you may want to also 'check the easy and common stuff first'. when the forklift repair gurus above asked if you were checking at the battery, it is at least in part because a dirty battery cable post terminal, right at the battery post, can cause an alternator to 'over charge' by putting out more voltage than the battery could use. the truck may still start most of the time, but the battery does not see the charge because of the insulation effect of the corrosion, so be sure you take the voltage reading right on the battery post, (as well as on the cable and at the alternator)
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.
GM, And new 1 has internal regulator. found potentiometer in control unit. where adjustment can be made to voltage. not a clark dealer so info isn't always at hand. thanks.
Where are you getting these readings- off the back of the alt or at the battery? Is the battery fresh? A faulty battery can greatly skew the readings of the charging system.
this a GM type alternator? or mitsubitshi? (not that it matters)
charging rate should be 13.2v to 14.8v
old style alternator with external regulator? or is it built in?
- If external then replace the voltage regulator box
- if internal then get another alternator
you may want to consider taking it to an alternator rebuild shop and let them test it for you.
good luck