Description
AUTOMATIC TRANSMISSION 4.0 LITREDESCRIPTION
The ZF 4 HP 24 E9 automatic transmission is similar to the ZF 4 HP 22 except for the following:
- Strengthened 'A' clutch,
- toothed disc and magnetic sensor in place of the mechanical governor,
- new valve block and spring assemblies,
- deeper oil pan to prevent foaming and an additional sleeve to improve oil suction,
- the 'kick-down' cable is replaced by a pressure switch under the throttle pedal.
The ZF 4 HP 24 E9 automatic transmission unit utilizes a micro-processor equipped Transmission Control Unit to control system functions by means of solenoid actuated valves, and features the following three-component principle:
1. Hydrodynamic (hydromatic) torque converter,
2. Four-speed epicyclic gear train,
3. Hydraulic automatic gear shift unit.
The unit is strongly built, compact and designed fora long operational life requiring minimum maintenance - only occasional oil level checks and oil changes being necessary.
Shift points have been planned for maximum fuel economy, particularly in city traffic, and economy has been further enhanced with the incorporation of a converter 'lock-up' clutch on the fourth (top) gear which is in itself a step-up or overdrive ratio.
Automatic selection can be over-ridden by the selector lever to obtain more engine braking, for example, on down-hill gradients.
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.
TRANSMISSION CONTROL MODULE (TCM)
The TCM continuously monitors the gear selected, the speed of the output shaft and the throttle position. This information plus input from the Engine Control Unit (ECM) of engine speed and load in conjunction with a pre-programmed control map, enables the most suitable gear for the prevailing conditions to be selected.
The TCM controls the gear shift speed with three solenoid valves and gear shift quality with a solenoid operated pressure control valve. Information is fed to the TCM from sensors, and if any electronic component fails, the basic functions will be performed by the hydro-mechanical system, i.e. Park, Reverse, Neutral, D3, or D4. Gear change quality on upshifts is improved by the TCM retarding the ignition momentarily to reduce the torque input as the gear change takes place.
Located in the front passenger footwell, behind the dash liner, the TCM is mounted adjacent to a decoder:
This decoder enables the rotary transmission switch to 'talk' to the illumination module and the TCM.
HYDROMATIC TORQUE CONVERTER
The hydromatic torque converter consists of an impeller, a turbine and a stator, the latter mounted on a one-way clutch.
These elements are enclosed within a common housing in such a way as to form a closed circuit through which the oil flows when the transmission is in operation.
The torque multiplication effect is at its greatest when the vehicle is driven away from a standstill. The stator bears against the housing through its one-way clutch, and deflects the oil so that its 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. The torque converter at this stage acts as a fluid coupling.
These characteristics make it possible to dispense with the clutch pedal. The engine cannot normally stall because it is not rigidly linked to the driven wheels. The converter's operating characteristics have been selected to suit those of the vehicle's engine.
FOUR-SPEED EPICYCLIC GEAR TRAIN
The converter drives a mechanical epicyclic gear train which provides four forward ratios and reverse. Top gear is a step-up or overdrive ratio for economy - further economy being provided by a 'lock-up' clutch which provides a rigid link between engine and the transmission output side - improving both efficiency and fuel consumption. 'Lock-up' is only available in top (fourth) gear.
Hydraulically actuated multi-disc clutches and brakes are used to select the various gear ratios. They are unaffected by wear and require no adjustment.
Built-in freewheels permit gear shifts without any interruption of the power flow. A low-loss oil pump in the front section of the transmission housing supplies the converter, the automatic gear shift and the lubrication circuit. A mechanical parking lock (at position 'P') is provided to prevent the vehicle from moving once it is stationary.
ROTARY TRANSMISSION SWITCH
Gear selection is achieved via a multi-contact mechanically controlled rotary transmission switch. The switch is housed in an aluminium casting attached to the side of the transmission unit and consists of a selector bar attached to a (squared) pivot, and a quadrant upon which are located copper segments. When a gear is selected, the selector bar moves across the quadrant to a pre-determined position to contact the copper segments. The copper segments are set out in seven bands across the quadrant, and each band provides an output or combination of outputs, via the decoder, to the TCM.
Outputs to the TCM are in the form of a three-digit (XYZ) code. The three-digit code takes up three bands on the quadrant, the remaining four bands are used for reverse, ground, and start inhibit.
