Control Systems
Introduction
Gear selection is achieved by controlling the flow of transmission fluid to internal multi-disc clutches.
The three solenoid valves direct the transmission fluid flow to the selected clutches and the pressure regulators control the fluid pressure to each component. One pressure regulator serves as a master pressure control for the entire system and a second is used exclusively for torque converter clutch lock-up operation.
The Transmission Control Module (TCM) controls the internal components thus determining gear selection and shift pattern.
In the event of an electronic system fault the basic functions Park, Reverse Neutral and Drive Fourth are retained by the hydraulic system.
Transmission Control Module (TCM)
The TCM is located in the same housing as the Engine Control Module (ECM) and is accessed in the same manner.
The TCM performs several 'self check' procedures to ensure correct operation. It is possible due to the nature of these faults that the module will fail to communicate with other nodes. However, condemnation of the TCM should not take place until any CAN or power supply related problems have been resolved.
There are three self check procedures for the TCM.
1. Check on ROM by calculating a checksum and comparing this with a known stored value.
2. Check on non-volatile diagnostic memory by writing a test pattern and then reading it back.
3. Internal 'watchdog' hardware to check whether the TCM has crashed.
Transmission Control Module Supply Voltage
The TCM monitors battery and ignition switched supply voltages.
A permanent supply is used to maintain a battery backed 'memory'. Should this supply be cut, due to battery disconnection perhaps, the 'adaptive shift' values will be lost. This will result in a small reduction in shift quality for a period until the adaptions are 're-learned'.
The TCM will adopt 'limp home' mode as a result of the supply voltage being > 16 Volts or < 7 Volts with an engine speed > 1600 rpm.
Should the ignition supply be > 7 Volts but < 9 Volts the TCM will hold the gear that it has currently selected. If after 2.5 seconds, with the engine speed > 1600 rpm, the voltage remains at this level, 'limp home' mode will be adopted. The 2.5 second delay is built in to prevent reaction to a momentary voltage fluctuation.
Operation
CAUTION: Disconnection of the TCM and/or the vehicle battery will cause system adaptions to be lost; this may be apparent by shift quality degradation. Following reconnection, a period of 'varied' driving will reinstate adaptions and thus normal operation. Please ensure that the customer is made aware that the adaption period is variable and may occur after handover, as the transmission re-learns the prevailing driving style.
NOTE: Should the TCM fail, please ensure that the control housing cooling fan is operating correctly. Failure of the cooling fan MUST be rectified before renewing the TCM and details of a fan failure should accompany the returned TCM.
The TCM processes information received in both analog and digital form, such as:
Transmission input speed
Transmission output speed
Throttle position
Pedal demand
Gear selector position
Engine torque
Engine speed
Transmission oil temperature
Mode switch
This information is then used by the TCM to control shift energy management and decide which shift program to implement and which gear to select.
The TCM uses the various sensors and inputs to monitor the correct operation of the system and is programmed to take default action and inform the operator when a fault occurs.
Safety Functions
The safety functions are designed to safeguard against inappropriate actions by the operator as well as against system malfunctions. The system prevents reverse gear from being engaged at high forward speeds and prevents manual downshifting at excessive engine speeds; these functions are not operational in mechanical limp-home mode.
The TCM constantly monitors the transmission for faults. In the event of a problem the TCM will adopt a 'limp home' mode in which only P R N D - (selector in D but only fourth gear is enabled) are available. The operator will be made aware of certain faults by an instrument panel warning.
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, i.e. the solenoid valve, and checking by means of redundant measured variables, i.e. engine speed, input speed and output speed.
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 through the illumination of the transmission warning lamp.