Rear Drive Axle and Differential
Rear Drive Axle and Differential
OPEN REAR DIFFERENTIAL
Component Location
OVERVIEW
The rear differential converts the 'angle of drive' through 90 degrees and distributes drive, via the rear drive halfshafts, to the rear wheels.
The output ratio of the rear differential varies depending on the engine variant. For additional information, refer to Specifications Rear Drive Axle/Differential
The unit is located centrally in the rear of the chassis and is mounted to the chassis via rubber bushes and bolts; two mounting points at the front of the unit and one at the rear.
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 with sealant and secured with 12 bolts. The cover and carrier have cast ribs, 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 differential is a conventional design using a hypoid gear layout, similar to the front differential. The ratio is changed by changing the amount of teeth between the crown wheel drive gear and pinion gear. For example, with a ratio of 3.54:1, the crown wheel drive gear will have 3.54 times more teeth than the pinion gear.
The carrier contains an oil drain plug.
OPEN REAR DIFFERENTIAL INTERNAL COMPONENTS
The differential comprises a pinion shaft and hypoid pinion gear and a crown wheel drive gear with an integral cage, which houses 2 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 by measuring the torque-to-turn, 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. 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 10 bolts. The differential case is mounted on taper roller bearings located in machined bores on each side of the pinion housing. Belleville washers are used 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 of the sun wheel bore and positively locates the output shaft. Oil seals are pressed into each side of the pinion housing and seals the output shaft.
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 degrees.
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 TORQUE MANAGED (ETM) REAR DIFFERENTIAL
Component Location
OVERVIEW
The Electronic Torque Managed (ETM) rear differential is available as an option on both petrol and diesel derivatives. The output ratio for the ETM rear differential remains the same as the open differentials for both petrol and diesel derivatives.
The ETM differential has the same functionality as the open rear differential but incorporates a locking feature.
ELECTRONIC TORQUE MANAGED (ETM) REAR DIFFERENTIAL INTERNAL COMPONENTS
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, e.g. parking.
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 ETM rear differential control module and locks the differential accordingly.
The ETM differential locking and biasing feature is actuated via a Direct Current (DC) motor, which is controlled by the ETM rear differential control module, via a Pulse Width Modulation (PWM) signal.
Multi-plate Clutch Assembly
The multi-plate clutch assembly for both center (transfer box) and ETM 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 counteract 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 rear differential, via the ETM rear differential control 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 five 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.
Electronic Torque Managed (ETM) Rear Differential Control Module
The ETM rear differential control module controls the multi-plate clutch actuation. The control module is mounted on a bracket located on the LH C-pillar, behind the trim.
The control module is connected on the Controller Area Network (CAN) bus and controls the differential operation using CAN messages from other control modules on the network.
The control module uses three connectors for all inputs and outputs. It receives a permanent power supply via a 40A fusible link located in the Battery Junction Box (BJB), and an ignition supply via fuse 24 located in the Central Junction Box (CJB).
The control module memorizes the position of the ETM rear differential motor when the ignition is switched off.
The control module controls the closed loop position sensing system within the motor and regulates the power supply to the motor.
If the control module is replaced, a Land Rover approved diagnostic tool must be connected to the vehicle and the ETM rear differential control module self-calibration procedure must be performed. This procedure must also be performed if the motor or differential assembly is replaced.
If a fault occurs with the ETM rear differential, the control module or one of the required input signals, i.e. road speed signal, the control module records an error code and a warning lamp, in the instrument cluster, illuminates permanently.
CAN Bus Messages
The CAN bus is a high speed broadcast network connected between various vehicle control modules. It allows the fast exchange of data between control modules every few microseconds. The bus comprises 2 wires, which are twisted together to minimize electromagnetic interference (noise) produced by the CAN messages.
The ETM rear differential control module is connected on the CAN 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, car configuration, axle ratios and mode inputs, are some of the main signals received by the control module.
The control module also sends messages via the CAN bus to tell other control modules on the network, the status of the ETM rear differential. The clutch torque and default mode status are some of the main signals sent out by the control module.
The following table shows the messages that can be displayed in the message center of a high-line instrument cluster relating to the ETM rear differential:
On vehicles fitted with the low line instrument cluster, in place of the message center there will be a status lamp, which has the following logic:
- Amber - Over temperature
- Red - Failure, stop vehicle
TERRAIN RESPONSE
The Terrain Response system allows the driver to select a program, which will provide the optimum settings for traction and performance for the prevailing terrain conditions.
Depending on vehicle specification the system is controlled by either a rotary control or rocker switch located on the floor console:
The system uses a combination of vehicle subsystems to achieve the required vehicle characteristics for the terrain selected. The following subsystems form the Terrain Response system:
- Engine management system (EMS)
- Automatic transmission
- Transfer box
- Brake system
- Air suspension
Each subsystem control module provides a feedback for the selected program so that the Terrain Response control module can check that all systems are controlling the system correctly. The exception to this is the ETM rear differential control module, which does not provide feedback to the Terrain Response system as it is a slave to the transfer box control module. For additional information, refer to Ride and Handling Optimization
SERVICE
The oil used in the ETM rear differential contains unique additives and friction modifiers, which enhance the differentials operation. No other oil must be used in the ETM rear differential. For additional information, refer to Specifications Rear Drive Axle/Differential
Electronic Torque Managed (ETM) Rear Differential Serviceable Components
- Halfshaft seals
- Needle roller bearing assembly
- Chassis bush/fixings
- Actuator motor
- Temperature sensor
- Control module and bracket
- Lubricant
DIAGNOSTICS
The ETM rear differential control module can store fault codes, which can be retrieved by connecting the Land Rover approved diagnostic tool using ISO-14229 protocol.
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 Land Rover approved diagnostic tool. 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 Land Rover approved diagnostic tool.
The ETM rear differential control module uses Diagnostic Trouble Codes (DTC), which relate to ETM rear differential electrical faults.