AC Issue

[Restrorob]

Hey guys, I'm having a issue with the AC in my 97 GMC.

When I bought it some 9 yrs. ago the AC didn't work due to no refrigerant in the system, I pulled it down and charged it to leak test the system and everything worked fine and it blew cold. I found the evaporator was leaking, I never replaced it because I only drive it to car shows in the area.

Fast forward to now, I'm ready to rip the whole dash apart for the evaporator replacement. I jumped the high and low pressure switches to make sure everything was still working okey, I found the compressor clutch kicks in when I push the AC switch on the dash but then it kicks back off when I turn the blower switch on, as long as the blower switch is off the clutch stays engaged.

In all my years as a auto tech. I've never ran across something like this, any ideas where to start diagnosing ?

Thanks !