A/C clutch wouldn't engage. Verified pressure with professional gauges. Verified solenoid is good. Verified voltage at low pressure switch is 5 volts. My buddy who knows more electrical than I do said there is resistance in the low pressure switch. Not sure what the 3rd wire is. On his 3rd Gen Trans Am he only has 2 wires. Took the solenoid out and jumped it. A/C clutch and fans run fine. I'm thinking that the low pressure switch isn't talking to the a/c solenoid correctly and not letting it send the signal to the clutch and fans to turn on. I wanted to ask if my theory is correct before I go buy a $25 switch. I have run the car around town and it blows ice cold air with the jumper in place. Pressure on the low side gauge is running around 48.

 

This may help. pressure sw does have some resistance as designed on the third wire.

 

Third gen cars have pressure switches. Fourth gen cars have variable voltage sensors. I think the voltage from the sensor (red/black wire) should be somewhere around 0.5-1 VDC with the compressor off, higher when on. It should never be 0 VDC or 5 VDC.
At 48 psi, it should be about 0.7 volts.

 

I just verified that the reference wire blue/black is 5vdc, and the red and black wire is 0.7vdc, so it appears the correct voltage is getting to the switch/sensor. Could it be this sensor that isn't letting the solenoid work?

 

I can't tell you what the setting is for a 97, but for my 94 bench-top setup, the minimum pressure for allowing the compressor clutch relay to operate is about 45-48 psi. You are very close to that limit.
This is how it works: The voltage from the sensor is fed to the pcm. The pcm reads the voltage, converts it to psi and then operates the relay if the AC switch is on and the pressure is high enough (45-48 or higher). A few other variables are required also, but that is the main one.

 

In the 96 service manual it is stated that the minimum pressure for good operation is 50 psi in a static condition (engine off, low side and high side equal pressure). I suspect it is just too low a charge to operate.