Automatic Gearbox - Operation

General
The gear position switch outputs are monitored by the BeCM and the EAT ECU. The BeCM outputs gear position signals to illuminate the position indicators each side of the gear selector lever and on the message center 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.

Selector Lever Interlock (where fitted)
The interlock solenoid on the selector lever is de-energized unless the foot brake is applied while the ignition is on. While de-energized, 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 BeCM energizes 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:

If the transfer box is in high range, the EAT ECU changes to the sport mode and illuminates the sport mode lamp on the selector cover and displays 'S' in the instrument pack message center. In the sport mode the gearbox is more responsive to accelerator pedal movement. Downshifts occur earlier and upshifts occur later.

If the transfer box is in low range, the EAT ECU changes to the manual mode and illuminates the manual mode lamp on the selector cover and displays 'LM' in the instrument pack message center. 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 lamp on the selector cover and removes the 'S' or 'LM' display in the instrument pack message center.

When the vehicle is in the default mode (i.e. high range and economy) and towing or driving up steep gradients, the EAT ECU will select a shift pattern appropriate to the driving conditions. If a heavy trailer is being towed or a steep gradient is encountered, the transmission will hold in the gears longer than in normal operation.

Shift Control
To provide the different driving characteristics for each mode of operation, the EAT ECU incorporates different shift maps of throttle position/road 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.

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 and select a gear to give the best available acceleration. When it detects a kickdown situation, the EAT ECU immediately initiates a down shift of one or two gears or will maintain the current gear to avoid engine overspend.

Torque Converter Lock-Up
The EAT ECU energizes 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 maneuvering 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 lock-up maps. These maps delay upshifts and torque converter lock-up similar to the sport mode if the inputs from the engine indicate:

A sustained high load on the engine, such as occurs when the vehicle is ascending a steep gradient or towing a trailer.

The EAT ECU monitors the engine inputs and selects the most appropriate adaptive map for the prevailing conditions.
^ On vehicles from 99MY, a lower than normal engine torque, such as occurs at altitude or high ambient temperatures.

Mechanical Limp Home
In the mechanical limp home mode in high range, gear engagement is controlled by the manual valve. The gearbox is fixed in 4th gear if the fault occurs while the vehicle is moving, or 3rd gear if the fault occurs while the vehicle is stationary. 3rd gear is also engaged if a vehicle is brought to a stop and the selector lever is moved out of, and back into, D.

Neutral and reverse gear are also available.

In the mechanical limp home mode in low range, depending on the severity of the fault, the engaged gear is held until the vehicle is brought to a stop. The gearbox then selects and holds 3rd gear.

Calibration Selection
EAT ECU's differ between NAS, UK/Europe and ROW markets and are identified by differentiation between the part numbers.

On V8 vehicles from 99MY, the ECU contains two calibrations for 4.0 and 4.6 litre engines. When a replacement ECU is fitted, the correct ECU calibration must be selected or the ECU will store a gearbox fault and 'GEARBOX FAULT' will be displayed in the message center. The vehicle can still be driven and is not in 'limp home mode'.

Removed EAT ECU's remember their calibration setting and if re-fitted to the same vehicle will not require calibration. A new EAT ECU will require calibration using Test Book.

If an ECU is fitted from another vehicle, the message center will not display the 'GEARBOX FAULT' message. The correct calibration level must be selected or premature gearbox failure will occur.