CONTROL SYSTEM OPERATION (Contd)
Transfer Case Control System Operation.
The transfer case control system converts four-wheel driving power into six-wheel driving power, provides smooth
shifting of transfer case into high or low driving ranges while vehicle is in motion, prevents transfer case from being shifted
with transmission in gear, and provides hydraulic power for auxiliary equipment through a Power Takeoff (PTO).
Six-wheel drive is achieved two different ways depending on the drive range (high or low) desired. In low
range, the transfer case shift linkage automatically moves a cam-actuated valve which dumps air into
the front drive cylinder. This forces a piston against the transfer case clutch to engage front-wheel
drive. In high range, front-wheel drive is engaged in the same manner except that the front-wheel drive
valve is manually actuated by the front-wheel drive lock-in switch on the instrument panel.
In order to shift the transfer case from one driving range to another, an interlock system working in
conjunction with the 5th-gear lock up solenoid is used. This system prevents the transfer case form
being shifted unless the transmission is in neutral.
With the automatic transmission, several actions must occur in order to shift the transfer case from one
driving range to another. Because of the interlock system, the transmission must be placed in neutral.
The transfer case shift lever switch must also be depressed.
The transfer case control system, through the use of a PTO driven by the transfer case, also provides
hydraulic power to operate the crane and rear winch on the M936/A1/A2 wreckers.
Major components of this system are: