I've just repurchased my 67 RS (the one I sold in 1976) and am working through a few bugs in the electrical system. I've read some of the threads here and hope I can get some guidance on what's going on.

All of these measurements are at the Battery. I see about 12.5V when the engine is off.

My GEN light is on when idling cold, but goes out if I rev above 1000 - 1500. Measuring at the battery, I see about 13.5V. Once the engine has been running several minutes the GEN light no longer lights at idle. Voltage still about 13.5V.

Turning on the blower fan, all is well until I hit high speed fan. At high speed, GEN light is on all the time - engine RPM makes no difference. At this time, I still see about 13.5VDC at Battery. But turning off the fan I instantly see charging happening as voltage jumps to nearly 15V for several seconds, then drops back to 13.5V. (GEN light goes out as soon as fan turned off) I can hear the change in sound as the voltage drops back from 15 to 13.5V.

If I turn on headlights the GEN light comes on and voltage gradually increases to about 15V. Interesting this doesn't jump up quickly. After it has been at 15V for 30 seconds or so - you guessed it - voltage drops back to 13.5V and GEN light goes out.

Car cranks and starts first time every time -- but with engine off and headlights and fan on high, I see Battery drop to about 11.5V. Is my problem actually a weak battery and nothing else? Interesting to me that with headlights on I see the charge occurring, but with fan on high I never see charge happen until fan is turned to a lower speed or completely off.

I'm awaiting the arrival of my wiring diagram which is stuck in USPS black hole for now, so I'm flying a bit blind for the time being. All guidance is much appreciated. (Several other electrical bugs <interior lights, cig lighter> have appeared which don't seem to be associated with the charging problem)

 

You don't have a weak battery. Voltage drop with engine off and running fan and headlights is normal. Your GEN light issue can be one of a few things. First may be a bad VR. You might consider replacing with a solid state one. If you want to keep original appearance, the cover rivets can be drilled off the replacement VR, and the Delco Remy cover from your old one screwed on. Could also be a bad diode in the alternator or a broken wire from the stator coil to the diode. Thought I'd seen everything until that last problem hit my 65 Vette. The ammeter showed it discharging at idle, and it just wouldn't produce enough to keep from discharging the battery with a bunch of stuff on (lights, fan). Pulled the alternator apart and found the broken wire. Jeez. Could also be a broken wire between the VR and the two-pin connector on the back of the alternator. Had that happen on the 65 Vette, too. Broke inside the insulation so it wasn't visible. Looked at it in the dark and saw sparking inside the insulation! Sum it up - 54 year old wiring can do some weird things.

 

Your symptoms, BIB, read normal. OE systems were intended to be started and driven for 15-20 miles at <2000 RPM.
As suggested, you may replace the mechanical VR for an electronic type and/or replace the present alternator for a 63 amp vice 37 amp.
Yes, 54 year old wiring can do strange things.