I have a 66 with a external reg. and the problem I am having is I am having some sort of a charging trouble. I have around 13.5 volts at the alt. when idling, but only about 12.5 at the battery and the amp gauge is showing a draw. This problem started after putting in an electric fan, but I completly disconnected it out of the system to how it was before, and I still have the problem. I have the 13.5 charge all the way to my main fuse before the 8 gauge + wire goes to my starter. At the starter I have the 12.5 just like the battery. My car was fine and the amp gauge showed a charge at idle untill I mess with the fan. Did something go wrong with the starter or battery? any help would be greatly appreicated. I have already spent about eight hours trying to figure it out.

This may not help yours, but my 66 Malibu still has the separate externally regulator, and every now and then, it quits charging, and the dash light comes on.

So far, wiggling the connector at the regulator, and cleaning the terminals has fixed it.

Only other suggestion is to clean all the connections, especially the grounds. Been a Ford guy forever, and grounds are usually the problem with old Fords. The 66 has some grounding issues with the taillights. I also ran the main power wire from the bat to the dist block vice the starter.

The MAD enterprises (I think) has an excellent explanation of how the GM charging system works, with the sense wire tied to the distribution block.

Only other time I had this, the alt lost a diode and was not charging fully.

I will keep checking all the grounds. I did have the alt. off the block and set it down on the wheel well with all the wires hooked up. btw I have a painless harness with it hooked up to their specs. I either grounded something out, our bumped something loose cause everything was fine till I started hooking up the fan.