Decelerator or Hand Throttle (STD cab) Position potentiometer voltage is too low
High range clutch is not calibrated
Aux 3rd Lower solenoid failed open or short circuit.
Low pressure switch (-) input shorted to power
Foot Throttle Position potentiometer voltage is too low (engine controller pin B30)
High range clutch is not calibrated
If Inadequate air flow is problematic, so clogged air filters, evaporator core air flow restricted, faulty blower fan motors, defective blower switch or faulty wiring or loose connections.