AUTOMATIC TRANSMISSION ZF 8HP70

The ZF 8HP70 transmission is an electronically controlled, hydraulically operated, eight speed automatic unit.

A single unit known as the Mechatronic valve block is located inside the transmission. The Mechatronic valve block contains the following control components:

• Hydraulic control elements
• Electronic control elements
• TCM.

The ZF 8HP70 transmission has the following features:

• Designed to be maintenance free
• Automatic Transmission Fluid (ATF) is 'fill for life'
• The torque converter features a controlled slip feature with electronically regulated control of lock-up, creating a smooth transition to the fully locked condition
• Shift programs controlled by the TCM
• Adaptive Shift Strategy (ASIS), to provide continuous adaptation of shift changes to suit the driving style of the driver, which can vary from sporting to economical
• Connected to the Powertrain Control Module (PCM) via the FlexRay systems bus for communications
• Default mode if major faults occur
• Diagnostics available from the TCM via the FlexRay systems bus.

The transmission selections are made using the linear Transmission Control Switch (TCS) in the floor console. For additional information, refer to:External Controls - Vehicles With: 8HP70 8-Speed Automatic Transmission AWD (307-05A Automatic Transmission/Transaxle External Controls - Vehicles With: 8HP70 8-Speed Automatic Transmission AWD, Description and Operation).

On vehicles featured with the stop/start system, the 8HP70 automatic transmission is fitted with a Hydraulic Impulse Storage (HIS) device to ensure pressurized Automatic Transmission Fluid (ATF) is available to the shift elements as quickly as possible when the engine is started.

TRANSMISSION

The transmission comprises the main casing which houses all of the transmission components. The main casing also incorporates an integral torque converter housing.

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

A fluid filter is located inside the fluid pan. If the Automatic Transmission Fluid (ATF) becomes contaminated or after any service work, the fluid pan with integral filter must be replaced.

The transmission does not have a Bowden cable for park lock operation. This is initiated electronically when the Transmission Control Switch (TCS) is moved to the 'P' park position. An Emergency Park Release (EPR) mechanism is provided to release the park interlock if a failure occurs.

A feature of the eight 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 new system disengages one of the transmission clutches and only a minimum rotating load remains. This has the effect of further reducing fuel consumption.

The ATF pump is driven by a simplex chain and two drive gears from the input shaft. The ATF pump is a double stroke vane cell pump which delivers 50 cm³ of ATF per revolution.

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 main casing contains the following major components:

• Input shaft
• Output shaft
• Mechatronic valve block which contains the solenoids, speed sensors and the Transmission Control Module (TCM)
• Three rotating multiplate drive clutches
• Two fixed multiplate brake clutches
• Four planetary gear trains.

TORQUE CONVERTER

A torsional damper torque converter is used on 8HP70 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 via 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 via 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 via the lock-up clutch and the twin spring sets. The turbine inertia is coupled between the two 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.

The torque converter comprises a torsional damper, an impeller, a stator and a turbine. The torque converter is a sealed unit with all components located between the converter housing cover and the impeller. The two components are welded together to form a sealed, fluid filled housing. With the impeller brazed to the converter housing cover, the impeller is therefore driven at engine crankshaft speed.

The converter housing drive plate has four threaded bosses, which provide for attachment of the engine drive plate. The threaded bosses also provide for location of special tools which are required to remove the torque converter from the torque converter housing.

 

Lock-Up Clutch

 

The torque converter lock-up clutch is hydraulically controlled by an Electronic Pressure Regulating Solenoid (EPRS), which is controlled by the Transmission Control Module (TCM). This allows the torque converter to have three states of operation as follows:

• Fully engaged
• Controlled slip variable engagement
• Fully disengaged.

The torque converter pressure valve reduces system pressure and guarantees the pressure needed for the torque converter. It also limits the maximum torque converter pressure, to prevent the torque converter from expanding.

