Unit Repair
General Description
The ZF Transmission covered in this section comprises a hydrodynamic torque converter, four speed gear train and electronic-hydraulic transmission control unit.
Transmission Overview:
The automatic transmission provides six gearshift positions:
- Position 'P': Park-in this position the driven wheels are mechanically locked at the transmission.
- Position 'R': Reverse - reverse gear selected.
- Position 'N': Neutral - engine disconnected from driveline and wheels.
- Position 'D': Drive - all four gears are selected automatically with lock-up available in top gear only.
- Position '3': automatic selection of the lowest three gears only.
- Position '2': automatic selection of the lowest two gears only, transmission is prevented from shifting up to the third and top speed ranges.
Immediate selection of a lower ratio is also available by 'kick-down' (pressing the accelerator down beyond the normal full throttle position) for example when overtaking.
Torque Converter
The torque converter serves two main functions: it acts as a fluid coupling between engine and transmission gear train and it provides multiplication of engine torque when required.
The torque converter consists of an impeller, a turbine and a stator which is mounted on a one-way clutch. The converter cover is fixed to the impeller to seal the three elements within a common housing through which oil flows when the transmission is in operation. The impeller is therefore connected to the engine and rotates at engine speed when the engine is operating.
Torque multiplication is at its greatest when the vehicle is driven away from standstill. The stator bears against the housing through its one-way clutch and deflects the oil so that the flow is accelerated. As road speed builds up, the impeller and turbine wheels turn at almost the same speed, so that the stator ceases to accelerate the oil flow and instead is disengaged from the housing and turns freely in the flow of oil. At this stage, the torque converter acts as a fluid coupling
The torque converter acts in all four forward gears and in reverse. The converter lock-up clutch closes in 4th gear at a point depending on road speed and accelerator position; engine power is then transmitted by purely mechanical means.
A low-loss oil pump in the front of the transmission housing supplies the converter, the epicyclic gear train, the hydraulic control system and the lubrication circuit; oil is drawn from a sump in the bottom of the gear train housing through a filter. The pump drive gear is keyed to the converter impeller hub and therefore turns when the engine is running, thereby pressurizing the oil.
Four-speed Epicyclic Gear Train
The torque converter drives a mechanical epicyclic gear train providing four forward ratios and reverse. Individual ratios are obtained by coupling together the various parts of the geartrain by means of solenoid-operated, hydraulically actuated multi-disc clutches and brakes; built in freewheels permit gear shifts without any interruption of the power flow.
For information on the operation of the solenoids, clutches, brakes and freewheels, refer to the table and the schematic diagrams shown under Operation of Gear Train Clutches.
A parking lock paw is provided at the rear of the gear train, operating on a toothed wheel attached to the output shaft; the parking lock is operated from the selector (position 'P') by actuator rod.
An electromagnetic sensor detects output shaft revolutions by means of a toothed disc attached to the shaft; one revolution of the shaft is equal to 36 pulses. The electrical signal from the sensor is passed, via a screened cable, to the TCM.
Operation Of Gear Train Clutches
Operation of the gear train clutches to provide the five gear ratios is as follows:
1st Gear
Clutches 'A' and 'E' are engaged.
The front planet gear carrier of gear set '1' is locked against the housing through freewheel '4' when the engine is pulling, but is overrun when the vehicle is coasting.
Epicyclic gear set '2' rotates as a solid block with the second planet set.
There is no engine braking in 1st gear.
2nd Gear
Clutches 'A','C1','C' and 'E' are engaged.
Freewheel '4' over-runs.
Clutches 'C1' and 'C' lock the sun gear to the housing.
3rd Gear
Clutches 'A','B','C' and 'E' are engaged.
Freewheels '4' and '5' are over-run.
Epicyclic gear sets '1' and '2' rotate as a solid block at a ratio of 1:1.
4th Gear
Clutches 'A' 'B','C' and 'F' are engaged.
Freewheels '3','4' and '5' are over-run.
Epicyclic gear set '1' rotates as a solid block.
The hollow shaft with the sun wheel of epicyclic gear set '2' is locked via clutch 'F' to the housing.
Above a predetermined road speed, lock-up clutch '6' locks torque converter '7' solid to prevent slip.
Reverse Gear
Clutches 'B','D' and 'E' are engaged.
As the front planet gear carrier of epicyclic gear set '1' is locked, the direction of output shaft rotation is reversed.
Epicyclic gear set '2' also rotates as a solid block.
