2003 Ranger 4x4, 4.0L, 162K miles.

Problem started slowly about a year ago. No charge over 4K RPMs. Fast forward to yesterday and at 3K it would stop but pick up once RPMS dropped. By the time I got home from work anything over 2200 RPMs cause a no charge situation. Would you all suspect the ALT since it is RPM related and not sudden no-charge situation?

I would normally do step-by-step diagnoses but I'm leaving town and I need it fixed before I leave so I don't leave a dead truck for someone else to deal with. Basically I have saturday afternoon to fix it. Thanks in advance.

 

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

 

This was the first time I've seen one fail like that as well. I didn't have a working multi-meter before I swapped the alternator but I could see the gauge drop and lights dim as the RPMs rose.

I was able to find a low milage used motorcraft unit for sale locally so I bought that and swapped out. Truck is back to normal with 14.14 volts at idle with everything on. Since the core was only $5 I wanted to research how the units work and see if I could fix this as a spare.