The EPRS is operated by Pulse Width Modulation (PWM) signals from the TCM to give full, partial or no lock-up of the torque converter.

The lock-up clutch is a hydro-mechanical device which eliminates torque converter slip, improving fuel consumption. The engagement and disengagement is controlled by the TCM to allow a certain amount of controlled 'slip'. This allows a small difference in the rotational speeds of the impeller and the turbine which results in improved shift quality. The lock-up clutch comprises a piston and a clutch comprising of friction and steel plates.

In the unlocked condition, the oil pressure supplied to the piston chamber is reduced and the pressure in the turbine chamber is allowed to push the piston back. In this condition the clutch plates are released and torque converter slip is permitted.

In the locked condition, the lock-up clutch spool valves are actuated by the EPRS. Pressurized Automatic Transmission Fluid (ATF) is directed into the lock-up clutch piston. The piston moves with the pressure and pushes the clutch plates together. As the pressure increases, the friction between the clutch plates increases, finally resulting in full lock-up of the clutch plates. In this condition there is direct mechanical drive from the engine crankshaft to the transmission planetary gear train.

The Transmission Idle Control feature is a standstill decoupling feature for the eight speed transmission. When the vehicle comes to a standstill with the brakes applied and the Transmission Control Switch (TCS) is in 'D' (Drive) position, the converter is disconnected from the driveline so that only a slight residual load remains. This further reduces fuel consumption. Decoupling is by actuating clutch B in the transmission, and is dependent on load and output speed.

AUTOMATIC TRANSMISSION FLUID PUMP

The Automatic Transmission Fluid (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 transmission cooler and the transmission to lubricate the gears and shafts.

The ZF 8HP70 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 torque converter. The pump has a delivery rate of 50 cm³ per revolution. The drive sprocket is driven at engine speed through a splined connection in the torque converter shell.

The Automatic Transmission Fluid (ATF) pump comprises:

• A sprocket
• A rear cover with bearing
• A front cover with bearing
• A cylinder
• A rotor shaft and a rotor with vanes.

A pressure relief valve is fitted in the pressure outlet gallery from the pump, but is not an integral part of the pump itself.

A sprocket is located around the transmission input shaft. Splines on the torque converter nose and the sprocket ensure a positive drive. A simplex chain transmits the rotation of the torque converter cover into rotation of the pump rotor shaft via a second sprocket fitted to the rotor shaft. The gearing of the two sprockets rotates the ATF pump rotor shaft at a speed slightly higher than the Revolutions Per minute (RPM) of the torque converter cover. The torque converter cover is directly connected to the engine crank.

The ATF pump contains seven vanes which are attached to the rotor and rotate within the cam shaped cylinder. As the vanes rotate, the eccentricity of the central hole in the cylinder causes the space between the vanes to increase. This causes a depression between the vanes and fluid is drawn into the space between the vanes via a suction port connected to the fluid pan. The fluid passes through the fluid pan filter before it is drawn into the ATF pump.

As the rotor shaft rotates further, the inlet port is closed by the vanes which have drawn in fluid, trapping the fluid in the space between the vanes. The eccentric hole in the cylinder causes the space between the vanes to decrease and consequentially compresses and pressurizes the fluid trapped between them.

Further rotation of the rotor shaft moves the vanes towards the outlet port. As the vanes pass the outlet port the pressurized fluid passes from the space between the vanes into the pressure gallery to the pressure relief valve.

As the ATF pump is a double stroke vane pump, this sequence is repeated twice per revolution of the rotor shaft.

The pressure relief valve controls the pressure and flow of ATF delivered to the transmission valve block, torque converter and other components. Pressure is controlled by a relief valve which limits the maximum system pressure to 32 bar (464 lbf/in²). The pressure control maintains a constant pressure of ATF irrespective of torque converter input shaft rotational speed. A metering orifice is subject to the pump output pressure. If the pressure in the orifice reaches a predetermined level, a spring loaded ball in the flow control valve is lifted from its seat and pressurized fluid is allowed to recirculate through the pump.