Transmission Control Unit - (Valve Body)
The electronic-hydraulic control unit comprising a series of valve blocks housing a cable-operated manual valve, control pistons and pressure valves is attached to the underside of the transmission housing; the unit controls the operation of the gear train clutches and directs oil pressure to the appropriate system components refer to the transmission description in it's entirety to understand the operation of the transmission.
The four valve blocks in which the control valves, pistons etc are located, are mounted and connected through a main gallery plate. The three solenoid valves MV1, MV2 and MV3 and the pressure regulator operating solenoid are located on the upper surface of the plate; electrical connections between the solenoids and the transmission and transmission control unit are via cable harnesses (refer to relevant diagrams and tables for details of the various pin connections).
Operation of the transmission control unit is through an electronic Transmission Control Module (TCM) which on receipt of electrical signals from transmission and engine management sources, activates the hydraulic pressure regulating and shift valves via the solenoid valves located on the valve block.
The gear selector in the vehicle transmits movement, via the selector cable, to the selector lever on the side of the gearbox; this rotates the selector shaft which controls the manual selector valve. The rotary position of the shaft is monitored by the rotary switch.
Transmission Control Module (TCM)
The Transmission Control Module (TCM) is an electronic control unit located in the front passenger footwell behind the underscuttle pad. It is electrically connected to the transmission and other components through a cable harness and multiple pin plug.
The TCM continuously monitors the gear lever position selected by the driver (via the rotary transmission switch), the speed of the output shaft (by speed sensor on the output shaft) and throttle position ('kick-down' switch). This information plus input from the Engine Control Module (ECM) of throttle angle and engine speed and load in conjunction with a pre-programmed control map, enables the most suitable gear to be selected.
The TCM, by operating solenoid valves MV1, MV2 and MV3, controls which gear is selected, and in conjunction with the solenoid operated pressure control valve, controls the gear shift quality. Gear shift quality on upshifts is improved by the TCM momentarily retarding the ignition to reduce the torque input as the gear change takes place. Information is fed to the TCM from sensors and if any electronic component fails, the basic shift changes will be performed by the hydro-mechanical system, ie Park, Reverse, Neutral, D3 or D4.
Transmission Rotary Switch
This switch is located on a square extension of the transmission selector shaft, which is linked to the shift lever via the selector cable. The switch comprises a selector bar which moves across a series of copper segments located on the switch quadrant. The copper segments are set out in seven bands, each band providing an electrical output or combination of outputs to the TCM.
The electrical outputs to the TCM are in the form of a three-digit code which takes up three of the seven bands of the quadrant; the remaining four bands are used for reverse, ground and start inhibit.
Decoder Module
The decoder module, located on the right hand side of the J-gate assembly, is used to translate the three-digit codes from the rotary transmission switch into single-line functions to feed the illumination module and to provide the following information:
- Cruise control, ie when cruise control can or cannot be engaged.
- Indication when the vehicle is not in 'P' Park.
- Provide the ECM with a Park I Neutral signal.
Performance Mode Switch
This switch, located on the shift lever surround, provides two alternative shift patterns:
- 'Normal Mode' - for everyday use,
- 'Sport Mode' - gear changes take place at higher road speeds in order to enhance performance.
Note: Torque converter 'lock-up' occurs in fourth gear in each mode.
'Kick-down' Mechanism
'Kick-down' is activated by the final travel of the accelerator pedal which contacts a floor-mounted switch located behind the pedal. Operation of the switch provides a signal to the TCM that a downward change is required. The switch is adjustable to ensure that the pedal does not overtravel and stretch the cable.
Reverse Safety Inhibit
If 'R' Reverse is selected when the vehicle is travelling forwards at more than 5 mile / h, solenoid valve MV2 will be energised to prevent engagement of reverse gear.
Gearshift Interlock
A brake pedal / gearshift interlock is incorporated in the gear selector mechanism. The gear selector lever may only be moved from the P - Park position if the ignition key switch is in position II and the foot brake is applied.
The ignition key cannot be removed from the ignition switch unless the shift lever is in the 'P' - Park position. Once the ignition key has been removed, the shift lever is locked in the park position.
Starter Inhibit Function
The starter inhibit function prevents the starter motor from being operated when the shift lever is not in position 'P' - Park or position 'N' - Neutral. The signal is generated from the rotary switch position.
Harness Connections
Transmission Control Module Connections - Pin Locations
Rotary Switch Harness
Main Harness Connectors - 12 way
Main Harness Connectors - 8 way