My a/c clutch engages only when I rev hard, and after a while, it will cut out.

Steve has suggested that it's a bad ground, so I have taken some voltage measurements.

Have measured the voltage from the positive terminal of the clutch, to the negative terminal (mounting bolts), and also 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 the clutch a tap to make it engage 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 with the clutch engaged, 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?

Thanks.

Roland