Introduction

Introduction

The Automatic Transmission Control Module (TCM) operates the solenoid valves in the gearbox to provide automatic control of gearshifts and Torque Converter Clutch (TCC) lock-up. Software in the TCM monitors hard wired inputs and exchanges information with the Engine Control Module (ECM) on a Controller Area Network (CAN) bus to determine gearshift and TCC lock-up requirements. Resultant control signals are then output to the gearbox solenoid valves.

Diagnostic Trouble Codes and Freeze Frames

While the ignition is ON, the TCM diagnoses the system for faults. The extent of the diagnostic capability at any particular time depends on the prevailing operating conditions, e.g. it is not possible to check TCC lock-up while the vehicle is stationary, or to check for a short circuit to earth if the circuit concerned is already at a low potential.

If a fault is detected, the TCM immediately stores a fault code and the values of three operating parameters associated with the fault. Depending on the fault, there are a few possible effects:
- The fault has liffle effect on gearbox operation or vehicle emissions. The driver will probably not notice any change and the warning lamps remain extinguished.
- The fault has little effect on gearbox operation but may effect vehicle emissions. On North American Specification (NAS) vehicles, if the fault is detected on a second consecutive drive cycle, the Malfunction Indicator Lamp (MIL) illuminates.
- All gears are available but kickdown does not function. The sport and manual warning lamps flash. The MIL remains extinguished. Limp home mode is selected and vehicle performance is greatly reduced. The sport and manual warning lamps flash. In all markets, if the fault is detected on a second consecutive drive cycle, the MIL illuminates.

After the detection of a fault, the effects remain active for the remainder of the drive cycle. In subsequent drive cycles, as soon as the TCM diagnoses the fault is no longer present, it resumes normal control of the gearbox. The conditions required to diagnose that the fault is no longer present depend on the fault. Some faults require the engine to be started, others require only that the ignition is switched on. After a fault has not recurred for forty warm-up cycles, the fault is deleted from the TCM fault memory. The ECM indicates via CAN that a warm up cycle has been completed. Only five different faults can be stored in the memory at any one time. If a further fault occurs, the fault with the lowest priority will be replaced by the new fault.

Mechanical Limp Home

In the mechanical limp home mode, the manual valve controls gear engagement. The gearbox is fixed in 4th gear if the fault occurs while the vehicle is moving, or 3rd gear if the fault occurs while the vehicle is stationary. 3r gear is also engaged if a vehicle is brought to a stop and the selector lever is moved out of, and back into drive. Neutral and reverse gear are also available.