Why does the air conditioning unit jam when switched on in a 1999 Isuzu Trooper when the front plate pulls in but won't turn?
The plate you refer to is part of the A/C clutch, when it is
energized it engages the clutch to turn the compressor. When it is
not energized (out position) the pulley freewheels and there is no
load on it. Your A/C compressor has either locked up due to an
internal failure or extremely high system pressures. Either way you
need to take it to a professional for repair.