Automatic Gearbox - Description
Electronic Automatic Transmission Control Schematic
1. Transmission high/low switch
2. Mode switch
3. Gear position switch connector
4. Solenoid valve/speed sensor connector
5. Electronic Automatic Transmission (EAT) ECU
6. Engine Control Module (ECM)
7. Diagnostic socket
8. Instrument pack
9. Transmission fluid temperature sensor
10. Body electrical Control Module (BeCM)
11. Battery power supply
12. Ignition power supply
General
The ZF4HP22 transmission is used on 2.5 litre Diesel and 4.0 litre petrol models. 4.6 litre petrol models use the ZF4HP24 transmission unit to accommodate the increased power output of the larger engine. Both units are of similar construction with the ZF4HP24 unit being slightly longer with an increased oil capacity. The operation of both units is the same.
Automatic transmission vehicles are fitted with an 'H-gate' selector mechanism. The selector mechanism combines the operation of the transmission selector lever and the transfer box high/low gear range selection. Selections on the selector lever assembly are transmitted by a selector cable to a gear position switch.
The gear position switch on the transmission passes gear selection signals to an Electronic Automatic Transmission (EAT) ECU located below the LH front seat, which outputs the appropriate control signals to an electro-hydraulic valve block in the transmission. A mode switch enables the driver to change the control mode of the EAT ECU between manual, economy and sport. The EAT ECU provides signals to the message center in the instrument pack to indicate the control mode and system status.
The gearbox features a pressure lubrication system and is cooled by pumping the lubricant through an oil cooler located in front of the engine cooling radiator.
From 99MY onwards, petrol models feature a revised EAT ECU with Controller Area Network (CAN) digital communications between the EAT ECU and the ECM.
H-Gate Selector Lever Assembly
The selector lever assembly consists of a lever and a cover attached to a cast base. The base is located on a gasket and secured to the transmission tunnel and has an 'H' pattern for the lever to move in. The lever is hinged to the base and a latch in the lever engages with detents in the base to provide positive location for the lever positions. The latch is disengaged by pressing a release button on the lever knob as shown in the lever illustration.
Except for lever movement between positions D and 3 (high range) and 4 and 3 (low range), the button must be pressed before the lever can be moved. In some markets, vehicles incorporate an interlock solenoid at the bottom of the lever, which prevents the lever being moved from P unless the ignition switch is in position 11 and the foot brake is applied.
The cover incorporates LED lever position indicators and the mode switch. The lever position indicators illuminate to show the position of the selector lever. The driver's side of the H-gate is labeled 'Hi' and is used to select the high range gears. The passenger side of the H-gate is labeled 'to' end is used to select the low range gears. Movement of the selector lever across the H-gate selects high and low transfer box gear ranges.
The LED indicators are controlled by the Body electrical Control Module (BeCM). A mode switch is located on the driver's side of the cover. The mode switch is used by the driver to select sport mode used in the high range gears and manual mode used in the low range gears. The mode switch is a non-latching hinged switch that, when pressed, connects an earth to the EAT ECU to request a change of mode. Sport and Manual indicator lamps on the cover illuminate to show the mode selected. The message center in the instrument pack also displays 'S' for sport mode and 'LM' for manual mode along with the selected gear.
An electrical connector at the rear of the cover connects the selector lever assembly to the vehicle wiring.
Selector cable
The selector cable is a Bowden type cable that connects the selector lever assembly to a selector lever on the gearbox. 'C' clips secure the ends of the outer cable to brackets on the selector lever assembly and the selector lever. The inner cable is adjustable at the connection of the inner cable with the gearbox selector lever.
Gearbox
The gearbox consists of a torque converter housing, an intermediate plate, a gearbox housing and a rear extension housing, bolted together in series. The rear of the gearbox is supported by a rubber mounting installed between a mounting bracket on the gearbox and the LH chassis rail. A heat shield is installed on the mounting to protect it from the exhaust.
Valve block
Torque converter housing
On 2.5 litre Diesel models a 260 mm (10.2 inch) diameter torque converter is used. On 4.0 and 4.6 litre petrol models a 280 mm (11 inch) diameter torque converter is used. On 4.6 litre petrol models up to 99MY the torque converter is longer than the torque converter used on 4.0 litre petrol models. From 99MY, both the 4.0 and 4.6 litre petrol models use the shorter torque converter previously used on up to 99MY 4.0 litre models.
The torque converter housing attaches the gearbox to the engine and contains the torque converter. The torque converter is connected to the engine drive plate and transmits the drive from the engine to the gearbox input shaft. When engaged, a hydraulic lock-up clutch in the torque converter prevents slippage, to give a direct drive from the engine to the gearbox for improved efficiency.
