Voltage Regulator ?

[boxrodz]

Well, if you have a one-wire alternator, then you can ignore the wires leading to the terminal harness since the only wire used is the larger battery wire (from the BAT terminal of the alternator to the battery). Typically, an one-wire alternator is has a special regulator that that senses a certain minimum RPM before activating the alternator. You'll have to bring up the RPM a bit to get the alternator to turn on. That's the downside of one-wires. You gotta keep the RPMs above a certain threshold to maintain the ~13.5 volt output. Take the truck out and slightly rev'ed the engine (in gear) to see if the alternator 'turns on'. (It sounds like it's turning on when you have the engine idling in PARK.) It should drop down again, once the engine RPMs drop, like at a stop light. The only wire to check would be the wire between the alternator and the battery.