Where is the voltage regulator located on a 1991 Dodge diesel?

On a 1991 Dodge diesel (likely a Cummins engine), the voltage regulator is integrated into the alternator. Unlike older systems where it was a separate component, the 1991 models used an internally regulated alternator. Therefore, you won't find a separate voltage regulator unit. If you're having charging issues, you'll likely need to test or replace the entire alternator.