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:

• Transmission Control Module (TCM)
• Electrical actuators
• Speed sensors
• Control valves
• Hydraulic Impulse Storage (HIS) (if fitted).

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

The Mechatronic valve block comprises the following components:

• TCM
• Seven Electronic Pressure Regulating Solenoids (EPRS)
• Two park lock solenoids
• Twenty one hydraulic spool valves
• Temperature sensor
• Turbine speed sensor
• Output shaft speed sensor
• Hydraulic Impulse Storage (HIS) (if fitted).

 

Electronic Pressure Regulator Solenoids

Seven Electronic Pressure Regulator Solenoids (EPRS) are located in the valve block. The solenoids are controlled by Pulse Width Modulation (PWM) signals from the Transmission Control Module (TCM). The solenoids convert the electrical signals into hydraulic control pressure proportional to the signal to actuate the spool valves for precise transmission operation.

EPRS A, B, D, E and WK supply a higher control pressure as the signal amperage increases and can be identified by an orange connector cap. The TCM operates the solenoids using PWM signals. The TCM monitors engine load and clutch slip and varies the solenoid duty cycle accordingly. The solenoids have a 12 V operating voltage and a pressure range of 0 - 4.7 bar (0 - 68 lbf.in²).

EPRS C and SYS supply a lower control pressure as the signal amperage increases and can be identified by a gray connector cap. The TCM monitors engine load and clutch slip and varies the solenoid duty cycle accordingly. The solenoids have a 12 V operating voltage and a pressure range of 4.7 - 0 bar (68 - 0 lbf.in²).

The resistance of the solenoid coil winding for EPRS is 5.05 Ohms at 20 °C (68 °F).

 

Control Solenoid (MV 1)

A shift control solenoid Magnetic Valve 1 (MV1) is located in the valve block. The solenoid is controlled by the Transmission Control Module (TCM) and converts electrical signals into hydraulic control signals to control clutch application.

The shift control solenoid is an open/closed, on/off solenoid which is controlled by the TCM switching the solenoid to earth. The TCM also supplies power to the solenoid. The TCM energizes the solenoid in a programmed sequence for clutch application for gear ratio changes and shift control.

The resistance of the solenoid coil winding for solenoid is between 10 to 11 Ohms at 20 °C (68 °F).

 

Control Solenoid (MV 2)

A control solenoid Magnetic Valve 2 (MV 2) is located in the valve block. The solenoid is controlled by the Transmission Control Module (TCM) and converts electrical signals into hydraulic control signals to control the electronic park lock function.

The control solenoid is an on/off solenoid which is controlled by the TCM by switching the solenoid to earth.

When the park position is deselected, control solenoid MV2 resets the parking lock valve in the Mechatronic valve block. This is achieved by the TCM providing the ground for the solenoid which is energized, releasing the claws from retaining the park lock piston. Main Automatic Transmission Fluid (ATF) pressure acting on the parking lock piston, pushes the piston back to release the lock.

When the park position is selected, control solenoid MV2 is de-energized. The ATF pressure at the parking lock cylinder piston is vented and the mechanical interlock of the piston is opened. A pre-tensioned torsion spring at the park lock disc pulls the piston into the 'park' position. In the 'park' position the piston engages with the control solenoid claws and is locked in the park position. An emergency release wire cable can be used to release the parking lock manually if an electrical failure occurs.

The resistance of the solenoid coil winding for solenoid is 25 Ohms at 20 °C (68 °F).

When the neutral "N" position is selected and the engine is turned off, the ATF pressure at the park lock cylinder piston is released. The current supply to the control solenoid MV2 remains. The park lock cylinder piston is still held in the unlocked position by the spring force acting on the park lock disc. This prevents the park lock plate from engaging the parking lock. This allows the vehicle to be moved when the engine is not running for a short time. Should the battery voltage fall below the level required to maintain the solenoid in the energized condition, the park lock will be engaged.

