Automatic Transmission/Transaxle: Description and Operation

AUTOMATIC GEARBOX

CIRCUIT OPERATION

Economy, Sport and Manual Modes
The Auto Gearbox Control Unit (Z255) has various different driving modes available, with Economy being selected automatically on start up. When the vehicle is in High-range, Sport mode can be selected. This will make gear changes more sensitive to throttle movement, thereby shifting down earlier and holding lower gears longer than the Economy mode. If the vehicle is in Low-range, Manual mode can be selected. This will make the gearbox change into the selected gear as quickly as possible and hold on to that gear at all speeds.

Interface to Petrol Engine Control Module (ECM) (Z132)
The Auto Gearbox Control Unit (Z255) uses engine torque, engine speed and throttle angle signals from the ECM (Z132) to calculate which gear is required. The Auto Box Control Unit supplies gear selection information to the solenoid valves which will then engage the correct gear. A torque reduction signal is sent from the Auto Gearbox Control Unit to the ECM which will retard the ignition point on each cylinder. This reduces engine power during gear changes. The combination of retarded ignition and reduced engine power delivers a smoother gear change.

Information about engine speed, engine torque, throttle angle and torque reduction is received from the ECM (Z132) by the Auto Gear Box Control Unit (Z255) via CAN bus "high" and "low" line on pins 16 and 44.

Interface to Diesel Engine Control Module (ECM) (Z132)
The engine speed is received by the Auto Gearbox Control Unit (Z255) via pin 3 and sent from the BeCM (Z238) via pin 11.

The throttle angle signal is received by the Auto Gearbox Control Unit (Z255) via pin 47 and sent from the ECM (Z132) via pin 40. The torque reduction signal is received by the ECM (Z132) via pin 43 and sent from the Auto Gearbox Control Unit (Z255) via pin 32.

Interface to Auto Gear Box Control Unit (Z255) Gear Box Position Switch - pins 14, 33, 50, Diesel (pins 8, 36, 37 Petrol)
The Gearbox Position Switch (X294) is mounted on the side of the gearbox and contains 3 internal switches. The switches are connected to the Auto Gearbox Control Unit (Z255) via pins 50, 14 and 33 for diesel (pins 8, 36, 37 for petrol). By analysing the status of these 3 switches, the Auto Gearbox Control Unit (Z255) calculates which gear the vehicle is in.

If a fault occurs with the switch, the Auto Gearbox Control Unit (Z255) may try to override driver requests and select a different gear.

Solenoid Valves - pins 5, 6, 19, 24, 42 Diesel (pins 5, 30, 32, 33, 53 Petrol)
The Auto Gearbox Control Unit (Z255) has control over 4 solenoid valves: MV1, MV2, torque converter lock up and pressure regulator. These are all supplied via pin 19 diesel (pin 53 petrol), which is activated via a relay in the Auto Gearbox Control Unit (Z255) itself.

MV1 - pin 5 Diesel (pin 30 Petrol), MV2 - pin 24 Diesel (pin 33 Petrol)
When the driver selects "D" (drive), the Auto Gearbox Control Unit (Z255) controls gear selection via two solenoid valves. If a fault occurs with one or both of the valves, a different gear than the one selected will result.

If the Auto Gear Box Control Unit (Z255) detects this fault the Limp home Function is activated to prevent damage but to maintain driveability. The common supply to all the solenoid valves is removed and the pressure regulator is set mechanically to maximum pressure. The effect on the transmission will depend on what position MV2 was in before the Limp home Function became active. The gearbox will mechanically shift to either third or fourth gear until the ignition is switched OFF.

MV2 is also used to prevent the driver from selecting reverse gear when the vehicle is moving forward at speeds greater than 8 km/h and from disengaging reverse gear at speeds greater than 6 km/h.

This function is known as "Reverse Safety" and will not be available if MV2 is non operational and/or the gearbox is in default mode.

Torque Converter Lock Up Solenoid - pin 42 Diesel (pin 32 Petrol)
The torque converter slips to allow smooth operation of the gearbox. Lock Up occurs when the vehicle reaches a speed of 45 mph or above in either third or fourth gears.

Pressure Regulator Solenoid - pin 6 Diesel (pin 5 Petrol)
Gearshift quality is controlled by modulating the pressure regulator which is controlled by the Auto Gearbox Control Unit (Z255). When energised the solenoid is in the closed position. If a failure occurs, the common supply to all the solenoids is removed and the pressure regulator is set mechanically to maximum pressure.

Output Shaft Speed Sensor - pins 2, 38, 20 Diesel (pins 42, 14, 15 Petrol)
The automatic gearbox system incorporates an output shaft speed sensor which provides an input to the Auto Gearbox Control Unit (Z255) on pin 2 diesel (pin 42 petrol) and pin 38 diesel (pin14 petrol) with a screen over the cable attached to pin 20 diesel (pin 15 petrol). It is important that this signal is screened correctly because it is a low amplitude analogue signal that can be affected by external interference.

MES 1 and 2 - pins 16, 31 Diesel (pins 25, 51 Petrol)
The two Manual/Economy/Sport (MES) lines indicate to the BeCM (Z238) which mode has been selected or if a fault within the transmission has occurred.

Interface to Transmission Gearbox Control Unit (Z256)

High/Low-Range Signal - pin 46 Diesel (pin 13 Petrol)
The High/Low signal is used by the Auto Gearbox Control Unit (Z255) to select the correct mode on application of the "MES" switch. Also, the gear shift points and strategies are very different in the different ranges.

Interface to H-Gate - pin 29 Diesel (pin 45 Petrol)
The H-Gate sensor module (Z254) displays selected gear information from the Gearbox Position Switch (X294), ratio information from the Transmission Gearbox Control Unit (Z256) and mode information from Auto Gearbox Control Unit (Z255). The H-Gate receives a signal from the ignition switch when it is turned to position II on a W wire. This is supplied via fuse 6 and is grounded to the console ground E0808 via a B wire.

Information on the selected gear is received from the Gearbox Position Switch (X294) on UB, UP and UG wires. These three lines also provide a feed to the Auto Gearbox Control Unit (Z255) and the BeCM (Z238).

Information on the gearbox mode (sport or manual) is received on a YG wire and a YR wire. These two lines also go to the BeCM (Z238).

The High-Ratio gear range position signal is received by the Auto Gearbox Control Unit (Z255), Transmission Gearbox Control Unit (Z256) and the BeCM (Z238). This signal is sent from the H-gate Sensor Module (Z254) on a NY wire via pin 8.

The Low-Ratio gear range position signal is received by the Transmission Gearbox Control Unit (Z256) and the BeCM (Z238). This signal is sent from the H-gate Sensor Module (Z254) on a NR wire via pin 11.

If the Auto Gearbox Control Unit (Z255) does not receive a High-Ratio gear range position signal it will assume that Low-Ratio gear range has been selected.

The illumination signal is on a RU wire.

The mode request is on the UO wire. This is sent from the H-gate Sensor Module (Z254) via pin 10 to the Auto Gearbox Control Unit (Z255) via pin 29 diesel (pin 45 petrol). Pressing the momentary action mode button while in High-Ratio gear range will activate or deactivate Sport Mode. Pressing the momentary action mode button while in Low-Ratio gear range will activate or deactivate Manual Mode. Changing from High-Range to Low-Range or from Low-Range to High-Range will automatically cancel Sport Mode or Manual Mode.