I know it's a odd question, but do you have the alternator case grounded? Sometimes a dedicated ground is needed beyond the chassis ground.
It is attached via an aluminum bracket to an aluminum engine plate bolted to a steel bracket that is welded to the chassis. My old alternator setup with elephant ears didn't have another ground, if it needing a separate ground wire is still believed to be a problem I have no issue adding one. Although it appears based on Powermaster literature that the 8172 needs an exciter wire or the IGN terminal on the engine sent to a keyed 12v source.
