I just purchased a 1990 535i and the a/c clutch doesn't engage. The a/c push-button on the dash lights, but the clutch does not engage. Normally I would expect this is due to loss of refrigerant and the low pressure switch is cutting power to the clutch.

Since I can't just take it over and get a shot of R-12, however, I wanted to try and trace down the power to the clutch first, and see if the problem is electrical. Are there any known failure modes on this system? The low pressure switch? Relay? a/c control button? Any insight would be appreciated before I head out with the multi-meter.

(Ordered a Bentley manual, but has not arrived yet, and I need to try and sort this out over the weekend)

Thanks to all.