starter or solenoid ...or control switch

[DiverRay]

Motorman:

......doubtful, since 78-81XS's had brushless type alt..

I think he is talking about the STARTER, not the Alt.... I would also loosen the three hold down bolts for the starter, and clean the surface between the two that go through the left side to the case. that IS the ground for the starter. You can get aluminum to oxidize and not conduct well between the starter body and the engine case.