1988 mustang gt 5.0 still pulling power when car is off killing batteries new alt but need to know where a third wire goes on the three prong plug on top of alt i know two go to batterie but where in the hell does the tthird one go

It goes from the battery to a bolt on the end of the box then straight to the alternator through a fuseable link protected wire. Another wire goes through the fuse box, but it has only a 20 amp fuse. Third wire goes to the instrument panel.

If you can,go to Autozone and have them test your alternator and battery,it's free and only takes a few minutes.

The ECU has no control on the voltage. Does your vehicle have a separate voltage regulator? If so that's your fault. A lot of alternators have the regulator built into them. The alternator needs to be put on a test rig away from the vehicle and the output tested, this will eliminate the vehicle, You could disconnect the wiring to the alternator, run three fly leads to the alternator and to a separate battery and run the engine and test the voltage on the spare battery. Wire 1: Battery neg to case of alternator Wire 2: Battery pos to + terminal on alt (the thick wire) Wire 3: Battery pos through a 5 watt bulb to the ind terminal (ignition light) 17 volt will blow another battery for sure.

The Charge lamp comes on with the Alternator fails to keep battery charge going. (lots of way to fail, and ALT Is 100% good) it's a system. each part of it must be good, end to end.
so...
i put my voltmeter across battery, i start car, i see 13.3 to 15vdc, alt is good.
no? , i see now, 12.2v. ALt is dead, (i didnt say bad) and battery is sightly discharged, 12.6v is charged. (key off, rested voltage)
This 12v tells me the ALT is shut down. only why is the question.

how can any charge system fail,
bad battery tops the list here, they LOVE to fail.
1: belt slips.
2: belt missing
3: bad alt.
4: bad wires to ALT . (corrosion on any wire or lug)
5: fuses blown , any listed below.:
your alt is not an ECU controlled, alt ,its 100% self contained

did you ever remove the ALT and blow the 100 amp fuseable link
the first step of any alt pul, is the pull the NEG, battery lug or BOOM.

im logged in to alldata.com so can you.

and what do i see.
a 5 wire ALT. one is the frame ground, so it must not be rusty or..... it fails. frame to engine block. no rust here.

wire L , yellow.goes to dash LEDs (alt failure idiot lamps) (this works)
wire. S white goes to ALT 5a fuse, is it blown?
wire :B B/G color, goes to the 100 amp link fuse. (blown>?)
wire IG Red-white, goes to a fuse marked 10A gauge.

i think the IG wire is the voltage sense wire.
no sense, no charge. I think the whole I/P dash cluster goes dead when this fuse blows.

The S wire is the boot strap voltage sense, it wakes up the regulator .
if open , the reg stays asleep, as does ALT.
Alternators can not self start, the regulator has boot strap device
that cures that. called self excitation. so this wire is critical.
All wires except the L wire is critical. in this case.

the B wire is the big wire, it charges the battery . 50amps or more..

any of the 5 paths open , kills the ALT dead.
The REG inside the ALT can also shut it down.
eg: battery bad, with huge cell resistances. total.
eg: overvoltge for any reasons. including attempts top run car with no battery, (dont ask)
eg: over current (battery cells 1 to 6 shorted) or 25 KC lamps added.
eg: over temperature shut down.

that is about it.

First, take several pictures of the serpentine belt arangement so you know how that goes back together.
Here are the steps I recommend :
- 1 ) Remove negative cable (black) from the battery.
- 2 ) Loosen the serpentine belt by pulling back the belt tensioner( you may need a special tool)
- 3 ) Remove the connector plug and( red) hot wire.
- 4 ) Pull the bolts out holding alt on bracket.
- 5 ) Pull out alt.
How do you know that the alt is bad ? A auto parts center can check it for you.
What are you replacing it with, new or used.
Please let me know how it goes.

for the most part its under the dash on the driver side give these websites a try www.autozone.co and www.alldatadiy.com if all fails stop by your local Ford dealership in the parts department and they will print you out pictures and diagram showing you where its located wish you the best Michiagn Man.

So, you have an integral regulator on your alternator.... so you have to connect the output of the regulator to your battery. You also have to connect between the regulator and the alternator. Hmmmmm how to describe....
using your descriptions of wires and prongs.....
Connect Wire Color to Prong

Yellow (w/wht dot) A+
White (w/blk tracer) STA
Black (w/orange tracer) Bat (alt output)
Ground wire Grd
Orange (w/Lt.Blue) F (field)
If battery was connected, the Black with orange tracer wire would have +12 Volts on it at all times

is your a 12v or 24v some imports are 24v and engine parts are different thanks brian

3 wires for the altenator all controll the voltage regulator. You need the ground as said and one is 5 volt constant and the other is 5 volt switched.

I had this problem and in the trunk on the driver side pull the inner panels down. There should be a little box and a 2 prong plug beside it. You need to bridge the 2 prongs on the plug over and the doors will lock then unlock. After they do then use the lock button on your remote and this should fix the problem. Good Luck