Rear Drive Axle and Differential
Rear Drive Axle and Differential
Overview
The open rear differential converts the 'angle of drive' through 90° and distributes drive, via the rear drive halfshafts, to the rear wheels.
The differential unit is mounted to the chassis via rubber bushes and bolts; two mounting points at the rear of the unit and one at the front.
OPEN REAR DIFFERENTIAL ASSEMBLY
Open Rear Differential - Exploded View
The cast iron casing comprises two parts; a cover and a carrier. The carrier provides locations for all the internal components. The carrier is sealed to the cover via an O-ring seal and secured with twelve bolts. The cover and carrier have cast fins, which assist mobility. A breather tube is fitted to the top of the carrier. This allows a plastic tube to be fitted and routed to a high point under the vehicle body, preventing the ingress of water when the vehicle is wading.
The carrier contains an oil drain plug. The differential unit contains a specified oil. For additional information, refer to Specifications Rear Drive Axle/Differential
The differential is a conventional design using a hypoid gear layout, similar to the front differential. The open rear differential is available in various ratios depending on engine installation. For additional information, refer to Specifications Rear Drive Axle/Differential Changing the number of teeth between the crown wheel drive gear and pinion gear changes the ratio.
The differential comprises a pinion shaft and hypoid pinion-gear and a crown wheel drive-gear with an integral cage, which houses two planet gears. Two sun wheels are also located in the cage and pass the rotational drive to the drive shafts.
The pinion shaft is mounted on two opposed taper roller bearings, with a collapsible spacer located between them. The spacer is used to hold the bearings in alignment and also collapses under the pressure applied to the pinion flanged nut. This allows the flanged nut to be tightened to a predetermined torque, which collapses the spacer, setting the correct bearing preload.
The pinion shaft has an externally splined outer end, which accepts and locates the input flange, which is retained by the pinion nut and retainer. The input flange has four threaded holes and mates with the rear drive shaft. Four bolts secure the rear drive shaft to the input flange. An oil seal is pressed into the pinion housing and seals the input flange to the pinion housing. The pinion shaft has a hypoid gear at its inner end, which mates with the crown wheel drive gear.
The crown wheel drive gear is located on the differential case and secured with ten screws. The differential case is mounted on taper roller bearings located in machined bores on each side of the pinion housing. Shims are retained in the casing behind the bearing cups, the shim thickness is selected to apply the correct bearing preload and hypoid backlash.
The differential carrier has a through hole, which provides location for the shaft. The shaft is supported by a sun gear and a needle roller bearing. The shaft is fitted with a snap ring at one end, which locates in a machined groove in the sun gear, locking the shaft in position.
The sun gears are located in pockets in the carrier cage and mesh with the planet gears. Spacers are fitted between the sun wheels and the carrier and set the correct mesh contact between the planet gears and the sun wheels. Each sun wheel has a machined bore with internal splines and machined groove near the splined end. The groove provides positive location for a snap ring fitted to the end of each output flange.
Each output shaft has a spline, which locates in each sun wheel. A snap ring fitted to the splined shaft locates in the groove the sun wheel bore and positively located the output shaft. Oil seals are pressed into each side of the pinion housing and seal the seal the output shaft.
Differential Operation
The operating principles of the front and rear differentials are the same. Rotational input from the drive shaft is passed via the input flange to the pinion shaft and pinion gear. The angles of the pinion gear to the crown wheel drive gear moves the rotational direction through 90°.
The transferred rotational motion is now passed to the crown wheel drive gear, which in turn rotates the differential casing. The shaft, which is secured to the casing, also rotates at the same speed as the casing. The planet gears, which are mounted on the shaft, also rotate with the casing. In turn, the planet gears transfer their rotational motion to the left and right hand sun wheels, rotating the drive halfshafts.
When the vehicle is moving in a forward direction, the torque applied through the differential to each sun wheel is equal. In this condition both drive halfshafts rotate at the same speed. The planet gears do not rotate and effectively lock the sun wheels to the differential casing.
If the vehicle is turning, the outer wheel will be forced to rotate faster than the inner wheel by having a greater distance to travel. The differential senses the torque difference between the sun wheels. The planet gears rotate on their axes to allow the outer wheel to rotate faster than the inner one.
