Sounds like you possibly have a high resistance somewhere in the wiring. A good place to check is where the grounds come together. Especially check where the battery big ground wire connects to the frame. You probably have the battery terminals clean. Some alternators have a sense wire that goes to the main buss somewhere. This is where the alternator gets its reading on system voltage and controls the voltage of the alternator,it needs to be tight and corrosion free.Check for loose connectors or bad looking corroded ring crimp terminals on the main power carrying wires at the alternator and your main power distrabution area. The size of the alternator pully does not matter unless it is too big..but then you would have low voltage. If the system voltage is higher than normal the lights will use more power which could cause your breakers to trip out. If whatever is causing your overvoltage gets worse it could damage alot of your electronics. A runaway malfunctioning unregulated alternator can put out 100 volts which fries lots of stuff. Connecting a volt across the batteries or the main buss will tell you better than the installed one. The max should be not much more than 14 with the engine running 14.2 is tops and it should be fairly steady. It may drop some as you turn on fans and lights. Our combine had an overvoltage conditon that fried alot of printed circuit boards. It has the 24volt start system so there are alot of connections and contactors to check but it turned out to be a bad regulator in the alternator that was intermittent.