ROTARY TRANSMISSION SWITCH LOGIC FUNCTIONS
PIN Z Y X
P 0 0 1
R 0 1 1
N 0 1 0
D 1 1 0
3 1 1 1 0 = Switch open
2 1 0 1 1 = Switch closed to ground
DECODER MODULE
The function of the decoder module is to 'translate' the three-digit codes from the rotary transmission switch into single-line functions to feed the illumination module, and also provide information regarding the following:
1. Cruise control - i.e. when cruise control can or cannot be engaged,
2. Indication when the vehicle is not in 'P' (Park),
3. To link the transmission TCM to the engine management ECM.
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 pedal is applied.
The ignition key cannot be removed from the ignition switch unless the gear selector lever is in the 'P' - Park position. Once the ignition key has been removed, the gear selector lever is locked in the park position.
GEARSHIFT INTERLOCK - MANUAL OVER-RIDE
The gearshift interlock may be over-ridden manually in the event of an electrical failure or when it is required to move the vehicle without power ie., when it is being towed.
Access to the release button is by use of a suitable implement, such as the ignition key. To deter inadvertant or habitual use, a two-handed operation is required - the release button is held depressed and the gear selector lever moved simultaneously out of the 'P' - park position. This operation is more difficult for the U.S.A. market vehicles in that it requires the removal of the J-gate to gain access to the release button.
DECODER MODULE LOGIC FUNCTIONS
INPUTS OUTPUTS
CRUISE
CONTROL NOT-PARK IDLE FUEL
X Y Z P R N D 3 2 INHIBIT SIGNAL SIGNAL
0 1 1 0 1 1 1 1 1 1 1 0
0 0 1 1 0 1 1 1 1 1 0 1
1 0 1 1 1 0 1 1 1 1 0 0
1 0 0 1 1 1 0 1 1 0 0 1
0 0 0 1 1 1 1 0 1 0 0 1
0 1 0 1 1 1 1 1 0 0 0 1
0 = Ground
1 = Open circuit
SPORT MODE SWITCH
The 'Sport Mode Switch' is located on the gear change lever surround and provides three alternative shift speed patterns:
1. 'Normal Mode' - designed for everyday use.
2. 'Sport Mode' - gear changes take place at higher road speeds in order to enhance performance.
3. 'First Gear Inhibit' - this inhibits the engagement of first gear and is to assist vehicle traction from rest during snow/ice or very wet/muddy conditions.
Note: Torque converter 'lock-up' occurs in fourth gear in each mode.
SPEED SENSOR
An electro-magnetic sensor detects output shaft revolutions by means of a toothed disc attached to the the shaft and sends a signal to the TCM via a screened cable. One revolution of the toothed disc is equal to 36 pulses.
SOLENOID VALVES
Gear selection and gear shift speeds are controlled by the manually operated selector valve, a solenoid operated pressure regulator and three solenoid valves. On receipt of signals from the TCM, the three solenoid valves MV1, MV2 and MV3, in various combinations with the selector valve, determine the appropriate gear range. The TCM on receipt of information of engine state and road speed determines the shift speed. The pressure regulator is factory calibrated and is not adjustable in service.
KICK-DOWN SWITCH
The 'kick-down' switch, located beneath the throttle pedal, is actuated by the final travel of the pedal and signals to the TCM that a downward change is required. The switch is adjustable for height to obtain the precise 'kick-down' speed.
Note: If 'R' (reverse) is selected when the vehicle is travelling forwards at more than 5 mile/h, solenoid valve MV2 will be energized to prevent engagement of reverse gear.
SELECTOR UNIT
If the vehicle is driven normally, without unduly high engine speeds being used, the selector unit will cause upshifts to take place at quite low engine speeds to ensure optimum economy.
In difficult driving conditions, for example in town traffic, fuel consumption is actually lower than with a conventional manual-shift gearbox, which is often left in too low a ratio by the driver, so that engine speed rises excessively.
The automatic control system, with selector valve, control pistons and pressure valves, is located in the lower part of the transmission housing.
Shift points are governed by accelerator pedal position and road speed. Pulses equivalent to road speed are derived from a centrifugal governor on the output shaft. The entire system operates hydraulically.
Gear shifts take place at predetermined speeds, depending on the amount by which the accelerator is depressed.
In special circumstances the system can be over-ridden by pressing the accelerator down beyond 90% of full throttle (ie. 'kick-down'), to select a lower gear immediately when maximum acceleration is required. In this event, the next upwards shift does not take place until almost the maximum engine speed - that is to say, at a far higher road speed than with normal driving.
This arrangement ensures that engine power can be utilized fully in all gears if so required.
Note: There is a maximum speed for each gear above which 'kick-down' will not be effective. Kick-down is set at 90% of full throttle. Full throttle pedal movement will, under normal driving conditions, operate the 'kick-down' successfully.