I have a 1985 R80RT that just wont charge in Mexico and I can't figure this out. First time was 5 yrs ago rideing thru Mex in 115 deg temps, the elect system would only put out 12. 5 V. Return to Colorado at 7500 ft and all was fine, 13.6 V. The next year same thing, 2 days of rideing and no start without a charge, return to Colorado alls well. Next year same thing, charge every 2 days. Now I have moved down here and it's time to fix this, today the gen light came on at 60mph for the first time, stayed on for 15 min at speed, I revved the motor and light went off but still only 12.6 across the battery terminals at 3500 rpm. What should I check first? Any suggestions?