Transmission Control Components

Transmission Control Module (TCM)
The TCM controls the transmission internal components thus determining gear selection and shift pattern.

Location





The TCM is located in the same housing as the engine control module (ECM) and is accessed in the same manner. With the two connectors unplugged the TCM may be withdrawn, complete with mounting bracket. The mounting bracket is secured to the TCM by 2 locknuts.

Operation

CAUTION: Disconnection of OTHER control module connectors and/or the vehicle battery, may introduce fault codes (DTCs). Following rectification and reconnection, ensure that no induced codes exist in the TCM memory.

The TCM processes input and output in both analogue and digital form which may be summarized in four categories:

Input Signals
Gear selector position (Dual Linear Switch x 4 inputs see 307.05)
Transmission oil temperature
Kickdown
Speed sensor #1
Speed sensor #2
Mode switch

CAN Input Signals
Traction status
ABS malfunction
ABS status
Engine torque status
Throttle position
Accelerator pedal position
Engine torque reduction confirmation
Engine speed
Engine coolant temperature
Cruise status
OBDII fault code clear request
Throttle malfunction warning RED or AMBER
All road wheel speeds
Token for network status ECM
Token for network status INST
Token for network status ABS
Diagnostic data in from external device PDU (Portable Diagnostic Unit)

Output Signals
Solenoid valve 1 <-> 2 and 4 <-> 5 shift
Solenoid valve 2 <-> 3 shift
Solenoid valve 3 <-> 4 shift
Solenoid valve converter lock-up clutch
Control valve - modulating pressure
Control valve - shift pressure

CAN Output Signals
Engine torque reduction request
MIL status (whether present DTC should operate MIL)
Transmission input speed
Transmission output speed
Converter slip
Kickdown status
Gear position (actual)
Gear position (selected)
Gear selection fault
Converter lock-up status
Current selected shift map
Transmission oil temperature
Transmission malfunction
Pecus flag (state of current TCM program)
Target for next gear position
Gear shift torque transfer progress at shift
MIL status (response to activate the MIL relevant to a DTC)
OBDII fault code clear acknowledgment
Transmission DTCs (P codes)
Token for network status TCM
Diagnostic data out to external device (PDU)

Emergency Running (electronic limp-home mode)
The TCM constantly monitors the transmission for faults. In the event of a problem the TCM will adopt the limp-home mode, in which the gear currently held remains engaged, until P is selected. The modulating and shift pressures will increase to maximum and converter lock-up will be inhibited.
Following the selection of P. the only ratios available will be 2 (irrespective of forward ratio manual selection) and R.
With the vehicle at rest, the procedure to manually select is as follows:
1. Select P.
2. Switch OFF the ignition.
3. Wait 10 seconds.
4. Start the engine.
5. Select R (reverse will be selected).
6. Select D (2 will be selected).
The limp-home mode will be retained until the fault is remedied or the fault code has been erased. Intermittent faults may be cleared by cycling the ignition OFF/ON. In certain cases the component may need to be operated before the fault code is cleared e.g.. a shift solenoid.

Emergency Running (mechanical/hydraulic limp-home mode)
Should slip be detected, due to a mechanical failure or loss of pressure, the transmission will either shift to, and hold 3 or shift to, and hold, the last gear which was known to be alright. This condition may be cleared by cycling the ignition OFF/ON following mechanical repair.
The operator will be made aware of certain faults by a warning message on the instrument cluster. Data concerning OBDII related transmission failures is stored in the ECM for access via the J1962 socket.

Safety Functions
These functions are designed to safeguard against inappropriate actions by the operator as well as system malfunctions. The electrical and diagnostic system has been designed such that system integrity is protected at all times, the safety concept being based on the following three points.
1. The hydraulic system has 'fail-safe' characteristics regarding its electrical operation, such that should the power supply be lost to the electro-hydraulic actuators the transmission will initiate a limp-home mode.
2. Recognition of critical shift operation by monitoring the last element in the signal path, is the solenoid valve, and checking by means of redundant measured variables relative to engine, transmission and road speeds.
3. Each time the vehicle is started there is a check on the entire safety hardware and the associated program parts and signal paths. A malfunction in this part of the system, or triggering of the safety circuit, is communicated to the operator by a warning message on the instrument cluster.

CAUTION: Do not engage R or P with the vehicle in motion.

Should R be engaged with the vehicle in forward motion, the transmission will default to neutral until either the vehicle speed decreases to 4 mph or D is selected.

Towing
The vehicle may be towed provided that:
Selector in position N
Speed < 50 kph
Distance < 50 km