my compressor will not engage why?

Double check that your system has enough 134A refrigerant. There is a pressure switch in the system that will not allow the clutch to engage on the AC pump unless that pressure switch is made. The only other thing that would not allow the pump to come on is a blown fuse or the wiring has been damaged or shorted.. Bad switch on your climate control?

I have 100 psi on both sides. fuse is good. That is what I was thinking (bad switch on climate control). how do I check it.

Check that you have 12vdc on one side of the switch to ground. Upon making the switch, there will be 12vdc on both sides of the switch. I will mention that you should check the AC Relay as well and perform the same test. If all is good, you may have a bad clutch on your compressor. You can try jumping 12vdc right to the compressor to see if it will engage. This will rule out the compressor being bad as well.

where is it located?