Automatic gearbox - ZF4HP22/24 (Kickdown): Description and operation

With time, the components in a gearbox wear and the duration of the gear shifts tends to increase, which has an adverse effect on the brake clutches. To counteract this, the EAT ECU applies a pressure adaptation to each shift. To calculate the adaptations, the EAT ECU monitors the pressure modulation used, and time taken, for each shift. If a subsequent shift of the same type, in terms of throttle position and engine speed, has a longer duration, the EAT ECU stores an adaptation for that type of shift in a volatile memory. The adaptation is then included in future pressure calculations for that type of shift, to restore shift duration to the nominal.

Kickdown

The EAT ECU monitors the input of the throttle position sensor to determine when kickdown is required. When it detects a kickdown situation, the EAT ECU immediately initiates a down shift provided the target gear will not cause the engine speed limit to be exceeded.

Torque converter lock-up

The EAT ECU energises the lock-up solenoid valve to engage the lock-up clutch. Lock-up clutch operation is dependent on throttle position, engine speed, operating mode and the range selected on the transfer box.

High range Unique lock-up maps, similar to the shift maps, are incorporated in the economy and sport modes for all forward gears. Engagement and disengagement of the lock-up clutch is dependent on throttle position and engine speed.

Low range To enhance off road control, particularly when manoeuvring at low speeds, torque converter lock-up does not occur when there is any degree of throttle opening. When the throttle is closed above a preset engine speed, the lock-up clutch engages to provide maximum engine braking.

Increased load/reduced torque compensation

To aid performance and driveability in the high range economy mode, the EAT ECU has three adaptive shift and lockup maps. These maps delay upshifts and torque converter lock-up similar to the sport mode if the inputs from the engine indicate: l A sustained high load on the engine, such as occurs when the vehicle is ascending a steep gradient or towing a trailer. l A lower than normal engine torque, such as occurs at altitude or high ambient temperatures.

The EAT ECU monitors the engine inputs and selects the most appropriate adaptive map for the prevailing conditions.

Diagnostics

While the ignition is on, the EAT ECU diagnoses the system for faults. The extent of the diagnostic capability at any particular time depends on the prevailing operating conditions, e.g. it is not possible to check torque converter lockup while the vehicle is stationary, or to check for a short circuit to earth if the circuit concerned is already at a low potential.

If a fault is detected, the EAT ECU immediately stores a fault code and the values of three operating parameters associated with the fault. Depending on the fault, there are four possible effects: l The fault has little effect on gearbox operation or vehicle emissions. The driver will probably not notice any change and the warning lamps remain extinguished. l The fault has little effect on gearbox operation but may effect vehicle emissions. On NAS vehicles, if the fault is detected on a second consecutive drive cycle, the MIL illuminates. l All gears are available but kickdown does not function. The sport and manual warning lamps flash. The MIL remains extinguished. l Limp home mode is selected and vehicle performance is greatly reduced. The sport and manual warning lamps flash. In all markets, if the fault is detected on a second consecutive drive cycle, the MIL illuminates.