Good point. wait till it gets good and dark & pull a batt cable & touch it back to the post & ANY spark is too much & as said means there is an unwanted key off draw. (unless you have a windup clock or a lighted digital display on a stereo etc & those are not near enough to run it down unless for a long period of idle time) which I don't think we are dealing with here.

Battery goes dead while vehicle is running. I ran a full field test today and the new alternator was putting out 15 volts and climbing. Now, for some reason, I don't have voltage to the sensing line. I did pull the bulkhead connector plugs today to examine them, perhaps I didn't get them back in good enough. I will check it in the morning.

A working alternator needs two things to generate electricity.

On a dual field terminal alternator you need battery voltage on one connection and ground on the other. The VR will vary the ground to control the battery voltage. The VR has two inputs, one is battery voltage, and it should be within a tenth of a volt of what you read at the battery, the other is a ground through the mounting bolts. The second wire at the VR goes to the alternator field terminal and it the control mentioned above. The large stud on the alternator is the output.

So with the key on, engine off, you had better read battery voltage at the VR and the alternator field terminal, usually this is a blue wire. They had best read within .1V of what you read at the battery positive, using the same ground, I usually use the battery negative post. If you jumper on field terminal to battery + and the other to battery - (stock wiring disconnected) the alternator will generate power out. As a very short term test this is ok. If either of the wires from the VR to the alternator doesn't have the correct signal on it the alternator will not work.

On a single field terminal alternator, the ground is handled thru the brush mount and the VR controls the battery voltage to the remaining field terminal, usually a blue wire. On the old mechanical regulator setup it simply switches the battery voltage to the field on or off. Electronic versions are a bit smoother than that. Disconnect the wire from the one field connection, jumper the terminal to battery + for the short term test.

Super, good description of testing the two styles but possibly confusing to the op.

Op, your truck originally had a dual field alternator, follow the first part of Supers post.

I might have to make a video, but I only have the single field terminal, mechanical setup. Be a good start, just gotta stick the engine back into the Cuda.

This morning I started tracing wires. I was checking for voltage at the alternator, when I found a small red wire coming out of the harness, not connected to anything. Looked like the wire had broken. Checked and it had power with the key on. Connected it to the sensing wire to the VR and the charging problem is solved. How much is too much voltage at the battery? I'm now getting 14.5 volts.

14.5 isn't too high especially with a run down battery. I'd run it for a bit to get a charge on the battery and recheck.

Most Mopar are around 13.5-14 with a charged battery.

Great, thanks. I was getting 12.8 at the battery with the key off, so I didn't think it was too high. Thanks to all for the info and great ideas. Had I found this wire to begin with, I could have fixed it in 5 minutes. However, I am now pretty familiar with the wiring and especially the charging system on a 77 D100!

12.8V at the battery, engine off is pretty much a fully charged battery. So looks like you are good to go.

I'd monitor it for a couple days. He may be on the verge of overcharged.

BTW, 12.8 volts at the battery sounds like just a surface charge. Turn n the headlights for a minute, turn them off and recheck battery voltage.