Help with high idle switch causing engine shutdown
 

2000 International S4900 digger derrick truck w/ DT466E & Allison trans. Suddenly yesterday discovered when trying to engage high idle to use the pto, the engine dies every time I flip the switch. This truck is utterly useless to me without the hydraulics working.

Anybody able to guide me here because I have no wiring diagrams for anything and cannot find anything at all online.