TM-9-2320-272-23-1
TRUCK, 5-TON, 6X6, M939, M939A1, AND M939A2 SERIES TRUCKS (DIESEL)
TECHNICAL MANUAL; FIELD MAINTENANCE MANUAL; VOLUME 1 OF 5
SEPTEMBER 2012
TM-9-2320-272-23-1 - Page 213 of 1313
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 PTO.
1.
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 vale 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.
2.
In order to shift the transfer case from one driving range to another, an interlock system working in
conjunction with the 5th-gear lockup solenoid is used. This system prevents the transfer case from being
shifted unless the transmission is in neutral.
3.
With the automatic transmission, several actions must occur in order to shift 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.
4.
The transfer case control system, through use of a PTO driven by the transfer case, also provides hydraulic
power to operate the crane and rear winch on M936/A1/A2 wreckers.
5.
Major components of this system are:
A. TRANSFER CASE SHIFT LEVER SWITCH
- When depressed with transmission in NEUTRAL, signals
interlock solenoid valve to exhaust air pressure from interlock air cylinder and actuates lockup solenoid.
B. TRANSFER CASE SHIFT LEVER
- Is pushed down to HIGH for light load operations, and up to LOW
for heavy load operations. Six-wheel drive is achieved automatically when transfer case shift lever is placed
in LOW.
C. TRANSFER CASE PTO CONTROL LEVER
- Manual control for engaging PTO.
D. TRANSFER CASE PTO
- Mounted and mechanically driven at rear of transfer case, the PTO drives a
pump to supply hydraulic pressure to power the rear winch and crane on the M936/A1/A2 wreckers.
E. FRONT-WHEEL DRIVE LOCK-IN SWITCH
- Manual control for activating front-wheel drive valve to
provide front-wheel drive with transfer case in HIGH drive range.
TM 9-2320-272-23-1
0003
0003-8
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