Intermediate plate
The intermediate plate supports the gearbox input shaft and provides the interface between the transmission fluid pump and the lubrication circuit. The pump attaches to the front of the intermediate plate and is driven by an impeller in the torque converter. The pump pressurizes transmission fluid drawn from the sump on the gearbox housing. The pressurized fluid then circulates through the torque converter and gearbox housing components for cooling, lubrication and gear shift purposes. Ports around the outer periphery of the intermediate plate provide the inlet and outlet connections to the fluid cooler and a pressure take-off point for servicing.
On ZF4HP24 gearboxes, the intermediate plate is 15 mm (0.6 inch) thicker than fitted to the ZF4HP22 gearbox to accommodate a larger fluid pump unit. To compensate for the increased length of the intermediate plate, the rear extension housing is 15 mm (0.6 inch) shorter than that fitted to the ZF4HP22 gearbox.
Gearbox housing
The gearbox housing contains two epicyclic gear sets on input and output shafts. Hydraulic clutches on the shafts control which elements of the gear sets are engaged, and their direction of rotation, to produce the P and N selections, four forward gear ratios and one reverse gear ratio.
Gear ratios
The lock-up and brake clutches are operated by pressurized transmission fluid from the valve block in the sump. A manual valve and four solenoid valves, also known as Motorized Valves (MV), control the supply of pressurized transmission fluid from the valve block:
The manual valve controls the fluid supply for P, R, N and D selector positions. The four solenoid valves operate accordingly to operate shift control, lock-up and shift quality.
Solenoid valves MV 1 and MV 2 control the supplies that operate the brake clutches for shift control. They are also used to prevent accidental engagement of reverse when moving forwards and a forward gear when moving backwards.
Solenoid valve MV 3 controls the supply that operates the lock-up clutch.
Solenoid valve MV 4 modulates the pressure of the supplies to the brake clutches, to control shift quality.
Operation of the manual valve is controlled by the selector lever assembly. In the gearbox, a selector shaft engages with the manual valve. The selector shaft is connected to the selector lever assembly via the selector cable and a selector lever on the left side of the gearbox. The selector shaft also operates a mechanism that locks the output shaft when P is selected.
Operation of the solenoid valves is controlled by the EAT ECU.
An output shaft speed sensor in the gearbox housing outputs a signal to the EAT ECU. The EAT ECU compares output shaft speed with engine speed to determine the engaged gear and output shaft speed with vehicle speed to confirm the range selected on the transfer box. The speed sensor signal is a diagnostic function and not essential for correct gearbox operation.
A bayonet lock electrical connector in the gearbox casing, to the rear of the selector lever, connects the solenoid valves and the output shaft speed sensor to the vehicle wiring.
A pressed steel sump encloses the valve block and collects transmission fluid draining from the gearbox housing. A suction pipe and filter on the underside of the valve block connect to the inlet side of the fluid pump. A magnet is installed in the sump to collect any magnetic particles that may be present. A level plug and a drain plug are installed in the sump for servicing.
Rear extension housing
The rear extension housing provides the interface between the gearbox housing and the transfer box. A splined output shaft transmits the drive from the gearbox to the transfer box. A seal in the rear of the housing prevents leakage past the extension shaft. A breather pipe, attached to the left side of the rear extension housing, ventilates the interior of the gearbox and rear extension housings to atmosphere. The open end of the breather pipe is located in the engine compartment at the right rear corner of the engine, against the bulkhead. On 99MY V8 vehicles, the breather pipe is also located against the bulkhead, but the open end is routed down the bulkhead and located below the converter housing.
Gear Position Switch X, Y, Z Outputs:
Gear position switch
The gear position switch outputs signals that are related to the position of the selector lever assembly. The switch is installed on the selector shaft on the left side of the gearbox. Slotted mounting holes allow the switch to be turned relative to the shaft for adjustment. A fly lead connects the switch to the vehicle wiring.
Movement of the selector lever assembly turns the selector shaft, which connects with three sliding contacts in the switch. The contacts are identified as the X, Y and Z.
When closed:
^ The X, Y and Z contacts output a combination of earth signals to the EAT ECU as shown in the table below.
^ The outputs of the X, Y and Z contacts are monitored by the EAT ECU, ECM and the BeCM to determine the position of the selector lever assembly.
The signals are interpreted by the EAT ECU for the correct gear selection. The ECM uses the signals to control for example engine idle speed etc. The BeCM uses the signals to illuminate the gear selection display on the selector cover, operate the reverse lamps, wiper reverse operation and message center display etc.
Fluid Cooler
Transmission fluid from the gearbox is circulated through a cooler located at the front of the radiator. Fluid lines from the transmission are connected to each end tank of the fluid cooler. A temperature sensor on the LH end tank provides the instrument pack with an input of transmission fluid temperature. If the temperature exceeds between 120 and 130°C, the instrument pack message center displays 'GEARBOX OVERHEAT'. The message remains displayed until the temperature of the fluid returns to between 82 and 88°C.