 

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 Transmission Control Module (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 Powertrain Control Module (PCM). Using a comparison of the two 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 Transmission Control Module (TCM) uses the temperature sensor signals to determine the temperature of the transmission fluid. 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 fluid cooling by controlling the transmission operation when high fluid temperatures are experienced. If the sensor fails, the TCM will use a default value and a fault code will be stored in the TCM.

 

Spool Valves

The valve block contains spool valves which control various functions of the transmission. The spool valves are of conventional design and are operated by Automatic Transmission Fluid (ATF) pressure.

Each spool valve is located in its spool bore and held in a default (unpressurized) position by a spring. The spool bore has a number of ports which allow ATF to flow to other valves and clutches to enable transmission operation. Each spool has a piston which is waisted to allow ATF to be diverted into the applicable ports when the valve is operated.

When ATF pressure moves a spool, one or more ports in the spool bore are covered or uncovered. ATF is prevented from flowing or is allowed to flow around the applicable waisted area of the spool and into another uncovered port. The ATF is either passed through galleries to actuate another spool, operate a clutch or is returned to the fluid pan.

HYDRAULIC IMPULSE STORAGE (IF FITTED)

The Hydraulic Impulse Storage (HIS) system if fitted to vehicles with the auto stop/start system.

The HIS system comprises a cylindrical accumulator which contains an electro-mechanical locking unit, a spring actuated piston and a one-way restrictor. The accumulator is located at the rear of the Mechatronic valve block and is secured in position with three screws and sealed into a port in the transmission casing with an O-ring seal.

The electro-mechanical locking unit comprises a low-current solenoid, a spring loaded keeper incorporating a magnetic core and a number of balls. The keeper has a detent into which the balls locate during filling of the HIS when the engine is running and the Automatic Transmission Fluid (ATF) pump is producing pressure.

The one-way restrictor is located in the inlet/outlet port of the accumulator. The restrictor provides a controlled charging of the HIS, to allow a small volume flow of ATF. This ensures that the operation of the transmission shift elements are not compromised by a sudden drop in ATF pressure. The restrictor allows a charging time for the HIS of approximately 5 seconds. When discharge is required, the restrictor allows full flow from the cylinder.

The filling process for the HIS has several steps:

• When the engine is running and the ATF pump is producing pressure, the one-way restrictor allows a controlled flow of ATF which acts on the piston.
• The ATF pressure moves the piston into the accumulator cylinder. A locking ring on the piston passes over the balls which are at this point located in the keeper detent.
• As the piston continues to move, a spring in the center of the keeper moves the locking cylinder and the magnetic core towards the solenoid windings and into the final fully charged position. The energized solenoid holds the magnetic core and the balls are lifted from the detent by the movement of the keeper, locking the piston in the charged position. The HIS is now electro-mechanically locked and ready for an engine stop to occur.
• When the engine is stopped, the ATF pump also stops and ATF pressure is decayed. The pressure acting on the piston is also decayed and the piston moves to be held in the locked position by the balls. The energy required for hydraulic filling during engine start is now stored in the tensioned piston spring. The solenoid remains energized to hold the keeper in position and the balls out of the detent to lock the piston.

Charging

Charged

Engine start process:

• When the engine is restarted, the solenoid holding current is removed, which starts the unlocking process.
• The magnetic core is released and the keeper is moved towards the piston under spring pressure. The balls fall into the detent in the keeper, releasing the piston.
• The piston is moved under spring pressure, pushing out the volume of ATF. The one-way restrictor opens fully to allow unrestricted flow of ATF from the accumulator cylinder into the transmission housing. The process is completed between 300 and 350 ms.
• Once the engine is started, the ATF pump produces flow and pressure to provide seamless transmission shift element engagement as soon as the engine is started.

