Model 378 ac not working

I've got a 1998 Peterbilt model 378 no power to the air conditioning compressor, no power to on off dash switch
also the blower ducts only blowing to floor
the pressure in the system was low so I jumped the compressor added 3 cans hoping The low pressure switch was keeping the compressor from coming on
anyone know of Typical failures with this system?
also would love to get a wiring schematic thanks

 

if you're talking about the hot(red) to cold(blue) knob / switch There's 4 black wires going to the back of it.
also I'm trying to determine which PDF file is the correct one for this model?
I'm working for my android & its hard to get to the wiring schematic list as posted above
but I did find my way to the PDF files list via a search engine
Thanks

 

got the system working
had a bad AC compressor relay at the fuse block
couple of missing fuses
several burnt / melted connectors somebody had connected a hot wire plug in connector to the ac compressor so they were bypassing factory power feed
which is probably How the connectors got melted, relay got overheated etc...