Chevrolet Silverado / GMC Sierra. Service manual — part 2904

Fig. 55: View Of 2WD Mode Power Flow
Courtesy of GENERAL MOTORS CORP.

The transfer case is shifted by an encoder motor (9) on the control actuator shaft. The control
actuator shaft rotates the 2/4 wheel drive actuator cam (10) and moves the high/low range shift
fork (12) or the 2/4 wheel drive mode shift fork (8). Depending on the selected mode or range,
the shift forks move either the high/low clutch (4) for the high/low range planetary assembly (3)
or the synchronizer sleeve (6) for the 2/4 wheel drive mode. In 2WD 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 (4). The high/low clutch
(4) is slip splined to rear output shaft (5). The engine power flows from the high/low clutch (4) to
the rear output shaft (5) and delivered to rear propeller shaft (7).

4HI Mode Power Flow

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Transfer Case - MP 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

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