Air conditioning compressor will not kick in,

What is the low side pressure?

Yes , there is a relay . It is energized by the PCM - engine computer . When you push the A/C button on the climate control panel ,it sends a voltage input to the BCM or strait to the PCM .When the PCM see's this it will energize the relay ,if it see's the correct input from the high an low pressure switch .
The Trainer 40 When the compressor won compress

could be low on coolant..in AC system..

First things first.....You must have the air conditioning system hooked up to an a/c system machine and then checked for leaks and also read low and high side pressures. If there are leaks then low Freon (R134a) content and low refrigerant oil can cause the a/c system to not engage the a/c compressor which basically means you will not have proper defrost functionality nor regular air conditioning. Most shops, when working with your air conditioning, will install air conditioning dye to assist them in locating leaks and is the first step in diagnosing all a/c systems. Post the findings after thw

IT COULD BE THAT YOUR AC IS OVERFILLED WITH FREON

CHECK THE COMPRESSOR FUSE AND RELAY.IF BOTH GOOD.PROBLEM COULD BE LOW REFRIGERANT LEVEL OR FAULTY AC COMPRESSOR CLUTCH COIL OR DAMAGE LOOSE WIRE TO CLUTCH COIL.FAULTY LOW PRESSURE SENSOR.

You may have created an over charged condition and at this point the a/c compressor will cut off on high head pressures. This can be verified by installing gauges to the systems high and low side service taps and reading the pressures.An overcharged system will start the compressor briefly but soon cuts off once pressure goes to 350 - 400 lbs on the high side. Acceptable pressures for this system should be no greater than 250 high and low suction should drop to 40-50 psi.You might recover the charge and evacuate the system,then try bringing the charge back to proper level.A sight glass in the reciever drier really helps determining proper fill.Once all bubbles have dissipated from the sight glass during charging ,one should stop at that point to avoid overfilling.Too much freon is worse than a low charge especially in hot ambient conditions....

Make sure your system is fully charged with r134. The low pressure switch won't allow it to activate if there isn't enough in it.

Is that your static pressure readings??? If that is your static pressure readings that's good. When the engine running and fan on high and a/c compressor clutch engaged you should have 25 to 35psi on your low side and 200 to 350psi on your high side. If the clutch is engaged and that is your readings, replace your a/c compressor. Keep me posted, be glad to help.

Is the system low on Freon? If not, you most likely have a bad A/C compressor, or a bad low pressure switch.