Ok, I have an 86 M939 truck under the SEORTM and I downloaded the manual last night - thank you SteelSoldiers! What I am unable to engage and disengage is the transfer case - lever is between the seats next to the parking brake. The PTO lever to the left of the driver seat moves. This is not the ECU truck of later years
We tried neutral, rocking the truck back and forth slightly and having the foot on the brake. The manual does not provide much enlightenment - it says put in neutral and pull the lever. I'm not sure if there is a lock out somewhere or if it is just stuck from lack of use. We did not try it while in gear.
I need to spend more time with the manual but it does talk about the following:
INTERLOCK AIR CYLINDER – Under air pressure, a piston in the
interlock air cylinder forces a shaft against one of three grooves in transfer
case shift lever. This prevents transfer case from being shifted with
transmission in gear.
INTERLOCK SOLENOID VALVE – Releases air from interlock air cylinder
when transmission is in neutral and transfer case shift lever switch is
depressed. This permits the transfer case high/low shift shaft to move.
Thanks for the help if anyone knows a specific sequence.
