Description Part 1
Four-Wheel Drive Systems
Transfer Box Component Location
Transfer Box Exploded View
GENERAL
The DD295 transfer box is a full time, permanent four-wheel-drive unit, with 50/50 torque distribution to the front and rear driveshafts. The unit is manufactured by Magna Steyr Powertrain in Graz, Austria and supports the following features:
^ Permanent four-wheel-drive with a bevel gear center differential, providing a 50:50 torque split
^ Selectable high and low range for optimum on-road and off-road performance
^ Two-speed, fully synchronized 'shift-on-the-move' system allows the driver to change the range without having to stop the vehicle
^ Electronically controlled multi-plate clutch providing a center differential lock and torque biasing function to give improved traction performance and vehicle dynamic stability.
A strategy, to electronically control the center differential multi plate clutch assembly, has been developed to provide:
^ a pre-loading function, increasing locking torque with increased driving torque
^ a slip controller to increase locking torque under off-road conditions and decrease locking torque for optimum comfort, e.g. parking.
The unit is located under the vehicle and is mounted on the cross-member, behind the transmission. The unit is identical for all engine derivatives.
The transfer box receives a torque input from the transmission output shaft, which is passed through the unit to two outputs for the front and rear drive shafts.
The input torque is equally distributed via a bevel gear type differential. In order to provide an optimal torque distribution to each wheel in all driving conditions, the unit is equipped with an electronically controlled locking and torque-biasing device. This device detects wheel slip via various vehicle system inputs to the transfer box control module and locks the differential accordingly. The locking torque is applied through a multi-plate clutch assembly.
A planetary gear set, located in the differential assembly, allows the driver to select high or low range whilst driving, this is known as 'shift on the move'. When in low range, the planetary gear set provides a ratio of 2.93:1, which gives the vehicle an extremely low crawl speed for off road driving and trailer towing. High range is a direct drive from the transmission output shaft and provides a 1:1 ratio.
Both the center differential locking and biasing and the 'shift on the move' features are actuated via a DC transfer box motor, which is controlled by the transfer box control module, via a Pulse Width Modulation (PWM) signal.
Transfer Box - Sectional View
Transfer Box Power Flow
The input torque, from the transmission, is transferred to the input shaft of the transfer box and then onto the planetary sun gear and planetary pinion gears. The planetary pinion gears are held in place by the planet pinion shafts, which are connected to the differential carrier, and drive the differential pinion gears. The torque is then distributed to both the front and rear carriers, which are connected to the outputs of the transfer box. The rear carrier is connected directly to the rear output flange-, the front carrier is connected to the sprocket and therefore to the chain drive, which provides the front output flange rotation.
TRANSFER BOX CASINGS
The front and rear casing assemblies are manufactured from cast aluminum.
Front Casing Assembly
The front casing assembly provides the location for the input shaft bearing and the front output flange bearing. It is also equipped with threaded holes to mount the chassis mounting bush, two lifting eyes and a breather cartridge for the transfer box breather pipe. The breather pipe allows an equalization between atmospheric and internal transfer box pressure.
Rear Casing Assembly
The rear casing assembly provides the location for the rear output flange bearing, the transfer box motor and the oil fill and drain plug. Fins are cast into the rear casing assembly to improve the heat dissipation. The unit number is also stamped into the rear housing.
OIL PUMP
An oil pump assembly is located in the front casing to provide lubrication for the bearings and rotary components through cross-drillings in the input shaft. A flat-sectioned coupling on the input shaft drives the rotor of the pump-, the stator is fixed to the front housing assembly. A tube is attached to the pump, which leads into a calm suction area at the bottom of the two casing assemblies. The collector magnet in the suction area of the pump collects any metallic debris.
CHAIN DRIVE
The chain-drive transfers drive from the center differential to the front output flange. A 3/8' pitch chain connects the sprocket on the transfer box input shaft with the sprocket on the front output flange. As both sprockets have the same number of teeth, the rotational speed of both sprockets is identical.
TRANSFER BOX MOTOR
One motor operates both the high/low range change and the differential locking and torque-biasing device (multi-plate clutch). The motor solenoid switches between the two functions, while the motor provides the rotational movement for both operations.
Transfer Box Motor Position For Clutch Control Mode
To actuate the multi-plate clutch, the transfer box control module energizes the solenoid (3). The solenoid pin pivots the solenoid shift fork (2), which engages the shifting sleeve (5) into the dogteeth on the clutch control disc (4). The rotational movement of the motor shaft (1) is then linked to the clutch control disc via the shifting sleeve.
This is the normal operating mode of the transfer box. In this position, the range change function is disengaged and mechanically locked.
