[ossiblue]

Have the shop (or do it yourself) measure the output from terminal D+ at varying rpms of the Marchal. I remember reading a thread about faulty connections inside the alternator to D+ but I can't be sure if that was in reference to generators or alternators. Regardless, that would be my first step, possibly even test both alternators at the D+ and compare the outputs since you know the Bosch works well.