No charge is when the Battery light comes on?
Have you tested the voltage at the battery at various RPMs
I have never heard of an alternator charging less as RPMs go up, opposite yes, all the time.
Ford added "Smart Charging" at some point in the early 2000's, not sure about your year.
These had a Smart Junction Box usually near the GEM(generic electronic module)
Couple of fuses in there for the charging circuit.
Alternators are simple to understand, voltage regulator feeds the rotor 5 to 9 volts, as the engine spins the rotor, an AC electric charge forms in the 3 Fields that are fixed in the alternators case.
Each field has 2 diodes that changes the AC volts to DC Volts, and this goes out to the battery/vehicle.
The amount of power sent to the rotor by the voltage regulator dictates the voltage going out to the battery, voltage decreases as RPMs go up, and voltage increases as power loads go up, like when headlights and heater fans are are turned on.
Regulator tries to maintain 13.6 volts while engine is running.
I have seen voltage regulators fail with 0 volts, so no power out, or get stuck on 5 volts so only provide above 13volts at higher RPMs.
Fields/diodes can fail, this would make less voltage at low RPMs but good voltage at higher RPMs
Last edited by RonD; 01-23-2015 at 07:06 PM.