a/c compressor clutch doesn't engage

[motuman]

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.

[duckloads]

Keep the multi-meter handy. Do you have a set a A/C gauges? You could hot wire the compressor and watch the system pressures to determine if it is low on R-12. Some people have topped them off with pure propane.

[632 Regal]

there s a low pressure cut off switch that you can jump to see if the compressor works. Check for power on one side and nothing on the other and then jump it.