Control System

General

The transmission is electronically controlled by a control unit (TCM, Transmission Control Module). The TCM (2) is mounted directly on top of the transmission (1) and has one connector to the car's electrical system and one to the transmission. The gear position sensor is Hall type and integrated in the control module. The TCM is mounted so that the selector lever shaft goes through the control module. The gear lever sensor position is calibrated in position N with a scan tool, there is no mechanical adjustment. TCM controls the solenoids to maintain or change gear. The solenoids are controlled with PWM (Pulse Width Modulation) to achieve a variable flow of oil.

Shifting

The determining factors for changing gear are two values; current gear and target gear. Current gear is the one engaged at the moment and the target gear is the one TCM wants to change to. As long as these to values are the same there will be no gear change. Gear changing is initiated only when the target gear differs from the current gear. It is mainly the vehicle speed and acceleration that determines the target gear in automatic transmissions. The vehicle speed is taken from a special sensor, output speed, while the acceleration is taken from the engine management system (ECM) via the high speed bus.

Gear changing must also follow a permitted schedule. If a direct change to the target gear is not present in the schedule, there will be an intermediate gear change. If, for example, the car is in 5th gear and the accelerator suddenly indicates a target gear of 2 (kickdown), the gear change will be 5 to 3 and 3 to 2. This is because 5 to 2 is not a valid gear change.

When a gear change is carried out, TCM will calculate a requested pressure for each solenoid circuit. The pressure is converted to a requested current that later results in a PWM ratio. The requested pressure is modulated according to a predetermined procedure so that the gear change is made comfortable without unnecessary wear. Current engine torque taken from ECM via the high speed bus is used for this purpose. A higher pressure is required for higher engine torque.

TCM reads the resulting current to each solenoid and can finely adjust the PWM ratio so that the requested pressure (current) agrees with the actual current. The transmission does not have a pressure sensor so current pressure cannot be sent back to the control system. For this reason, the solenoids are calibrated very precisely.

Manual gear changing can be accomplished with the selector lever in position D using the up/down switch in the lever. A manual gear change must be fully completed before TCM will accept a new change command. The current gear is shown in DIC. The highest permitted gear for pulling away is 3rd. For safety's sake, there will be a change down from gear 6, 5 and 4 automatically after kickdown and engine speed below 2000 rpm.

TCM sends continuous information on the high speed bus concerning the engine torque allowed by the transmission. When changing gear this value drops and ECM limits engine torque if it exceeds this value.

Lock Up

The torque converter clutch is activated at steady speed in gears 3-6 when there is no need for any torque amplification. The slipping in procedure is regulated by comparing the speed on the input shaft with the engine speed obtained from ECM via the high speed bus. Lock up will not work at temperatures below 15°C (56°F). If the brake is depressed or acceleration increases, the function will be activated.

Safety Features

A reverse detent function prevents reverse gear being engaged if the car is travelling faster than 7 km/h (4.3 mph) when the selector lever is moved from N to R. Overrevving in gear position M will be prevented by not allowing changing down to take place if the speed is too high.

Special Gear Change Program

For high engine loads without a corresponding increase in speed one of two hill-climbing programs will be chosen. Gear changing is done at higher engine speeds and the gear will remain engaged for longer periods until the extra load ceases. These gear change programs are used to avoid changing up and down unnecessarily, e.g. when driving up long inclines with trailer and to avoid overheating the transmission oil. The Sport Mode can be engaged by a switch. Upon detecting the sport mode request TCM adapts the shift lines in order to enable a more progressive driving. Special gear change programs are not activated at speeds above 150 km/h (93 mph). If a special gear change program is active when the speed increases to over 150 km/h (93 mph), the program will continue, however, until the load has normalized.

Diagnostics

The control module has diagnosis for internal faults. The solenoids are diagnosed with respect to open circuits and short circuits. They are also pulsed and the inductive voltage peak is analyzed to make sure the coil is not partially shorted. The speed sensor and temperature sensor circuits and plausibility are checked. High speed bus communication is checked to make sure no necessary messages are missing. Mechanical faults (slipping or wrong gear) are detected by comparing speed sensor values with the engaged gear.

Limp Home

If the power supply to TCM or to the solenoids is cut, the car can still be driven. Gear position D/M will give 2nd gear and P-R-N will work as normal but the gear will be engaged roughly as there is no regulation.

In case of a major fault in the transmission, mechanical or electrical, TCM will cut the supply to the solenoids in a controlled manner. In most cases, the control module will attempt to maintain the current gear until the car has stopped. Subsequently, the control module will attempt to engage 2nd gear if D/M is selected. After this, the supply to the solenoids will be cut and the transmission will lose all electrical control.

If an invalid gear ratio or the wrong gear is detected, the engine torque will be limited to 170 N.m in reverse and 150 N.m forward. For other faults, the torque will be limited to higher values depending on failure.