AC compressor not getting power to engage.

[dmeere]

I have an 87 2.5 2 wheel drive that I can't get the ac compressor to come on with the a/c control lever.  Blower works fine and if I add 12 volts directly to the single wire that comes off the compressor, it engages.  any thoughts on where I'm losing the 12 volts in the normal circuit?

[eaglescout526]

Are you sure you’re losing voltage? Has the system been charged recently? 
The system has a pressure switch that will let the 12v through to power the relay to engage the clutch. If the system has no pressure, it won’t pass the voltage on. 

[ghetdjc320]

 Get yourself a set of AC gauges and check the low pressure side. It could be you have a bad pressure switch but verify the pressure first