if that is a GM type alternator with an internal voltage regulator then there is no adjustable output voltage. All that is done internally by the alternator voltage regulator. If your output voltage is too high you need to remove the alternator and take it to a repair shop and have them fix it or just get a rebuilt one and replace the unit.

I don't know what this potentiometer you speak of actually is but i doubt it has anything to do with the output voltage. It may adjust some other voltage on the truck itself and by tampering with it you may effect other functions of the truck inadvertently and cause other problems later on down the road.

It could be changing the truck voltage during operation but if you check the output terminal on the alternator itself and find it is still trying to overcharge the battery, check it at the battery and see if that part has changed. If it is still trying to charge the battery at 18 volts then adjusting that potentiometer was pointless. You will still cook the battery until you fix the alternator.