Some problems I found with the alternator arrangement on the GD:
GD1021 chassis cars had a smaller alternator drive pulley on the crank, GD 1022 chassis cars (here anyway) had a larger one to speed up the alternator, which makes a difference at idle speed.
I have found on other cars that Mitsubishi alternators seem to have a regulator that is not good a delivering current at slow rotational speeds compared with Denso and Bosch equivalents.
So, do you want to increase the idle speed current delivery, or is the alternator output completely inadequate at all speeds ? The latter usually results in the battery going flat after a long drive despite the alternator working properly.
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