13.9 is not bad. What year car? If it's 69/ earlier, should be a big thick black (the output) to a stud/ nut and one green "push on" smaller wire which is the field from the regulator
If it's 70/ later, should have one more (blue) "push on" wire which is ignition "run" 12V from the ignition switch
Put under what conditions did you measure the 13.9? Check that the V belt is not slipping. Then warm it up and allow the battery to "normalize." Now measure at the battery with engine running to simulate "medium cruise" RPM. Now turn on whatever accessories you can, hazard flashers, heater blower, lights, etc. It should maintain an absolute minimum of 13.5, no lower