ELECTRONIC REAR DIFFERENTIAL ASSEMBLY
Electronic Rear Differential - Exploded View
The electronic rear differential has the same functionality as the open rear differential but incorporates a locking feature.
An electronically controlled multi-plate clutch provides a rear differential lock and torque biasing function to give improved traction performance and vehicle dynamic stability.
A strategy to electronically control the rear 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, for example when parking the vehicle.
The unit receives a torque input from the transfer box output-shaft, which is passed through the unit to two outputs for the rear drive halfshafts.
The unit detects wheel-slip via various vehicle system inputs to the differential locking module and locks the differential accordingly.
The electronic rear differential locking and biasing feature is actuated via a DC motor, which is controlled by the differential locking module, via a PWM (pulse width modulation) signal.
Multi-plate Clutch Assembly
The multi-plate clutch assembly for both centre (transfer box) and electronic 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 ABS (anti-lock brake system). The transfer box control module monitors the driver's demands through primary vehicle controls and automatically sets the slip torque at the rear differential via the differential locking module. 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 rear differential and optimizes the torque distribution in the driveline. The clutch assembly biases the torque from the differential to the wheels with the higher grip and prevents the wheels with the lower grip from spinning.
By turning the input actuator disc, via the motor shaft, the output actuator is rotated. This movement acts on 5 balls in a ramp mechanism between the input and output actuators and gives a defined axial movement. The movement forces the pressure disc to induce friction between the sun gear and differential case via the clutch plates supported by the sun gear and the plates supported by the clutch basket on the differential case. This frictional force inhibits the differential rotation; the differential case and left hand differential side gear are locked together.
Differential Locking Module
The differential locking module controls the multi-plate clutch actuation. The locking module is mounted on a bracket located on the LH C-pillar, behind the trim.
The module is connected on the high-speed CAN (controller area network) bus and controls the differential operation using CAN (controller area network) messages from other control modules on the network.
The module uses three connectors for all inputs and outputs. It receives a permanent power supply from the EJB (engine junction box), and an ignition supply from the CJB (central junction box).
The module memorizes the position of the differential actuator motor when the engine is switched off.
The locking module controls the closed-loop position sensing system within the motor and regulates the power supply to the motor.
If any of following components are replaced:
- differential locking module
- differential actuator motor
- differential assembly.
An approved diagnostic system must be connected to the vehicle and the differential locking module self-calibration procedure performed.
If a fault occurs with the electronic differential, the locking module or one of the required input signals, for example; road speed signal, the locking module records an error code and a warning lamp, in the instrument cluster, illuminates permanently.
CAN Bus Messages
The high-speed CAN (controller area network) is a broadcast network connected between various vehicle control modules. It allows the fast exchange of data between control modules every few microseconds.
The differential locking module is connected on the high-speed CAN (controller area network) bus, via the transfer box control module, and controls differential operation using CAN messages from other control units on the network. Wheel speed, steering angle, automatic transmission speed, temperature information, vehicle configuration, axle ratios and mode inputs, are some of the main signals received by the locking module.
The locking module also sends messages via the CAN bus to tell other control modules on the network, the status of the electronic rear differential. The clutch torque and default mode status are some of the main signals sent out by the locking module.
The following table shows the messages that can be displayed in the message centre of a high-line instrument cluster relating to the electronic rear differential:
On vehicles fitted with the low line instrument cluster, in place of the message centre there will be a status lamp, which has the following logic:
- Amber - Over temperature
- Red - Failure, stop vehicle
DIAGNOSTICS
The electronic rear differential locking module can store fault codes, which can be retrieved using an approved diagnostic system.
The information is communicated via a diagnostic socket. The diagnostic socket allows the exchange of information between the various control modules on the bus systems and the diagnostic equipment. The information is communicated to the socket via the CAN bus. This allows the retrieval of diagnostic information and programming of certain functions using the diagnostic equipment.
The electronic differential locking module uses DTC (diagnostic trouble code), which relate to electronic rear differential electrical faults.
ELECTRONIC DIFFERENTIAL CONTROL DIAGRAM
NOTE:
A = Hardwired; D = high-speed CAN bus.