Transfer Box Motor Position For High/Low Range Mode
To actuate the high/low range change, the transfer box control module de-energizes the solenoid (3). A spring in the solenoid retracts the solenoid pin and rotates the solenoid shift fork (2). This engages the shifting sleeve (4) to the dogteeth on the high/low actuation cam (5). The rotational movement of the motor shaft (1) is then linked to the cam.
In this position, the multi-plate clutch is open, the differential cannot be locked and torque cannot be biased. Once the range change is complete the system returns to clutch control mode. In the event of an electrical failure, the motor will default to this position.
center DIFFERENTIAL ASSEMBLY
The center differential assembly is the primary feature of the transfer box. Torque is transmitted through the center differential carrier and distributed to the differential gears and the front and rear output flanges. The planetary gear set, for the high/low range change function, is also an integral part of the center differential assembly.
The assembly comprises 3 differential pinion gears (4) and shafts (5), which are equally spaced within the center differential carrier (3). The differential shafts have a rigid connection to the differential carrier. Located between the pinion gears are 3 planetary pinion gears (6) and shafts (7). The planetary sun gear (8) and two differential side gears (10) are located in the center line of the carrier.
The planetary ring gear (2) is supported in both directions by the differential casing and the differential cover (9). The planetary ring gear is connected to a shifting sleeve, which is engaged in either high or low range.
The multi-plate clutch basket (11), which is welded to the differential casing, supports the friction plates, the dogteeth (12) for high range engagement and the synchronization cup and spring (1) for the 'shift-on-the-move' function.
When high range is engaged, the shifting sleeve (4) connects to the differential carrier via dogteeth (1). The planetary ring gear (3), via the shifting sleeve, and the planetary pinion gears (5), via the planetary shafts, are also attached to the differential carrier. The planetary gear set rotates as one unit and therefore turns the differential side gear with a 1: 1 ratio.
In low range the motor moves the shifting sleeve (4) in the direction of the low range dogteeth (5). The low range dogteeth, with the synchronization cup and spring, are fixed to the rear carrier assembly (6). When the shifting sleeve is engaged with the low range dogteeth, the planetary ring gear (3), via the shifting sleeve, is stationary and the planetary pinion gears (2), via the planetary bolts, turn the differential side gears with 2.93: 1 ratio.
High range actuation sequence
The rotational movement of the motor shaft turns the shifting cam (3) to high range position. The shifting cam then moves the shifting sleeve (1), via the high/low shifting fork (2), into the high range position. After the synchronization sequence, the planetary ring gear is connected to the high range dogteeth, via the shifting sleeve, on the differential carrier. In this position, the input speed equals the output speed, which equates to a high range ratio of 1:1.
Low range actuation sequence
The rotational movement of the motor shaft (4) turns the shifting cam (3) into low range position. The shifting cam then moves the shifting sleeve (1) of the center differential assembly via the high/low shifting fork (2) into low range position. After the synchronization sequence, the planetary ring gear is connected to the low range dogteeth, via the shifting sleeve, on the rear carrier assembly. The output speed is then reduced to a ratio of 2.93: 1.
MULTI-PLATE CLUTCH ASSEMBLY
The multi-plate clutch assembly for both center and rear differentials act in a similar way. The aim of the multi-plate clutch assembly is to prevent excessive differential slip and therefore maximize the traction performance of the vehicle. This is fundamentally different from the 'braked' traction control, which can only counter act differential slip when it occurs.
A certain amount of differential slip is required to allow the vehicle to turn corners and to remain stable under control of the Anti-lock Braking System (ABS). The transfer box control module monitors the driver's demands through primary vehicle controls and automatically sets the slip torque at the differentials. The system is completely automatic and does not require any special driver input.
The multi-plate clutch assembly actively controls the torque flow through the center differential and optimizes the torque distribution in the driveline. The clutch assembly biases the torque from the transmission to the axle and wheels with the higher grip and prevents the wheels with the lower grip from spinning.
The multi-plate clutch assembly comprises the sprocket (7), which is connected to the front differential side gear, the motor levers (5) with the ball ramp mechanism (6), the clutch hub (1) as support for the clutch plates (3), the clutch piston (4) to generate friction between the clutch plates, and a pack of cup springs (2) to return the clutch piston into its original position.
One set of friction plates are connected to the clutch hub-, the other set of friction plates are connected to the multi-plate clutch basket, which is welded to the center differential housing.
Multi-plate Clutch Actuation
Transfer box motor levers in initial position, multi-plate clutch open condition
Transfer box motor in end position, multi-plate clutch closed condition
By turning the clutch control disc (3), via the motor shaft (2), the motor levers (4) are rotated relative to each other. This relative movement acts on 5 balls (5) in a ramp mechanism between the two levers and give a defined axial movement. The movement forces the clutch piston (1) to induce friction between the plates supported by the clutch hub and the plates supported by the clutch basket on the differential carrier. This frictional force inhibits the differential rotation-, the differential carrier and front differential side gear are locked together.
