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Chevrolet Silverado / GMC Sierra. Manual - part 2904
Fig. 56: View Of 4HI Mode Power Flow
Courtesy of GENERAL MOTORS CORP.
In 4HI mode, the engine power from the transmission (1) is delivered to the input shaft (2). The
engagement teeth on the inside of the input shaft (2) are engaged to the outer teeth on the
high/low clutch (3). The high/low clutch (3) is slip splined to the rear output shaft (4). During a
shift mode from 2HI to 4HI, the cam profile on the 2/4 wheel drive actuator cam (13) pushes the
2/4 wheel drive mode shift fork (9) and the synchronizer sleeve. The sleeve, engaging the
synchronizer (8), equalizes the front propeller shaft (12) and rear propeller shaft (7) speeds. The
sleeve then engages to the drive sprocket (6). The engine power flow for driving the front
propeller shaft (12), now travels from the rear output shaft (4) to the synchronizer hub (5), which
is splined to the rear output shaft (4). The power flows from the synchronizer hub (5) to the
sleeve and then to the drive sprocket (6). The power flow is transmitted by a chain (10) to the
driven sprocket on the front output shaft (11) and to the front propeller shaft (12). The front and
rear propeller shafts are being driven at equal RPM, therefore giving a four wheel drive mode.
This mode should only be used for extra traction. Using this mode on dry pavement causes tire
scuffing when turning.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
4LO Mode Power Flow
Fig. 57: View Of 4LO Mode Power Flow
Courtesy of GENERAL MOTORS CORP.
During a range shift request to 4LO, a separate cam profile, in the high/low shift fork spring
assembly (12), moves the high/low shift fork (13). The high/low shift fork (13) moves the
high/low clutch (6) rearward. The high/low clutch (6) outer teeth disengage from the input shaft
(2) high-speed teeth. The high/low clutch (6) outer teeth then engage in the high/low planetary
carrier teeth (5). The power flow is now from the transmission (1) to the planetary teeth on the
input shaft (2) to the planetary gears (3) in the high/low planetary carrier assembly (5). Rotating
the planetary gears (3), which are engaged in the high/low internal gear (4), rotates the high/low
planetary carrier assembly (5). The high/low planetary carrier assembly (5) delivers the power to
the high/low clutch (6). The high/low clutch (6) then drives the rear output shaft (7), providing a
2.68:1 gear ratio reduction to the rear output shaft (7). The sleeve is still engaged to drive the
drive sprocket (9), as in the 4HI position, providing the engine power to the rear propeller shaft
(10) and to the front propeller shaft (11).
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
NEUTRAL Mode Power Flow
Shifting to the NEUTRAL position, the high/low clutch is centered between the input shaft and
the high/low planetary carrier. The high/low clutch is not engaged with either the input shaft or
the high/low planetary carrier teeth. The 2/4 wheel drive synchronizer is in the 4HI mode
position.
Electrical Components
Below is a list of major components that make up the automatic transfer case (ATC) system.
Front Axle Actuator
The front axle actuator engages and disengages the front axle. The front axle actuator consists of a
permanent magnetic (PM) motor, a worm gear controlled plunger, a front axle switch and an
electronic control circuit. When a range shift to 4HI, or 4LO is requested, the transfer case shift
control module requests engagement of the front axle by grounding the axle actuator control
circuit through a current limiting driver. The front axle actuator also sends a return signal to the
transfer case shift control module indicating when the 4WD is engaged.
SERVICE Indicator (4WD)
The SERVICE indicator (4WD) is an integral part of the cluster and cannot be serviced
separately. This indicator is used to inform the driver of malfunctions within the ATC system. The
SERVICE indicator (4WD) is controlled by the transfer case shift control module via a Class 2
message or by a service indicator control circuit.
Transfer Case Two/Four Wheel Drive Actuator Assembly
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
Fig. 58: Transfer Case Two/Four Wheel Drive Actuator Assembly
Courtesy of GENERAL MOTORS CORP.
The transfer case two/four wheel drive actuator assembly consists of a PM DC motor and gear
reduction assembly. It is located on the left side, drivers side, of the transfer case. When
activated, it turns the sector shaft of the transfer case clockwise or counter clockwise to shift the
transfer case. The encoder motor is controlled by the transfer case shift control module. This
circuit consists of a driver on both the motor control A and motor control B circuits. The encoder
motor is bi-directional, to allow the motor to shift the transfer case from 2HI or 4HI to
NEUTRAL and 4LO positions. The transfer case encoder motor can be fully driven clockwise or
counterclockwise using a scan tool. You may also monitor motor control A and B circuits using a
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
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