I have a 1999 3,8L ford mustang my tester indecates the alt is overcharging,but the batt light is coming on but it goes off! is there anything i can do to besides replace the alt?

Your alternator-mounted voltage regulator has failed. Luckily, your voltage regulator is
replaceable separately from the alternator (most voltage regulators are integral to the alternator,
and cannot be separately serviced).

Note: this works for 3.8L cobra alternators/voltage regulators only - in the 4.6L model, the
voltage regulator is an integral component of the alternator.

1999 Ford Mustang Cobra -3.8L engine

3.8L Engines

1. Disconnect the negative battery cable.

2. Remove 4 Torx® head screws holding the voltage regulator to the alternator rear housing. Remove the regulator, with the brush and terminal holder attached.

Fig. 1. The regulator and brush/terminal holder are mounted on the backside of the alternator




Fig. 2. The regulator and brush/terminal holder are secured by 4 Torx® head machine screws




Fig. 3. Hold the regulator while removing the screws to prevent the regulator from dropping outi have a 1999 3,8L ford mustang my tester - archaeology_72.jpgbrushes





3. Hold the regulator in one hand and pry off the cap covering the A terminal screw head with a small prybar.

4. Remove 2 Torx® head screws retaining the regulator to the brush holder. Separate the regulator from the brush holder.



Fii have a 1999 3,8L ford mustang my tester - archaeology_73.jpggulator out-the brushes will move out of the holder




Fig. 5i have a 1999 3,8L ford mustang my tester - archaeology_74.jpgrminal holder can be separated from the regulator once it is removed from the alternator




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To install:

1. Install the brush holder on the regulator with 2 retaining screws. Tighten the screws to 25-35 inch lbs. (2.8-4.0 Nm).

2. Install the cap on the head of the A terminal screw.

3. Depress the brushes into the holder and hold the brushes in position by inserting a standard size paper clip, or equivalent tool, through both the locatioi have a 1999 3,8L ford mustang my tester - archaeology_75.jpglator and through the holes in the brushes.

Fig. 6. When installing the regulator, press the brushes back into the housing

Fig. 7. Use a thin piece of soft material, like this plastic wire tie, to hold the brushes in position while replacing the regulator, or use a paper clip inserted in the hole





4. Install the regulator/brush holder assembly and remove the paper clip. Install the attaching screws and tighten to 20-30 inch lbs. (2.3-3.4 Nm).

5. Connect the negative battery cable.

That could be the voltage regulator and most likely is. You'll either have to replace the alternator, most likely case, or replace the voltage regulator. Occasionally, you can actually replace the voltage regulator in these but in most cases it is built into the Alternator itself and is difficult to replace. I'm not sure if the 99 ford alternator had voltage regulators you can access on the outside of the alternator.