The first thing you need to look at is the voltage on the large, bolted-on output terminal on the back of the alternator. That has to have full battery voltage. Did the truck come with a 100 amp alternator originally? If it did, I have to follow a different part of the wiring diagram.
We can forget about half of the output circuit because besides going back to the battery positive post, it feeds a number of other circuits through their individual fuse link wires. Those aren't dead, so we know the wiring is okay up to that splice. There is a five-pin connector that the output wire runs through. The two smaller wires, red and green, go through it too. Look for that connector and measure the voltage on the black output wire on both sides. Overheated connector terminals are the weak point in the circuit and are good places to find problems.
Once you get full battery voltage at the alternator, if it's still not charging, recheck the voltage on the green wire. If it's nearly the same as on the red wire, unplug the two-wire connector from the voltage regulator, then measure the voltage on both terminals in that plug. The red wire should again have full battery voltage, and in this condition you should have full battery voltage on the green wire too. If you do have those two voltages, turn on the head lights so you have a visual indicator to watch, or put the voltmeter right across the two battery posts, then use a jumper wire to ground the green wire in the regulator's plug. That will "full-field" the system and make the alternator charge wide open. If you see battery voltage rise a bunch, or the head lights get bright, and you hear the engine load down from the alternator working hard, remove the jumper. That proves the entire circuit except for the regulator is working. It is important while full-fielding that you do not raise engine speed above idle. All generators are very inefficient at low speeds, which works to our advantage during this test. If you raise engine speed, it is possible for system voltage to climb past 16 - 18 volts. That will destroy the radio, any lights that are turned on, the ignition module, and on newer vehicles, some of the many computers. You only want to do this long enough to verify the alternator and its wiring are okay. You can ground the green wire at the alternator or at the regulator, but it's better to do it at the regulator for two reasons. First, doing it there includes the green wire in the test, so we'll know it is okay too. Second, on newer Chrysler products, the two small wires on the alternator go through a black plastic block with noise suppression circuitry inside, and come out on the two terminals. There is no way to know which terminal corresponds to which wire, (unless I share my secret later).
Wednesday, June 23rd, 2021 AT 6:49 PM