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

Operation

Refer to illustration. AUTOMATIC GEARBOX - ZF4HP22 - 24, DESCRIPTION AND OPERATION, Control schematic.

When the ignition is switched on, a bulb check is performed on the transmission temperature warning lamp and the mode warning lamps by the instrument pack and the EAT ECU respectively. The warning lamps are illuminated for approximately 3 seconds and then extinguished.

The gear position switch outputs are monitored by the BCU and the EAT ECU. The BCU outputs gear position signals to illuminate the position indicators each side of the gear selector lever and on the odometer LCD in the instrument pack.

In D, 3, 2, and 1, the EAT ECU outputs control signals to the gearbox to select the required gear.

In D, all forward gears are available for selection by the EAT ECU. In 3, 2 and 1, a corresponding limit is imposed on the highest gear available for selection. When R is selected, reverse gear only engages if the vehicle is stationary or moving at 5 mph (8 km/h) or less. When R is deselected, reverse gear only disengages if the vehicle is moving at 4 mph (6 km/h) or less.

Selector lever interlock (where fitted)

The interlock solenoid on the selector lever is de-energised unless the foot brake is applied while the ignition is on. While de-energised, the interlock solenoid allows the selector lever to move through the range unless P is selected. On entering the P position, the interlock solenoid engages a latch which locks the selector lever. When the ignition is on and the foot brake is applied, the BCU energises the interlock solenoid, which disengages the latch and allows the selector lever to be moved out of P.

Economy, sport and manual modes

During the power-up procedure after the ignition is switched on, the EAT ECU defaults to an economy mode. Pressing the mode switch causes the EAT ECU to change between the economy mode and the sport or the manual mode, depending on the range selected on the transfer box: l If the transfer box is in high range, the EAT ECU changes to the sport mode and illuminates the sport mode warning lamp in the instrument pack. In the sport mode the gearbox is more responsive to accelerator pedal movement. Downshifts occur earlier and upshifts occur later. l If the transfer box is in low range, the EAT ECU changes to the manual mode and illuminates the manual mode warning lamp in the instrument pack. Kickdown is disabled and the EAT ECU maintains the gearbox in the gear selected on the selector lever (D = 4th gear) to give improved off road performance. Downshifts occur only to prevent the engine stalling. From a standing start, the vehicle pulls away in 1st gear and, if a higher gear is selected, upshifts almost immediately to the selected gear (shifts of more than one gear can occur).

After a second press of the mode switch the EAT ECU reverts to the economy mode, for the range selected on the transfer box, and extinguishes the related mode warning lamp in the instrument pack.

Shift control

To provide the different driving characteristics for each mode of operation, the EAT ECU incorporates different shift maps of throttle position/engine speed. Base shift points are derived from the appropriate shift map. When a shift is required, the EAT ECU sends a request to the ECM for a reduction in engine torque, in order to produce a smoother shift. The percentage of torque reduction requested varies according to the operating conditions at the time of the request. When the EAT ECU receives confirmation of the torque reduction from the ECM, it then signals the shift solenoid valves in the gearbox to produce the shift. To further improve shift quality, the EAT ECU also signals the pressure regulating solenoid valve to modulate the hydraulic pressure and so control the rate of engagement and disengagement of the brake clutches.