Medium and Heavy Trucks
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Yes, starting is controlled through the shifter.
Place a jumper wire between the 307 orange wire and the 306 green wire the try starting engine. If it cranks, the circuit is good and the shift is likely the issue.
Also verify shifter control is getting power on the C993 white wire (pin 1) and ground on the 202 black wire. The neutralizer (Trans disconnect) buttons power this control.
See attachment for all pin outs.