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No the alt should get a ground through the engine the 3 wires it should have are
1. the heavy charge wire
2. feild wire, I think its orange and goes to the reg.
3. stator wire, wht and black usually runs the elec choke
As KF said, the alternator grounds through the engine block. However, most alternators do have a ground stud which is simply integral to the casing. This can be used to wire a direct ground between the alternator and one of the regulator mounting bolts to give a slightly more accurate sense (otherwise the regulator ground is shifted by any resistance in the ground connection between the inner fender sheetmetal and the engine block).