1988 XJ6 just started "over charging", when this happens charge voltage goes up to 17 and headlights get very bright. After a moment everything goes back to normal, then repeats often. Is this the altenator or what....help

 

Get this sorted PDQ. 17volt on your system is likely to provoke a sudden electronic failure. This is MISERY. As you say, the alternator is the culprit if the regulator sytem is contained within it as it usually is. On some cars, the regulator is not inside the alternator. The idea is to maintain 14.4volt at the o/p terminal and of course the battery under a constant-voltage charging system, appropriate for lead-acid batteries.
BUT -- sort this immediately.
Leedsman.

 

Thanks for the heads up! I keep thinking it must be the altenator and you can hear a sort of whine the same time as the voltage increases. I'll change it out ASAP