alternator discharging under load

[3024V]

I AM THE ORIGINAL OWNER OF A LOW MILEAGE 1987 GT. WHEN I TURN THE FAN SPEED TO THE HIGHEST SETTING THE AMPERAGE DROPS ON THE ALTERNATOR AND THE ENGINE RPM ALSO DROPS. IT USUALLY REBOUNDS BUT NOT TO WHERE IT IS NORMALLY WITH THE FAN OFF. IT DOES NOT MATTER IF THE CONTROL IS SET FOR HEAT OR AIR CONDITIONING. I HAVE REPLACED BOTH THE ALTERNATOR (ADJUSTING IT) AND THE BATTERY WITH HEAVY DUTY ONES, BUT IT MADE NO DIFFERENCE. ANY HELP IS APPRECIATED. THANKS IN ADVANCE.

 

[YawnY]

Voltage regulator is probably sticking.

 

[4rdnut]

The wires that plug into the alternator could be the problem, check to see if the rubber plug gets real hot, there was a recall on the alternator plug where resistance would build up in the plug and catch fire.

 

[Mustang5L5]

What sort of state is the rest of the electrical system? Might not hurt to freshen up all the connections by replacing the terminals, cleaning the connectrions on the solenoid and just overall restoring the electrical system, especially if the charging system is testing out fine.

1987 is a unique year in that the 5-spds got a 65A alt and the AOD's and 1988+ Mustangs got a 75A alt. So a little voltage drop at idle causing the idle to drop is somewhat normal. But if it's doing it badly, you are losing a lot of voltage somewhere else

 

[HISSIN50]

You might also want to have the alt tested for a dropped diode. Alts right out of the box have failed a load test.