wiring/charging issue

Do you have an internal (alternator mounted) regulator or an external regulator for say, a 1 wire alternator?

If you're backfeeding through the alt then the diode in the regulator is bad. Doesn't matter if it's internal or external. I just figured you might be running an external if you've replaced the alt a few times already.

where did you get your alternator from, is it a rebuilt, or new ones that you are replacing>?
I recently had the same problem. I bought a alternator from advanced auto parts about 4 months ago, and it went bad when I got to the track to do a little drag racing. I made it home barely. Went to advance auto the next day got another one under warrantee, put it on. Didn't drive it till today, but when I started my car up it sounded like I had a vortech t-trim blower on it......so I went ahead and risked it going to work. Every time I got on it and took it past 4000rpms my volt gauge on my dash went down to about 12volts, lights went dim and lost power, but as soon as I got out of it, it charged like normal. So I went back to advance today I got my 3rd one, but upgraded from the standard rebuilt unit to the fully rebuilt unit, I'm gonna see how this one works out.
Anyway point to my story here is. I got the cheapest one I could the first time, they (advance) said it was rebuilt, but the only thing thats rebuilt on the standard alternator it what ever initailly took it out in the first place say a winding, or stator, or regulator. On the fully rebuilt one everything is new except for the housing.

Remove the alternator and have it checked. Most auto parts stores will do it for free. The problem
sounds like a bad diode inside the alternator.

If the alternator checks good, look for voltage on the small Lt green/red wire that plugs into the alterantor.
With the key off, there should not be voltage present on Lt green/red wire. With the key on, you should see voltage.
That is the wire that tells the regulator to turn on and supply power to the field windings. If there is voltage with the key off,
someone has miswired the ignition switch. See the diagram below.

Diagram courtesy of Tmoss & Stang&2birds


Before putting a new alternator on the car, check the secondary power ground 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. [
B]Any car that has a 3G or high output current alternator needs a 4 gauge ground wire running f
rom the block to the chassis ground where the battery pigtail ground connects. [/B] The 3G has a
130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the
ground side handles just a much current, so it needs to be 4 gauge too.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif