Have measured the voltage from the positive terminal of the clutch, to the negative terminal (mounting bolts), and using the intake manifold as a "real ground". Both yield about 12.9volts, with no significant differences. This is when the clutch is not engaged.
If I give it a tap or rev the engine hard to engage the clutch, the voltage will drop to just below 12volts, about 11.9volts.
Taking a reading between the mounting bolt and the intake manifold also verifies that there's no voltage difference.
Hence, what can I conclude? A mechanically faulty pulley?
BTW, what kind of current (amperes) does the clutch draw? Is there a difference using the compressor body or the clutch mounting bolts as ground?