On Vehicle
GENERAL DESCRIPTION
This section provides information relating to the automatic transmission fitted to the 4.0 liter normally aspirated engine (ZF 4 HP 24 E transmission). The general arrangement of the unit is shown in Fig. 2. The electronic-hydraulic control of the ZF 4 HP 24 E unit provides for optimized shift points and shift quality based on engine and transmission data received by the Transmission Control Module (TCM). The automatic transmission comprises a hydrodynamic torque converter driving an epicyclic gear train which provides four forward ratios and reverse. Gearshift selection is made by a hydraulic (or electronic-hydraulic) transmission control unit. Six gearshift positions are provided:
Position 'P' (Park) - the driven wheels are mechanically locked at the transmission.
Position 'R' (Reverse) - reverse gear selected.
Position 'N' (Neutral) - engine disconnected from drive-line and wheels.
Position 'D' (Drive) - all four speed ranges are selected automatically with lock-up available in top gear only.
Position '3' - automatic selection of the lowest three speed ranges only.
Position '2' - automatic selection of the lowest two speed ranges only; the transmission is prevented from shifting up to the third and top speed ranges.
Immediate selection of a lower ratio is also available, within mapped limits, by 'kick-down' (pressing the accelerator pedal down beyond the normal full throttle position) for example when overtaking.
A brake pedal / gearshift interlock is incorporated in the shift lever mechanism. The shift 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. The gearshift interlock may be over-ridden manually in the event of an electrical failure or when it is required to move the vehicle manually for access, ie for removal of the propeller shaft.
Gear Selection (ZF 4HP 24 E)
Shift Speeds
Gearshift selection causes the appropriate gear to be selected through a cable operated shift lever on the side of the transmission unit; the shift lever also operates a rotary switch attached to the side of the transmission unit. When a gear is selected, the rotary switch provides an output or combination of outputs to the TCM, which continuously monitors the gear selected in addition to output shaft speed and transmission oil temperature. Information from the Engine Control Module (ECM) representing engine speed, load and throttle position is also fed to the TCM to enable the most suitable gear to be selected.
Gear selection and gearshift 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 safety valve, determine the appropriate gear range. The TCM, on receipt of information of engine state and road speed, determines the shift speed.
The Performance Mode switch, located on the shift lever surround, provides two alternative shift speed patterns:
1. 'Normal (Economy) Mode' - designed for everyday use.
2. 'Sport Mode' - gear shift takes place at higher road speeds to enhance performance.
The 'kick-down' switch, located beneath the accelerator pedal, is actuated by the final travel of the pedal and signals to the TCM that the next lower gear is to be selected.