Mystery remains why the voltage fell off while driving and it wasn't the alternator. Whacking the charging solenoid seemed to make the voltage jump back to 14v, and all was good for the six hour drive home. But whacking that should not have made any difference.
More like you drove through a big puddle and the belt was slipping? Then it dried for the rest of the trip after you stopped?
What is the situation now? If not driven since, you could do some tests of the engine charging system.
a. disconnect the house side wire from the solenoid so there can be no confusion
b. start and run the engine.
c. observe voltage at the battery too, not just at the dash in case they are different (dropping at the dash but ok at the battery eg)
Is there something between the alternator output and the engine battery in the charging circuit that could have been going wrong but the alternator was ok?
Did whacking the solenoid fix that by vibration never mind what it did in the solenoid?
After whacking the solenoid, you now maybe got house voltage to the dash, but the engine batt still low. (they should have been ganged with voltage somewhere between the two voltages) Did it stay at 14 all the way as though house batts being charged at 14? Solar?
Or was that 14 from alternator ok to both sides?
If all seems well reconnect the house side of the solenoid but disconnect the little ignition wire so the solenoid is open (as it might have been in pre-whack stage for some mystery reason). Does engine voltage fall off like it did? If so, does reconnecting the ignition wire close the solenoid (without having to whack it) and does that make the voltage come back up? All to see if what happened before is repeatable.
