At wits end with this problem. '68 SS, charging system in inoperative. I tested and figured it was the regulator, since the alternator would full-field. New regulator, same problem. I even rewired from the regulator to the alternator pigtail, thinking the old stock wires may have broken. Same problem. I take apart the alternator, it seemed that the plug on the back (on the inside of the alternator) was loose. I replace the alternator. Same problem. Battery is also new within a few months. This problem just came out of the blue. I have power on the battery terminal on back of the alternator, no power at all on either wire on back of the plug. What gives?

Oh yeah, I forgot to add, my GEN light does NOT come on during any of this, even though the battery is near dead and the alt. is not charging. Is this problem related somehow?

Do these tests , then post back

This was written by another member, a very good write up on how it works and how to troubleshoot a externally regulated charging system.

I dug this out from before, hopefully it help. If not, just post or send me your questions.

I'll try to provide a simplified explanation of how the externally regulated alternator and regulator work here. This explanation starts from a parked car with engine off state.

The regulator brown #4 wire is connected through the light bulb to 12V whenever the key is in the ON position. The other end goes through a set of points in the regulator and then out the F terminal to the field winding in the alternator. So some current begins flowing in the field winding from this connection. The little bit of current flowing produces a weak magnet field in the alternator and also lights the bulb.

When you then start the car, the weak magnetic field from above begins rotating which makes the alternator producing a little bit of voltage. This voltage is fed from the alternator to the regulator on the white regulator #2 wire. This wire connects to a coil in the regulator and turns on a contact when the voltage goes above 3.2V.

Once the contact turns on in the regulator the #3 and #4 terminals are connected together. Terminal #3 is battery power so this puts battery power on both sides of the bulb and turns it off. This means the brown #4 wire should go to +12V. Also, this connects the field terminal right to battery power but still going through the points.

When the battery voltage goes above a set level, the F terminal is disconnected from the battery power by the points I kept mentioning. Then the voltage drops and the points close again. This on-off cycling happens rapidly and is how the voltage gets regulated.

From the above (KOEO = key on engine off and KOER = key on engine running).
F or #1 terminal (field) - KOEO = #4 slight voltage, KOER = 9-12V typically
#2 terminal (sense) - KOEO = 0V, KOER = >3.2V
#3 terminal (Battery) - KOEO = 12V, KOER = 12V
#4 terminal (light) - KOEO = F slight voltage, KOER = 12V

By 12V above I mean battery voltage, which may be 12V when engine is off but could be up to 14.5V when engine is running.

Check the light: Ground the brown #4 terminal wire at the regulator. When grounded the light should come on. If you don't have a light then skip this test. Test this with the connector off the regulator.

Checking the alternator: Jumper the blue wire F terminal to the battery post on the back. This should make it easily crank out 16+ volts. Next, connect a troublelight between the battery post and the F terminal. The light should come on and you should measure > 3.2V at the other alternator terminal. If it passes these tests, then it's in the regulator or wiring.

Checking the wiring: At the regulator connector jumper the brown light wire (#4) to the F terminal (#1) and you should be able to measure >3.2 volts on the other alternator terminal or terminal #2 of the regulator connector. If these tests pass, it is in your regulator.

When doing these tests, make sure you turn off or pull the fuses for any added electronics that you can. The alternator test can produce enough voltage to damage stuff.

Peter

Well, turns out my NEW $29 (insert parts house name here) alternator is bad. I can't get it to full-field by jumping the regulator F to 3 wire test. The light works fine when you turn the key ON, it just doesn't light when the car is running. Had to jump it to start it and find out the alternator is bad. Oh well the only frustrating part is that, of course, said parts store is not open today (Thanksgiving) so it has to wait until tomorrow. The old alternator would full-field, but the connector on back of the alternator was coming loose which was why I replaced it. I guess you get what you pay for.

Another option is to convert the charging system to a internally regulated one. This is very easy to do and is much better than the old external system. Searching this forum will have a lot of info, or post back if interested. Personally on any car that I owned that had a external system, if something needed to be replaced, I converted it to a internal system.