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)