Land Rover Defender: Description

AUTOMATIC TRANSMISSION

The automatic transmission comprises the main transmission housing which contains all of the transmission components.

A fluid pan is attached to the lower face of the transmission housing and is secured with bolts. The fluid pan is sealed to the transmission housing with a gasket. Removal of the fluid pan allows access to the mechatronic valve block and the electric ATF pump. The fluid pan has magnets located at the rear which collects any ferrous metallic particles present in the ATF.

The ATF filter is located inside the fluid pan. If the ATF becomes contaminated or after any service work, the fluid pan with integral ATF filter must be replaced.

The transmission does not have a bowden cable for park lock operation. This is initiated electronically when the TCS is moved to the PARK (P) position. An EPR mechanism is provided to release the park interlock if a failure occurs.

A feature of the 8 speed transmission is decoupling of the transmission when the vehicle is at a standstill. Normally the transmission remains in gear with the torque converter slipping and the vehicle is prevented from moving by applying the brake. The system disengages 1 of the transmission clutches and only a minimum rotating load remains.

This has the effect of further reducing fuel consumption.

The integral torque converter housing provides protection for the torque converter assembly and also provides the attachment for the transmission to the engine. The torque converter is a non-serviceable assembly which also contains the lock-up clutch mechanism.

The transmission housing contains the following major components:

• Input shaft.
• Output shaft.
• Mechatronic valve block which contains the solenoids, speed sensors and the TCM.
• 3 rotating multi-plate drive clutches.
• 2 fixed multi-plate brake clutch.
• 4 planetary gear sets.

TRANSMISSION SECTIONAL VIEW

1. Transmission housing
2. Gear set 1
3. Gear set 2
4. Gear set 3
5. Gear set 4
6. Output shaft
7. Clutch D
8. Clutch C
9. Clutch E
10. Mechatronic valve block
11. Brake B
12. Brake A
13. Fluid filter
14. Fluid pan
15. ATF pump
16. Torque converter
17. Input shaft

TORQUE CONVERTER

A torsional damper torque converter is used on 8HP76 transmissions.

The torsional damper enables a more direct engine connection for highly efficient power transmission when starting and shifting. The inner spring set is connected directly to the transmission, the outer spring set responds to torque from the engine through the lock-up clutch. When the lock-up clutch is applied, the hydro-dynamic circuit is by-passed and torque is transferred directly to the transmission input shaft. In comparison to conventional torque converters, the torsional damper applies the lock-up clutch earlier. This improves driving comfort at low engine speeds and increases fuel efficiency by enabling the engine to be operated at lower Revolutions Per Minute (RPM) levels.

When the converter lock-up clutch is not applied, torque is transferred hydro-dynamically. Power from the engine is transmitted through the pump and turbine to the transmission input shaft. When the converter lock-up clutch is applied, the hydro-dynamic circuit is by-passed, torque is transferred directly through the lock-up clutch and the tandem spring sets. The turbine inertia is coupled between the 2 spring sets which improves disengagement, helps reduce wear on the transmission and improve noise insulation.

The torque converter is the coupling element between the engine and the transmission and is located in the torque converter housing, on the engine side of the transmission. The driven power from the engine crankshaft is transmitted hydraulically and mechanically through the torque converter to the transmission. The torque converter is connected to the engine by a drive plate attached to the rear of the crankshaft.

AUTOMATIC TRANSMISSION FLUID PUMP

The mechanical ATF pump is an integral part of the transmission. The ATF pump is used to supply hydraulic pressure for the operation of the control valves and clutches. The ATF pump also passes the ATF through the ATF cooler and to lubricate the gears and shafts.

The mechanical ATF pump is a double stroke, vane type pump and is located below the transmission input shaft. The pump is driven by a chain drive from a sprocket located on the input shaft.

MECHATRONIC VALVE BLOCK

The mechatronic valve block is located in the bottom of the transmission and is covered by the fluid pan.

The valve block houses the following components:

• The TCM
• Speed sensors
• Electrical actuators
• Control valves.

The above components provide all electro-hydraulic control for all transmission functions.

The Mechatronic valve block comprises the following components:

• The TCM
• Park lock solenoids (quantity 2)
• Hydraulic spool valves (quantity 10)
• Temperature sensor
• Turbine speed sensor
• Output shaft speed sensor.

SENSORS

Speed Sensors

The turbine speed sensor and the output shaft speed sensor are Hall effect type sensors located in the mechatronic valve block and are not serviceable items. The TCM monitors the signals from each sensor to determine the input (turbine) speed and the output shaft speed.

The turbine speed is monitored by the TCM to calculate the slip of the torque converter clutch and internal clutch slip.

This signal allows the TCM to accurately control the slip timing during shifts and adjust clutch application or release pressure for overlap shift control.

The output shaft speed is monitored by the TCM and compared to engine speed signals received on the FlexRay system bus from the PCM. Using a comparison of the 2 signals the TCM calculates the transmission slip ratio for plausibility and maintains adaptive pressure control.

Temperature Sensor

The temperature sensor is also located in the mechatronic valve block. The TCM uses the temperature sensor signals to determine the temperature of the ATF. These signals are used by the TCM to control the transmission operation.

The TCM uses the temperature signals to promote faster warm-up in cold conditions. The TCM also uses the temperature signal to assist with ATF cooling by controlling the transmission operation when high ATF temperatures are experienced. If the sensor fails, the TCM uses a default value and a Diagnostic Trouble Code(s) (DTC) is stored in the TCM.

MULTIPLE CLUTCH BRAKE - TYPICAL

There are 3 drive clutches and 2 brakes used in the transmission. Each clutch comprises a number of friction plates dependent on the output controlled. A typical clutch consists of a number of alternating steel plates and plates with friction material bonded to each face.

The clutch plates are held apart mechanically by a diaphragm spring and hydraulically by dynamic pressure. The pressure is derived from a lubrication channel which supplies fluid to the bearings and clutch cooling. The fluid is passed through a drilling in the input shaft into the chamber between the baffle plate and the piston. To prevent inadvertent clutch application due to pressure build up produced by centrifugal force, the fluid in the dynamic pressure equalization chamber overcomes any centrifugal pressure in the piston chamber. It holds the piston off the clutch plate assembly.

When clutch application is required, main pressure from the mechanical ATF pump is applied to the piston chamber from the supply port. This main pressure overcomes the low pressure fluid present in the dynamic pressure equalization chamber. The piston moves, against the pressure applied by the diaphragm spring, and compresses the clutch plate assembly. When the main pressure falls, the diaphragm spring pushes the piston away from the clutch plate assembly, disengaging the clutch.

PLANETARY GEAR SETS

The 8 forward gears and the reverse gear are produced by a combination of 4 simple planetary gear sets, 3 clutches and 2 brakes. The front 2 gear sets share a common sun gear. Power is output always through the planetary carrier of the 4th gearset.

The 5 shift elements comprising 3 clutches and 2 brakes, are responsible for all 8 forward and reverse gears. High efficiency is achieved by the use of only 2 shift elements disengaged in each gear which reduces drag and so increases the efficiency.

OVER TEMPERATURE MESSAGES

The transmission clutch temperatures are modeled within the TCM. The 8HP76 transmission incorporates additional monitoring of the torque converter as this is worked hard on low speed crawl situations. The TCM uses algorithms to detect high load on the launch clutch and when detected provides a message to the driver to select low-range. Further monitoring then provides a critical warning to the driver with a notification sound from the Instrument Panel Cluster (IPC) to select low-range if this first message is ignored.

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