[boxhead]

1/ check actual alternator output with known good voltmeter.
2/ if your alternator has an internal regulator then check all earths from engine to chassis and chassis to battery.
3/ if your alternator is fitted with an external voltage regulator then check to make sure your regulator is earthed via its body
4/ if you have the external regulator then where is your voltmeter wired too? is it on the alternator output or the regulator output?
5/ make sure there is no resistance in the alternator body to battery negative circuit, (should be earthed through the earth harness and chassis harness)
6/ if all this is ok then it would seem you have a faulty regulator.