First check the belt to be sure it's tight. If it's slipping, the pulley will get hot. Next, have the new generator tested on the truck with a professional load tester. If "ripple" is very high and all you can get is one third of its rated capacity, it has a bad diode. That is a common repeat failure that can be avoided by replacing your perfectly good battery at the same time. These generators develop a lot of nasty voltage spikes that can destroy the internal voltage regulator and diodes. As the battery ages, it loses its ability to dampen and absorb those spikes. That should not be a problem if your battery is less than about two years old.