The most common problem with a Denso alternator on a '89-'02 Dodge Turbo Diesel truck is that the brushes wear out. Without brush-to-armature contact there is no electrical charge.
Some truck owners replace the Denso alternator after if fails and leaves them stranded. Budget conscious owners rebuild the alternator with new brushes and bearings prior to complete failure.
The alternator housing should be marked with the alternator type so you purchase the correct rebuild kit.
GOULD Alternator Rebuild Kit (DENSO) - DODGE ('89-'02)