You have a voltage drop problem in ignition and charging circuit not a voltage regulator malfunction. It is regulating just fine based on bad information.
Somewhere there are poor or rather corroded connections reducing harness voltage the regulator is seeing. When voltage regulator sees low harness voltage (bad information) it directs the alternator to output higher voltage to make up the difference. In other words if harness is one volt low say about 12.5v, regulator will bump up alternator output one volt higher than normal so you will see 15.5 v at battery.
Some where there are poor or rather corroded connections reducing harness voltage the regulator is seeing. When voltage regulator sees low harness voltage it directs the alternator to output higher voltage to make up the difference. In other words if harness is one volt low say about 12.5v, regulator will bump up alternator output one volt higher than normal so you will see 15.5 v at battery.
To reduce voltage drop one has to remove any voltage reducing corrosion, and inadequate wire causing excess resistance in charging & ignition circuits. This includes the ground side of circuit as well as positive side. In other words there cannot be any voltage difference between negative battery terminal and chassis of regulator, spark controller if equipped, and alternator case. On plus side of circuit all connections need to be clean and fee of electrical resistance.
Check for voltage drop by measuring difference between battery terminal, and ant point in charging circuit. This is done by placing black VOM lead on (+) battery terminal, and the red on any point in that circuit. If you see more than 0.1 volt you have voltage drop at that point. Same method works for ground side, place black lead on (–) battery post, and red on any ground point in circuit; more than 0.1 volt, and something need to be cleaned and refastened to correct the problem.
Clean and remake battery cable and engine ground connections, ground head to fire wall, and make sure all grounded cases of components are electrically as battery negative terminal. On the plus side do the same, make sure brass connections at both sides of bulkhead connector are clean & tight, and then look for voltage drop at ignition switch.
If ground side is problematic you can make up a ground loop with #14 wire connecting mounting screws of voltage regulator, spark controller, alternator, and back to negative battery terminal. This will eliminate all bad connections in ground side of circuit.