Discharging

DRIVE CLUTCHES

There are three drive clutches and two 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 via 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 and holds the piston off the clutch plate assembly.

When clutch application is required, main pressure from the Automatic Transmission Fluid (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 TRAINS

The eight forward gears and the reverse gear are produced by a combination of four simple planetary gear sets, three clutches and two brakes. The front two gear sets share a common sun gear. Power is output always through the planetary carrier of the fourth gearset.

Five shift elements comprising three clutches and two brakes, are responsible for all eight forward and reverse gears. High efficiency is achieved by the use of only two shift elements disengaged in each gear which reduces drag and so increases the efficiency.

 

Planetary Gear Sets 1, 2 and 3

The planetary gear sets 1 and 2 comprise:

• Sunwheel - shared by both gear sets
• Three or four planetary gears per gear set
  • NOTE: Ingenium I4 2.0L Diesel engine fitment has three planetary gears in gearset 2, all other engine variants have four planetary gears.
• Planetary gear carrier (spider) per gear set
• Ring gear per gear set.

The planetary gear set 3 comprises:

• Sunwheel
• Three planetary gears
• Planetary gear carrier (spider)
• Ring gear.

 

Planetary Gear Set 4

The planetary gear set 4 comprises:

• Sunwheel
• Four planetary gears
• Planetary gear carrier (spider) - output shaft
• Ring gear.

TRANSMISSION CONTROL MODULE

The Transmission Control Module (TCM) is an integral part of the Mechatronic valve block which is located at the bottom of the transmission, within the fluid pan. The TCM is the main controlling component of the transmission.

The TCM processes signals from the transmission speed and temperature sensors, Powertrain Control Module (PCM) and other vehicle systems. From the received signal inputs and pre-programmed data, the module calculates the correct gear, torque converter clutch setting and optimum pressure settings for gear shift and lock-up clutch control.

POWER FLOWS

Operation of the transmission is controlled by the Transmission Control Module (TCM), which electrically activates various solenoids to control the transmission gear selection. The sequence of solenoid activation is based on programmed information in the TCM memory and physical transmission operating conditions such as:

• Vehicle speed
• Throttle position
• Engine load
• Transmission Control Switch (TCS) position.

All gear shifts from 1st to 8th and 8th to 1st are known as 'overlap' shifts. Overlap shifts are during a gear shift one clutch must remain capable of transmitting torque at a reduced main pressure until the other clutch is ready to accept the torque.

Engine torque is transferred, via operation of single or combinations of clutches to the 4 planetary gear trains. All gear trains are controlled by reactionary inputs from brake clutches to produce the eight forward gears and one reverse gear. The ratios are as follows:

The shift elements, clutches and brakes are actuated hydraulically. Fluid pressure is applied to the required clutch and/or brake, pressing the plates together and allowing drive to be transmitted through the plates. The purpose of the shift elements is to perform power-on shifts with no interruption to traction and smooth transition between gear ratios.

INSTRUMENT CLUSTER

The virtual display Instrument Cluster (IC) is connected to the Body Control Module/Gateway Module (BCM/GWM) on the High Speed (HS) Controller Area Network (CAN) comfort systems bus. The Transmission Control module (TCM) is connected to the BCM/GWM on the FlexRay systems bus.

Automatic transmission status is transmitted from the TCM to the virtual display IC via the BCM/GWM. Transmission status is indicated to the driver by a message in the message center or illumination of a warning indicator. For additional information, refer to:Instrument Cluster (413-01 Instrument Cluster, Description and Operation).

 

The Thin Film Transistor (TFT) display instrument cluster is connected to the TCM via the HS CAN powertrain systems, the BCM/GWM and the FlexRay systems bus. Transmission status is transmitted by the TCM and displayed to the driver in one of two displays in the instrument cluster. For additional information, refer to:Instrument Cluster (413-01 Instrument Cluster, Description and Operation).

