happened, because the issue was intermittent when driving anyway. When I took it to the shops the first time, it was not acting up. Then I took it home, turned the car on and off a few times while testing and it started acting up again. So when I took it in the last time, it failed.