Alternator problem on 1994 Mustang GT - Ford Mustang Forum
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post #1 of 17 (permalink) Old 03-31-2010 Thread Starter
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Alternator problem on 1994 Mustang GT

hi, 1994 mustang gt 5.0L. The problem is that the alternator will be charging, then just drop to 11.7 volts, it will do that for a while, then just jump back to 14.5volts. I checked the grounds and checked the brushes. The voltage regulator checked out fine. I drove it fine for about 30 minutes. It sat for about 2 hours, then i started it up, and it was sitting at 11.7 again. So it doesnt appear to be heat related(as in eletronics or etc). So any ideas???????
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post #2 of 17 (permalink) Old 03-31-2010
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Had the same problem with a F150 truned out to be bad main power wire on alt going back to battery.
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post #3 of 17 (permalink) Old 03-31-2010 Thread Starter
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Thanks for the input, does anyone else have any ideas?
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post #4 of 17 (permalink) Old 03-31-2010
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Take it to one of the big chain autoparts stores. They can test the alternator for you. That will eliminate it as the cause so you can start chasing bad wires.
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yea, forgot to add, i had it tested, it tested out fine. how about a fusible link?
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post #6 of 17 (permalink) Old 04-01-2010 Thread Starter
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Bump for some more answers
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post #7 of 17 (permalink) Old 04-03-2010
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Do you run underdrive pulleys? If so, that could be your problem. With the alternator spinning slower, it is generating less amps. Therefore, if your A/C compressor is cycling, it will read 11.7 when its clutch is engaged, then back to normal when it disengages.
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post #8 of 17 (permalink) Old 04-03-2010
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The 94 has a 135a 3G alternator that should be able to produce well even with underdrive.

While there are several wires involved, the wiring is really simple. I just re-wired my 65 to accept one of these alternators.

If the alternator is really not the problem, the only thing other than a simple loose connection at the alternator, or something in the harness close by is the "I" signal that "tells" the alternator that the car is on and that it needs to get to work.

On some cars this is simply tied to the ignition switch and goes positive when the ignition switch is in position 2, or "run".

On other cars, this positive voltage is supplied THROUGH the idiot light in the instrument cluster. An intermittent connection at the bulb, or a flaky bulb can keep the alternator from operating.

This circuit is traditionally the cause of the most frustration with a non-producing, or intermittent alternator.

I am attaching a diagram of the connectors involved and a wiring diagram of a typical 3G setup.

http://inlinethumb60.webshots.com/46...500x500Q85.jpg

The 3G has a "stud" on the back that is the "fused" charging line to the battery. This is fused in case the ALTERNATOR shorts out and provides a dead-short to ground for your battery. The charge line will glow red-hot and start a fire if your diodes short out and this fuse doesn't exist. This "battery" connection is usually one of the studs on the starter solenoid/relay.

Next, there is a 3-pin connector:

The "A" signal goes to a battery connection. Sometimes it is looped over to the charging "stud" on the back of the alternator, but usually it follows the charge lead to the stud on the starter relay.

The "S" signal simply loops over and connects to the single-pin connector labeled "S".

The "I" signal is the one mentioned above that receives a positive 12 volts to tell the Alternator that the car is on and that it needs to get to work.

Those are ALL of the connections needed for a 3G alternator to work!

Below is a wiring diagram that shows this pictorally.

http://inlinethumb53.webshots.com/41...600x600Q85.jpg

Stan -
1965 5.0 Coupe /MAF/EFI/AOD/B303 Roller Cam

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post #9 of 17 (permalink) Old 04-03-2010 Thread Starter
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Thank you very much for the detailed and though answer.
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post #10 of 17 (permalink) Old 04-08-2010 Thread Starter
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Update: the guy replaced the voltage regulator and it *seems* to have fixed the problem. my question is, if it was the voltage regulator that failed; would it have caused any damage to other components? he said he noticed that the voltage would read a little high sometimes and low others. He said that it was a few letters away from the max area of *NORMAL* operation on the gauge in dash. When a voltage starts to fail, is there a fail safe to keep from shorting everything else out because of voltage spikes? Im pretty sure it didnt cause any damage, i just would like others input on this.
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Quote:
Originally Posted by mustang1010 View Post
Update: the guy replaced the voltage regulator and it *seems* to have fixed the problem. my question is, if it was the voltage regulator that failed; would it have caused any damage to other components? he said he noticed that the voltage would read a little high sometimes and low others. He said that it was a few letters away from the max area of *NORMAL* operation on the gauge in dash. When a voltage starts to fail, is there a fail safe to keep from shorting everything else out because of voltage spikes? Im pretty sure it didnt cause any damage, i just would like others input on this.
Well, in theory, everything is fused to prevent such problems from occurring. If you don't have any issue with driveability, then I would assume it didn't mess anything up.

Matt

2006 GT - Vortech polished w/ intercooler, Eibach suspension, Hurst short-throw, some other goodies - tuned by Paul's High Performance. It moves...

and it collects dust admirably while parked for repairs in the garage...

SOLD - 1995 Black Mustang GT, AODE. Loved it, but I had to let it go...

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I know the post is 6 months old...but

I had a similar problem.Carwould run fine(13-14.4V) then the gauge would lazy it's way down to the "N" (normal - onthe gauge cluster), this would show 11.4-11.8 with multimeter.
After thinking that I had it fixed by re-wiring the NEW Napa alternator w 4g battery cable over to the fuse box stud.(Battery is also remote mounted-trunk- and run with 2/0 weld cable and a 2g 12" long ground) I then started looking around after the prob continued, found the "exciter/stator" wire had 0.3 volts(dunno what it is supposed to have, and that the plug-in showed 12v on BOTH yellow and gr/red wires, but the plug-in side only showed the yellow had 2.4v.......a long story short, look at this image...it works!!!
http://www.fordfuelinjection.com/pub...ternator05.jpg
and this one....
http://inlinethumb53.webshots.com/41...600x600Q85.jpg

Quit buying the cheapest CRAP you can get,mail order, from China, ...and start spending your hard earned money on quality parts, from reliable sources....like the people building engines,building cars, and making reliable power through hard work and ingenuity! You will appreciate it more and know that it works from their experience. remember: Cost Effective, not CHEAP.
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I had a 1986 thunderbird that the wiring was identical to the mustang, but am changing all my stuff into a 1988 & the new wires are a yellow w/white on right, green w/red center & black wire on left tha was cut about 4" long. Cant find diagram anywhere & don't know if I can hook the wires up like the old one. Can you help me?
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Hey SPeace-Atl can you help me out on the wiring question that I posted about my 88 tbird wiring for the G3 alternator? Seems like you are very knowledgable about that alternator but I dont know if you know the car. I know its not a stang , but it is a fox body & everything stang bolts right on.

If your not breakin' parts your not building enough power. Big cube small blocks ROCK!
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On a 3G alternator, the black/white wire is usually about 4 inches long and loops from the 3-pin connector to the single pin connector, but the locations of the 3 wires you describe do not match the locations in the diagram I posted above...

On some applications the yellow/white wire is also short in length and connects along with the 6-gauge charge line to the stud on the back of the alternator instead of connecting at the starter solenoid. This wiring is functionally equivalent to the diagram.

If you are sure that you have a 3G and can identify the wire locations by the chart above, it will work if wired as shown in the chart regardless of the color of the wires, but if it is a Ford harness, the colors should match the chart.

Stan -
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