I need some help with a charging problem on a 04 gt

Russ Cochran

New Member
Nov 26, 2016
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North GA Mountains
I have looked thru the threads on here and found some good information, but nothing has worked! Okay, here is what is going on and what I have done. I have completely rebuilt a 04 gt that I bought wrecked. I did an engine swap, added long tube header, EVE intake, cold air kit and car has x pipe with PIPES muffler. I put a 130amp alternator and a 765cca 95r battery.
Car cranks, runs, but the alternator isn't charging. I have what appears to be dead short somewhere, but where?
Battery is full 12,5 amps, as I said brand new alternator (tested good off the car). Battery light on dash is on. Check voltage on Alternator connector on top connector 0. Take ground off of Battery, put probe on ground post and then the alternator reads 12.45? I have traced the ground from the battery to the block and it looks fine. Traced the positive all the way to the starter, still nothing.
Next, did a fuse test with with test light, power to all fuses and all fuses good.
Then I decided to check for parasitic drop, took ground off of battery, hooked up DMM and DAMN IT, 12,4v on volt side and 0.15amp on other. Like I said before, touch probe to ground post of battery with out the cable and the power goes to alternator.
ASK me any question and I will try to provide what I have done.
HELP!!!
 

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wmburns

SN Certified Technician
Aug 14, 2009
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The first problem I see is that the motor grounding strap from the left hand motor mount to the car's frame rail is just hanging unconnected.

When you said that the top connector has zero volts does that mean the large main B+ connector or the small LG/RD wire?

Are you positive that the correct alternator has been used for this application? Ford makes several types of alternators with different control systems that look the same externally.

IF the LG/RD and YE/WH wire has zero key on volts THEN that is why the alternator likely isn't putting out. For a test you could try taking a fused +12 volt jumper from the battery positive to the alternator's LG/RD terminal. IF this makes the alternator to start charging this "proves" why the alternator isn't charging.

The next series of tests will help narrow down where the circuit is breaking down. The LG/RD wire starts from fuse F2.5 and then through the ballast resister and battery warning light in the cluster.

Confirm there is +12 volts "always on" power at fuse F1.20. And key on power in fuse F2.5.

1999-2004 MY fuse panel schedule:
https://www.allfordmustangs.com/forums/4-6l-tech/1077091-2000-gt-blown-26-fuse.html

The next steps will depend upon the above test results.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/
 
Last edited:

Russ Cochran

New Member
Nov 26, 2016
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1
60
North GA Mountains
Okay, once the K-member was reinstalled after the long tube headers I was able to connect all the frame grounds. I have checked to make sure all the grounds are connected. I did a continuity test on all the fuses like I said, from ground to hot, everything is showing connections. I haven't done a key on test for 12v. The thing I don"t understand is, when I have the ground strap of the battery, reading from the negative side of the battery post to the alternator is 12.4 but the ground back on and it goes away.
Any idea's .

thanks,
Russ
 

Russ Cochran

New Member
Nov 26, 2016
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1
60
North GA Mountains
Oh, I meant to mention the large wire on top of the alternator show 12.4 when ground is disconnected and I do the voltage test from the ground post. WHEN both cables are on the battery nothing on the alternator?
 

wmburns

SN Certified Technician
Aug 14, 2009
5,811
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Houston Texas
I gave you my best ideas and tests based upon solid trouble shooting and experience helping others. Need to know IF there's key on voltage at the small LG/RD and YE/WH alternator control wire. The alternator needs those control circuits to tell it to turn on. The LG/RD circuit is used to turn off the cluster battery warning light.

I also gave you a valid work around test that only requires a short piece of scrap wire.

It doesn't help to mention again some odd operating condition with the ground wire disconnected which is not a valid way to test a modern alternator.

Otherwise I just need to drop off.

Here's someone else that was able to trouble shoot the LG/RD battery warning circuit with an open in the cluster.
https://www.stangnet.com/mustang-forums/threads/96-altenator-issue.909742/
 
Last edited:

Russ Cochran

New Member
Nov 26, 2016
13
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1
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North GA Mountains
Oh, one other thing I forgot to mention. I replaced the factory fan with twin electric fans, not sure if that has any impact or not? The original radio was missing and replaced it with aftermarket blue tooth radio.
 
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