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 212 of 1313
TRANSMISSION CONTROL SYSTEM OPERATION
The transmission control system permits shifting of transmission, prevents starting of engine with transmission in
gear, and prevents shifting of transfer case unless transmission is in neutral. This system also permits engagement
of the transmission Power Takeoff (PTO) to provide hydraulic power for auxiliary equipment on M925/A1/A2,
M928/A1/A2, M929/A1/A2, M930/A1/A2, M932/A1/A2, and M936/A1/A2 vehicles. Major components of the
transmission control system are:
A. TRANSMISSION 5TH-GEAR LOCKUP SOLENOID VALVE
- Activated by transmission control switch
and transfer case switch, the 5th-gear lockup solenoid valve directs main oil pressure of transmission to the
transmission governor system. This puts transmission in 5th-gear, creating less drag on transfer case
synchronizer which permits smoother shifting from one transfer case drive range to another. Refer to
TRANSFER CASE CONTROL SYSTEM OPERATION, for further details.
B. TRANSMISSION NEUTRAL START SWITCH
- The neutral start switch, wired to the starter switch,
prevents the engine from being started with transmission in gear.
C. TRANSMISSION PTO
- Driven by the transmission, the PTO drives the hydraulic pump which provides
hydraulic pressure to power the front winch on M925/A1/A2, M928/A1/A2, M930/A1/A2, M932/A1/A2, and
M936/A1/A2 vehicles, and to power the dump body on M929/A1/A2 and M930/A1/A2 vehicles. The PTO is
mounted on the right front side of the transmission.
D. TRANSMISSION CONTROL SWITCH
- Actuates transmission lockup solenoid valve when transmission
selector lever is placed in NEUTRAL and transfer case shift lever lockout switch is pressed.
E. TRANSMISSION SELECTOR LEVER
- Is used to select vehicle driving gear range.
F. PTO CONTROL LEVER
- Engages transmission power takeoff to provide power for auxiliary equipment.
F
E
D
A
B
C
G0018DAA
Figure 6.
Transmission Control System Components.
TM 9-2320-272-23-1
0003
0003-7
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