[tincan1966]

If the fan is running, you have about a 15-20 amp draw on the system, so the alt is going to kick in to keep up. If still running an external alt system, you may actually lose voltage at idle with the fan running. I'd suggest about a 63 amp or higher alternator with internal regulator, unless you've already done that conversion.

I am sure there are other fixes with external regs, but I am not familiar with those, I always replace with internal reg alts.