[Tommy]

12-13 volts is what you'd expect from a fully charged battery with the engine not running. 12-13 volts at the alternator with the engine running sounds like it is not charging. If the voltage on the meter is different from the voltage at the battery on a multimeter, then perhaps the meter is bad. If the voltage at the battery (using a multimeter) does not increase above 13 volts when the engine is running at a fast idle, the charging circuit is not working.