 

Malfunction Indicator Lamp

 

The Malfunction Indicator Lamp (MIL) is located in the virtual display Instrument Cluster (IC). Transmission related faults which may affect the vehicle emissions will illuminate the MIL.

The MIL is illuminated by the Powertrain Control Module (PCM) on receipt of a relevant fault message from the Transmission Control Module (TCM) on the FlexRay system bus. The nature of the fault can be diagnosed using approved diagnostic equipment which reads the fault codes stored in the TCM memory.

 

Transmission Status Display

 

The transmission status display is located in the virtual display Instrument Cluster. The display shows the Transmission Control Switch (TCS) or the selected gear when in manual and sport modes.

The following table shows the displays and their descriptions.

 

Message Center

 

The message center is located in the virtual display Instrument Cluster (IC). The message center display relays vehicle status and operating information to the driver and can display messages relating to a number of vehicle systems. If a transmission fault occurs, the message GEARBOX FAULT is displayed in the message center. For additional information, refer to:Instrument Cluster (413-01 Instrument Cluster, Description and Operation).

TRANSMISSION CONTROL MODULE

The Transmission Control Module (TCM) outputs signals to control the hydraulic operation of the transmission. Signals are sent to the shift control solenoid valves and the Electronic Pressure Regulating Valve's (EPRS).

The TCM processes signals from the transmission speed and temperature sensors, the Transmission Control Switch (TCS), the Powertrain Control Module (PCM) and other vehicle systems. From the received signal inputs and pre-programmed data, the TCM calculates the:

• Correct gear
• Torque converter clutch setting
• Optimum pressure settings for gear shift and lock-up clutch control.

The PCM supplies the engine management data over the FlexRay systems bus. The TCM requires engine data to efficiently control the transmission operation, for example:

• Flywheel torque
• Engine speed
• Accelerator pedal angle
• Engine temperature.

The steering angle sensor and the Anti-lock Brake System (ABS) control module also supply data to the TCM on the FlexRay systems bus. The TCM uses data from these systems to suspend gear changes when the vehicle is cornering and/or the ABS control module is controlling braking or traction control.

The Body Control Module/Gateway Module (BCM/GWM) supplies steering wheel paddle data over the FlexRay systems bus. The TCM uses this to schedule driver requested upshifts and downshifts.

Using the signal inputs and the memorized data, the TCM control program computes:

• the correct gear and torque converter lock-up clutch setting
• the optimum pressure settings for gear shift and lock-up clutch control.

Special output-side modules (power output stages, current regulator circuits), allow the TCM to control the solenoid valves and pressure regulators. This allows precise control the hydraulics of the automatic transmission. In addition, the amount and duration of engine interventions are supplied to the engine management via the FlexRay systems bus.

The TCM determines the position of the TCS using signals from the TCS via the High Speed (HS) Controller Area Network (CAN) systems bus.

The TCM transmits the position of the TCS and any manual gear selected on the FlexRay systems bus to the BCM/GWM. This information is shown in the gear selector display in the instrument cluster.

Engine Stall

If the engine stalls it will coast down in gear, with the transmission providing drive to the engine. A restart can be attempted at this point and the engine may start and the driver can continue.

If the coast down speed reduces such that the speed of the engine is less than 100 Revolutions Per Minute (RPM), the transmission will go to neutral, D illumination will flash in the instrument cluster. The driver needs to select neutral or park and then press the brake pedal to restart the engine.

If the start/stop switch is pressed when driving, the message ENGINE STOP BUTTON PRESSED is displayed in the message center but there will be no change to the ignition state. If the driver requires to switch off the engine, the start/stop switch must be pressed for a second time. The engine will be stopped and will be back driven by the transmission as the vehicle coasts down.

A = HARDWIRED: AN = HIGH SPEED (HS) CONTROLLER AREA NETWORK (CAN) POWERTRAIN SYSTEMS BUS: AP = HS CAN COMFORT SYSTEMS BUS: AX = FLEXRAY SYSTEMS BUS.