His problem is the compressor will not come on even thought the low pressure switch is closed and there is a charge on the system. The compressor is not getting +12 volts. What he's looking for is the air conditioning control diagram showing ECU connections, compressor relay and pressure sensors. Kevin
on my 90 I had the same issue. My head unit worked but the A/C clutch would not engage. If your head unit works go to the relay and see if you have 12 to both wires when the A/C is on, ground the small brown one , if it turns on the clutch it's probably the ECM unit. The ECM unit grounds that wire to engage the clutch. If your not getting 12 volts to both relay wires when the A/C is turned on it's probably the relay. On mine it was the green wire that goes to the pressure switch. it had been cut. The above info was at the courtesy of toptech and twisterup. If they jump in, follow their lead.
Your '90 had somewhat different problem with an open in the Low Pressure Switch wiring, though if this '93 had a loose or intermittent connection at the switch (common on the '94), it would cause the same condition. Otherwise, if the contacts are closed, the connection is good, and the system has gas - the reasons the Relay won't drive the compressor clutch are: a stored trouble code (most common) - a bad relay - a bad control panel or programmer (Electronic Air) - or a bad ECM - or bad CCM (Electronic Air - though that is really rare since the link is for diagnostics) - or a defective Pressure Sensor (on some GM's it will trigger a PCM code). Also, the second schematic is for Manual Air - Electronic is a bit different with the Programmer sending the a/c request to the PCM and there should be another schematic for it - there's only the Control panel to Programmer shown in the Electronic Air schematic.
I didn't have any codes, so basically I chased 12 volts around until I got to the last point where it would no longer read 12 volts. Then I ran a test jumper from the green wire to the switch and the compressor worked.
once I determined I had 12 volts past the ECM I was pretty sure that was the only thing left.