TRANSFER BOX CONTROL MODULE
The transfer box control module controls the high/low 'shift-on-the-move' actuation and the multi-plate clutch actuation. The control module is located in the E-box, next to the Engine Control Module (ECM), behind the battery in the engine compartment. The position of the control module changes with LH and RH drive vehicles.
The control module is connected to the Controller Area Network (CAN) bus and controls the transfer box operation using CAN messages from other control modules on the network.
The control module memorizes the position of the transfer box motor when the ignition is switched off.
The transfer box control module uses the same actuator to control both range change function and application of center differential locking torque. The module uses position feed back from the actuator to provide smooth range changing capability and graduated application of locking torque appropriate for the current driving conditions. Range change can be carried out while moving providing the transmission is in neutral and the vehicle is below the speed necessary for the requested range change.
The control module uses three connectors for all inputs and outputs. It receives a permanent power supply via a 30A fusible link located in the Battery Junction Box (13,113), and an ignition supply via fuse 24 in the Central Junction Box (CJB).
The control module uses a series of programmed shift maps to control the synchronization speed and ensure that a maximum shift time of approximately one second is achieved.
If the control module is replaced, T4 must be connected to the vehicle and the transfer box control module self-calibration procedure must be performed. This procedure must also be performed if the transfer box motor assembly is replaced.
Default/Limp-home Strategy
If a fault occurs with the transfer box, the transfer box control module or one of the required input signals i.e. road speed signal, the control module records an error code and will respond appropriately to provide the highest level of system capability under the specific fault conditions. The following fault states are possible:
If a driveline over temperature condition has occurred, after the driveline has been allowed to cool, clutch control will be re-enabled and the warnings will disappear. There is no need to seek service assistance following an over temperature event.
If clutch control or Range change is not possible due to a permanent fault the driver must seek service assistance at the earliest opportunity.
If the system suffers a fault, which causes the transfer box to fail in neutral, the control module is designed to continue attempting to engage the requested range or return to its original range for a fixed number of attempts. If this has not been successful and the low range lamp is still flashing the driver should bring the vehicle to a halt and attempt the range change again while stationary. If this does not work after a number of attempts, key off for 30 seconds, restart engine and request range change again while stationary. The driver must seek service assistance at the earliest opportunity.
Transfer Box Control Module Pin Out Details
Connector C1319
Connector C1854
Connector C1855
TRANSFER BOX CONTROL MODULE INPUTS
The transfer box control module receives the following inputs:
^ Range change selection switch
^ High/low position sensor
^ Transfer box actuator motor temperature
^ Transfer box actuator motor position sensor
^ CAN bus messages
^ Gear position sensor (manual transmission only)
^ Transmission output shaft speed sensor (manual transmission only).
CAN Bus Messages
The CAN bus is a high speed broadcast network connected between various vehicle control modules. The CAN network carries an extensive list of messages between the different control modules enabling more sophisticated control with reduced complexity. Data on the network is packaged for efficient communication and prioritized according the urgency and importance of the Messages. The bus comprises two wires, which are twisted together to minimize electromagnetic interference (noise) produced by the CAN messages.
The transfer box control module is connected on the CAN bus and controls transfer box operation using CAN messages from other control units on the network. Wheel speed, vehicle acceleration, engine torque and speed, gear information, from the automatic transmission, temperature information, car configuration, axle ratios and Terrain Response TM mode inputs, are some of the main signals received by the control module.
In the event of a CAN bus failure the following symptoms may be observed:
^ Shift from high to low or low to high inoperative
^ Instrument cluster low range warning lamp inoperative
^ warning messages or lamps displayed in instrument cluster.
Gear Position Sensor (Manual Transmission Only)
The transfer box control module uses positional information from the manual gear position sensor to determine which gear the transmission is in. This information is broadcast on the CAN bus for display on the instrument cluster and for use by other vehicle systems. Vehicles fitted with automatic transmission use a similar message broadcast by the Transmission Control Module JCM). Vehicles fitted with manual transmission have a learning function, which compares the positional information from the sensor with the gear ratio calculated from the ratio of engine speed to transmission output shaft speed. The transmission learning is carried out at end of manufacture. If a new transmission is fitted during the life of the vehicle the learning algorithm needs to learn the characteristics of the new transmission.
The instrument cluster displays the selected gear as determined by the transfer box. The transfer box also uses this to check the vehicle is in neutral before attempting a range change.
Manual Transmission Output Shaft Speed Sensor
The output shaft speed sensor is located at the rear of the transmission and measures the speed of the transmission output shaft.
The transfer box is designed to allow range changes when the vehicle is moving, providing the transmission speed complies with the preset thresholds determined by the control module. The control module calculates the optimized synchronization timing through the speed of the transmission output shaft and the wheel speed of the vehicle.