Transmission Control Module

WARNING: This page is about a different variant/trim than selected.

The Transmission Control Module (TCM) outputs signals to control the shift control solenoid valves and the Electronic Pressure Regulator Solenoids (EPRS) to control the hydraulic operation of the transmission.

The TCM processes signals from the following:

Transmission speed sensors
Transmission temperature sensors
Transmission Control Switch (TCS)
Powertrain Control Module (PCM)
Other vehicle systems.

The PCM supplies the engine management data over the FlexRay systems bus. The TCM requires engine data to efficiently control the transmission operation, for example drive plate torque, engine speed, accelerator pedal angle, engine temperature. The steering angle sensor and the Anti-lock Brake System (ABS) control module also supply data to the TCM on the FlexRay systems bus. The TCM uses data from these systems to suspend gear changes when the vehicle is cornering and/or the ABS control module is controlling braking or traction control.

The paddle switches are hardwired to the right steering wheel switchpack. Operation of the paddle switch completes a ground path to the switchpack. The switchpack converts the completed ground signal into a Local Interconnect Network (LIN) bus signal which is passed via the Clockspring (CLKSPG) and the Steering Wheel Module (SWM) to the Body Control Module/Gateway Module (BCM/GWM) assembly. The BCM/GWM converts the signal into a FlexRay bus signal to the Transmission Control Module (TCM).

Using the signal inputs and the pre-programmed data, the TCM control program calculates the correct gear and torque converter lock-up clutch setting. The optimum pressure settings for gear shift and lock-up clutch control are also controlled by the TCM. 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 FlexRay systems bus.

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 100 RPM, the transmission will go to Neutral (N), Drive (D) illumination will flash on the IC. The driver needs to select Neutral (N) or Park (P) and then press the brake pedal to restart the engine.

If the ignition switch is operated 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 ignition 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.