old truck--later alternator!..
Up until 1972,GM used "external" regulators..after that,the used regulators that were built in the case of the alternator..its easy to swap the newer one into an old truck.(both bolt right in the same way)...some folks "cheat" and use the old wires at the regulator to wire up the 2 prong plug on the later style altenator...they might have done it wrong,or maybe your old wires are gangrene,shorted,or broken-here's what to do..
Turn the key on..you should have 12 volts at BOTH wires on the 2 prong plug on the alernator--use a test light,or multimeter to see..I bet the smaller brown or white wire is "dead",and has a short or break in it somewhere..no cuurent there = no charging!...
If the thick red wire at the plug is dead,look for burnt fusible links at the solenoid,and near the brake booster,there should be one at the plastic junction block there,and inspect the wire itself....the thick red wire that bolts to the stud on the rear of the alternator should have 12V constantly,even with the key off..if that wire is dead,its either cut,shorted,or the fuse links are toast..
With the engine running,put the test light from the battery positive to the brown or white wire on the plug,if it had no juice during the earlier test..I bet the alernator kicks in and starts charging,because you just gave it the 12V it needed to energize the feild coils to make it charge..
It sounds like someone "upgraded" your truck to the newer style alternator,but wired it in with the old harness,and now it has 2 regulators ,or its not getting power to the wires I just described correctly..I re-read your post after writing a whole page on how to wire up the later alternator in your truck--then I saw "new wiring harness"
...if you still have trouble ,just ask,I'll try to help..
