Transmission Control Module

WARNING: This page is about a different car, the 2019 Jaguar XJ and 2018 Jaguar XJ. However, it is still accessible from the selected car via links, so may be relevant.

The TCM outputs signals to control the shift control solenoid valves and the EPRS (Electronic Pressure Regulating Solenoid)'s to control the hydraulic operation of the transmission.

The TCM processes signals from the transmission speed and temperature sensors, the TCS, the ECM and other vehicle systems. From the received signal inputs and pre-programmed data, the TCM calculates the correct gear, torque converter clutch setting and optimum pressure settings for gear shift and lock-up clutch control.

The ECM supplies the engine management data over the HS CAN Powertrain bus. The TCM requires engine data to efficiently control the transmission operation, for example; flywheel torque, engine speed, accelerator pedal position, engine coolant temperature. The ABS (Anti-lock Brake System) control module also supplies data to the TCM on the HS CAN Powertrain bus. The ABS control module receives the steering angle data from the SASM (Steering Angle Sensor Module) via the HS CAN Chassis bus, converts the message and forward it to the TCM via the HS CAN Powertrain bus. The TCM uses data from these systems to suspend gear changes when the vehicle is cornering and/or the ABS is controlling braking or traction control.

The CJB (Central Junction Box) supplies steering wheel paddle data over the HS CAN Powertrain bus. The TCM uses this to schedule driver requested upshifts and downshifts.

Using the signal inputs and the memorized data, the TCM control program computes the correct gear and torque converter lock-up clutch setting and the optimum pressure settings for gear shift and lock-up clutch control. Special output-side modules (power output stages, current regulator circuits), allow the TCM to control the solenoid valves and pressure regulators and consequently precisely control the hydraulics of the automatic transmission. In addition, the amount and duration of engine interventions are supplied to the engine management by way of the HS CAN Powertrain bus.

The TCM determines the position of the TCS using signals from the TCS on the HS CAN Powertrain bus and a hardwired connection for the 'P' (park) position determination.

The TCM transmits the position of the TCS and any manual gear selected on the HS CAN Powertrain bus. This information is shown in the transmission status display in the IC.

Engine Stall

If the vehicle stalls it will coast down in gear, with the transmission providing drive to the engine. A restart can be attempted at this point and the engine may start and the driver can continue.

If the coast down speed reduces such that the speed of the engine is less than 400 RPM, the transmission will go to neutral, D illumination will flash in the IC. The driver needs to select neutral or park and then press the brake pedal to restart the engine.

If the Stop/start switch is pressed when driving, the message 'ENGINE STOP BUTTON PRESSED' is displayed in the message center but there will be no change to the ignition state. If the driver requires to switch off the engine, the Stop/start switch must be pressed for a second time. The engine will be stopped and will be back driven by the transmission as the vehicle coasts down.

Cold Start Neutral

NOTE: The Cold Start Neutral (CSN) function has been introduced on 8HP45 transmissions for all petrol engine variants from 18MY onwards.

Cold Start Neutral (CSN) is a software function. The software function operates when the engine is cold. When the driver selects drive or sport with their foot on the brake, the transmission delays the engagement of gear. This allows the engine speed to remain high. If the driver releases the brake the engine speed reduces to idle and the transmission engages the gear. The function is also exited and the transmission engages the gear if while the driver has their foot on the brake the engine reaches operating temperature.

Previous transmissions would engage the gear when the driver selected drive or sport. This resulted in a noticeable gear engagement. The introduction of CSN will delay gear engagement until either the driver releases the brake or the engine reaches operating temperature. The purpose of the function is to assist in maintaining engine management control during the warm up cycle. Some customers may indicate that there is a delay in gear selection or an elevated engine speed (circa 1600rpm) but this is normal with CSN.