Control Systems

CONTROL SYSTEMS
The XK8 electronically controlled automatic transmission contains a hydraulic valve body, a torque converter, three solenoid valves and five pressure regulators. 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.
In the event of a system fault the basic functions Park, Reverse Neutral and Drive Fourth are retained by the hydraulic system.
The Transmission Control Module (TCM) controls the internal components thus determining gear selection and shift pattern.

Transmission Control Module

NOTE: Should the TCM fail, 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 is located in the same housing as the Engine Control Module (ECM) and is accessed in the same manner.

Operation
The TCM processes information received in both analogue 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
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 driver when a fault occurs.

Interface
The Transmission Control Module 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, and Drive 4th (selector in D position but only fourth gear enabled) are available. The driver will be made aware of certain faults by a warning lamp. Data concerning OBD II related transmission failures is stored in the ECM for access via the J1962 socket.

Safety Functions
The safety functions are designed to safeguard against inappropriate actions by the driver 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.

NOTE: These functions are not operational in mechanical limp-home mode. 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:
^ 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.
^ 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.
^ 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 driver through the illumination of the transmission warning lamp.