Have a 66 SS, dual electric fans, new 100 amp alternator, have not changed regulator. My problem is when I start engine, voltage will jump to 14 volts, when the car has heated and fans come on (with headlights, full load etc.) the voltage drops to 13 (seems normal) when cruising, and just below 12 when at idle (seems low). Should I adjust regulator up, get new regulator, leave as is, or could something else be causing problem?

In my opinion the best thing you can do is to replace the alternator with an internal reg alt. I like the 108 A CS144 . The advantage to this alternator is that it can drive all your accessories at idle. I don’t know if you have self service salvage yards or not. The last alternator I bought cost me $7.50 including the remote sensing connector.

The alternator you have will work fine, voltage drop at idle is a characteristic of the first design alts.

How have you connected the fans (to the battery, horn relay)? Did you upgrade the alt output wiring? Where are you measuring the voltage? Trying to determine if you’re overloading some poor little wire.