Your 13.3 reading is in line with what my cars show with both headlights and blower on.

Max output is supposed to be 14.8 volts, according to the Bentley manual. Mine puts out 14.7 according to the dash gauge and a multimeter. Your 15.5 volt readings are way too high.

I would check the electrolyte levels in your battery ASAP. I suspect you will find them low, due to overcharging and consequent boil-off. Top off the cells with distilled water (fill each tube until you see the meniscus form as the level reaches the bottom of the tube). Then pull your regulator, make sure all contacts are clean, and clean the blue alternator ground wire connections. Retest system voltages. If you still showing numbers above 15, you likely have a bad diode and will need a new alternator.

This overly-high voltage scenario happened to me recently with a car I fix for a friend. A replacement used alternator off a junkyard 940 plus a new regulator/brush assembly from Volvo did the trick.

John