Auto 4WD Power Flow
Power is delivered from the transmission (1) to the input shaft (2).
The input shaft (2) is splined to the rear output shaft (7).
To deliver the power to the rear propeller shaft (9), the power is
constant through the rear output shaft (7). When power is required
for the front propeller shaft (11), a command is sent to the encoder
motor (13). The encoder motor (13) rotates the shift detent
lever (12), which is cam shaped. The cam action moves the clutch lever (3).
The clutch lever (3) pivots on the clutch lever pivot studs, and moves
toward the clutch apply plate to engage the clutch. As more pressure is applied
to the clutch apply plate, the clutch discs are compressed. Using the inner
clutch discs, which are engaged with the clutch hub (4), and the outer
clutch discs, which are engaged with clutch housing (5), the power
flow is delivered to the clutch housing (5). The clutch hub (4)
is splined to the rear output shaft (7). The clutch housing (5)
rotates on a needle bearing on the rear output shaft (7). The chain
drive sprocket (6) is splined to the clutch housing (5). The
power flows from the drive sprocket (6), through the chain (9),
to the chain driven sprocket. The chain driven sprocket is splined to the
front output shaft (10). The power flow is delivered to the front
propshaft (11) through the front output shaft (10).
New Venture Gear 136 Automatic Transfer Case
During normal driving situations, the Auto 4WD mode is active. During
the Auto 4WD mode, the transfer case shift control module monitors rear
wheel slip speed, based on the inputs from both the front and rear propshaft
speed sensors. When the vehicle experiences a rear wheel slip condition,
the transfer case shift control module sends a pulse width modulated
(PWM) signal to an electronic motor, transfer case encoder motor. This
motor rotates the transfer case sector shaft, applying a clutch pack.
This clutch pack is designed to deliver a variable amount of torque,
normally delivered to the rear wheels, and transfers it to the front
wheels. Torque is then ramped up to the front wheels, until the front
propshaft speed sensor matches that of the rear propshaft speed sensor.
Torque is then ramped down, until torque is completely removed from
the front wheels or until rear wheel slip is once again detected. The
process then repeats.
View the list of major components that make up the automatic transfer
case (ATC) system below.
SERVICE Indicator (4WD/AWD) Lamp: The SERVICE indicator
(4WD/AWD) lamp is an integral part of the cluster and cannot be serviced
separately. This lamp is used to inform the driver of malfunctions within
the automatic transfer case (ATC) system. The SERVICE indicator (4WD/AWD)
lamp is controlled by the transfer case shift control module via a
Class 2 message or by a Service Indicator Control circuit.
Transfer Case Encoder Motor: The transfer case encoder
motor consists of a permanent magnet (PM) DC motor and gear reduction assembly.
It is located on the left hand side of the transfer case. When activated,
it turns the sector shaft of the transfer case to shift the transfer
case and to apply the clutch that applies the front propshaft. The
encoder motor is controlled with a pulse width modulated (PWM) circuit
provided by the transfer case shift control module. This circuit
consists of a power supply relay, Motor Control A, and supplies voltage
to the motor. The Motor Control B circuit is a PWM driver that varies
the duty cycle to control the amount of current through the motor to
ground. The transfer case encoder motor can be turned ON and OFF using a
scan tool. You may also monitor Motor Control A and B circuits using a scan
tool.
Transfer Case Shift Control Module: The transfer case
shift control module uses the VIN information for calculations that are
required for the different calibrations used based on axle ratio, transmission,
tire size, and engine. The system does not know which calibration to
use without this information. This information is provided to the transfer
case shift control module via Class 2 data bus from the powertrain control
module (PCM). The transfer case shift control module monitors front and rear
propshaft speed as well as controlling the operation of the transfer case
encoder motor assembly.
Transfer Case Speed Sensors: There are three speed sensors
mounted on the transfer case. 2 speed sensors are mounted on the rear output
shaft and one on the front output shaft. Each speed sensor is a permanent
magnet (PM) generator. The PM generator produces an AC voltage. The
AC voltage level and number of pulses increases as speed increases.
Front Propshaft Speed Sensor - The transfer case shift control
module converts the pulsating AC voltage from the front transfer case speed
sensor to front propshaft speed, in RPM to be used for calculations,
and to monitor the difference between the front and rear sensor speed.
It is also used in the AUTO 4WD mode to determine the amount of slip
and the percent of torque to apply to the front axle. The front propshaft
speed can be displayed with a scan tool.
Rear Propshaft Speed Sensor - The transfer case shift control
module converts the pulsating AC voltage from the rear transfer case speed
sensor to a rear propshaft speed, in RPM to be used for calculations.
The rear propshaft speed can be displayed with a scan tool.
Vehicle Speed Sensor - One of the 2 speed sensors on the rear
output shaft is the vehicle speed sensor (VSS) input to the powertrain control
module (PCM). The PCM sends this information to the transfer case
shift control module via the Class 2 serial data bus.