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I have replace my Battery and alternator on my 2000 mustang GT. But the batttery keep draining down and will not charge

I can jump it off, let it run next day same thing, don't know what to do next in GA.

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Get a wiring diagram free from Just register your car and look at the wiring diagrams and fusebox layouts. Sometimes the charging circuit has a fuse or uses a Relay in the Powerbox.

Hope you did a benchtest on the old unit and sometimes is a good idea to test the replacement. The Alternator amplifies the voltage and uses the small brush wires to provide power to the stator. The ignition key will either directly feed the brushes or activate an intermediate Relay to power the brushes. If you lose this power to the brush clip, the Alternator will not work.

The drainlng issue can be another matter, aside from bad diodes in the replacement Alternator. Sometimes the keyswitch will simply not turn off the power. You could also have a stuck Relay. The ignition switch turns on several Relays each time you start a car. They are suppose to disengage when power to them is cut.

Check a switch on the Brake pedal. Usually it interrupts the starter from working, but the switch can easily be smashed by a drivers foot. There is no telling how it shorts out and what circuits are crossed. It could be responsible for the drain.

At this point you need to get the Alternator working or have it bench tested. You can temporarily limit the overnight discharge by disconnecting the battery. Then connect an Ohmmeter across the 2 disconnected battery cables.

Turn off every control. The main computer and clock should be all that is on. Review the fuse diagrams and remove those fuses. Now everything should be off. There should be no "loop" possible as everything is switched off.

If the Ohmmeter shows something is still connected, that will be the short. Have a helper disconnect each fuse and return same to holder. When you find the circuit that is the problem, the connection will break and the meter will drop to zero or blank. Then investigate that circuit.

Please rate my info.

Posted on Dec 17, 2012


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Posted on Jan 02, 2017

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SOURCE: 96 ford mustang gt charging problem

I have had nearly exactly this problem in a different car. The starter was the culprit. It was worn down to a point where it was just threshold acceptable electrically and mechanically to turn and do its work. Yet it drained very high amperage during this work. If you have the time, remove the starter and disassemble it. Then cleant the copper commutator thoroughly using ScotchBrite pad, diesel fuel and strong hands.Then ensure the brushes are long enough and brush springs are ok, then clean brushes and holders with a stiff brush and fuel. Then blow dry with compressed air the run a bench test...clamp the starter down and apply a well-charged car battery and an ammeter. It should only draw about 5 to 10 amps steady after startup, but much heavier at startup under load...could be up to 500 amps depending on your bench test....difficult to apply loads here...main point is low current under no load.
A second possibility, other than a wire harness default, is the car's computer has gone bad...try a diagnostic code checker...

Posted on Jun 08, 2008

  • 6982 Answers

SOURCE: My battery keeps dying but my alternator is fine

First, DON"T check your alternator that way anymore. It's not good to do on newer systems! The best, quickest check I can tell you is to put a volt meter across the battery. Running with accessories off you should see voltage of about 13.5 to 14.5.
With all accessories and headlamps on high turned on, it should not drop below 12.5. (and that is the absolute low end!).
It sounds like something in the car is staying on. check the glovebox light and interior light delay and make sure they are OK. Otherwise, you will have to start checking entire system for a draw that's taking out the battery overnight. To check, you can pull one fuse at a time and see if draw goes away, then check system that fuse serves.

Posted on Jan 07, 2009

  • 6674 Answers

SOURCE: slow to fast battery drain problem when engine is off.

If you connect a 12VDC test light in line, between your + battery post & + battery cable,(cable off battery obviously) the light will stay lit brightly as long as there is a draw. Keep in mind your radio will cause some due to memory function, clock etc. Pull stereo/radio fuse to eliminate that draw, then start removing fuses, and putting them back, watch light. If it dims, or goes out, you've found the source of your current draw. I have had to resort to pulling them all,(after making notes for anything I needed to remember about where they were) to get light to go out, then re-installing one by one & watching light. When it comes on nice & bright, note which fuse slot, and pull it back out and continue on, in case of more than one draw.

Posted on Mar 01, 2009

  • 477 Answers

SOURCE: 2003 Ford Mustang GT Battery Light

Your alternator should power everything when engine is running. Battery is only for start up. Test voltage with a multimeter when engine is running , should be 14V or more . Any less indicates low alternator output.

Posted on Jun 03, 2009

  • 1 Answer

SOURCE: Got a 2000 Cougar that has a charging problem, new


Posted on Jun 16, 2010

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I am seeing a lot of mustangs are having battery draining issues and I wish to share some simple solutions found while working on a friends 03 GT 4.6ltr.

In a quiet area, key switched off, hood up, Listen carefully close to alternator. I heard a high pitched hissing/whistle sound at low volume, came from back of alternator. This sound was the voltage regulator/rectifier staying on when key is off due to faulty diodes inside the voltage regulator/rectifier circuit, can cause battery to go dead after car not ran in about 2-7 days.

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