[jeffcoslacker]

When the system is at rest (compressor not running) the high and low sides equalize. 120 psi is not excessive for a static reading.

When the compressor starts, that will drop if that was the low side reading.

Chances are it is just low enough on charge to disable the compressor. You can force the compressor clutch to engage a number of ways, but I'm not gonna recommend it (nudge, wink)

Possible the relay for the system is stuck too. If you can locate it in the underhood power center, and have one of the same type in there, you can swap them and see what happens. If you don't have a donor, try tapping the relay lightly on a hard surface...sometimes that'll wake them up if the contacts are stuck for some reason. If that works, I'd buy a new relay.

I'm assuming you are working with one of those 134a suicide hoses with gauge that shows low side pressure?

You can try tapping a can, hook it up to the port, and with the engine running and A/C on MAX, spray your condensor with water from a hose (this cools the condenser and lowers the static pressure) and try valving in some 134a. If it sucks it in readily, the compressor will probably start within a few seconds. Once it starts, it will devour the can pretty fast. If it doesn't, or pressure continues to climb with the comp engaged, there is a blockage or something.