Interface With Other Systems

Interface With Other Systems

The TCU interfaces with several other ECUs, ECMs and sensors. The general relationships between these components are outlined below.

Engine Management System (EMS)

NOTE: 1995 MY vehicles do not have fault line communication between the TCU and the EMS ECM and will not set codes. 1996 MY and later vehicles have the capability to determine faults with the transfer gearbox control systems and circuits.

On 1996 MY and later vehicles, three EMS codes can be set by problems with the Transfer Gearbox Control System:
- 1701-Transfer gearbox line fault.
- 1703-Transfer gearbox line open circuit fault.
- 1708-Transfer gearbox line short circuit fault.

The EMS stores a fault code when the fault line is activated. The MIL (Malfunction Indicator Lamp) is illuminated upon completion of two successive trips where a fault is recorded.

At ignition ON, the link between the TCU and the TCM is driven near ground, and then near battery voltage and stays there if no faults are present. If a fault is present, the line is driven near ground.

TCU Readiness Checks
When the transfer gearbox receives a range change request, the TCU carries out the following three checks:

NOTE: Code 1701 will be set if any of these checks are failed. Speed Sensor Check

The resistance of the speed sensor is checked. If the speed sensor is found to be open or short circuited, the fault output line is activated. If the speed sensor passes the resistance test, it will be used to determine the speed of the vehicle. The TCU compares this speed with a value stored in memory and decides if a range change is allowed. Presently the range change speed is set at five mph (8 kph) High to Low and Low to High.

Speed Sensor Check
The resistance of the speed sensor is checked. If the speed sensor is found to be open or short circuited, the fault output line is activated. If the speed sensor passes the resistance test, it will be used to determine the speed of the vehicle. The TCU compares this speed with a value stored in memory and decides if a range change is allowed. Presently the range change speed is set at five mph (8 kph) High to Low, and Low to High.

Shift Control Motor Position Check
The TCU reads the position of the shift control motor as a binary code produced by the four shift control motor encoder switches. Given this information the TCU can determine the position of the shift control motor and where the motor should move to engage the requested range. If the shift control motor should move outside the normal range (i.e., Left of High) then the TCU can move the motor back to the requested position.

The TCU will attempt three times to move the shift control motor to High Range. If High Range position is not reached, the fault output line is activated.

Shift Control Motor Plausibility Check
As the shift control motor moves from one range to the next, the TCU monitors the signals from the four motor encoder switches. If a position is sensed that does not conform with the position expected, the motor stops and the fault output line is activated.

Changing Ranges

NOTE: Range changes should be made with the vehicle stationary. It may be possible to change ranges with the vehicle traveling at a very slow speed, however this is not recommended.

To change range, reduce speed to below 5 mph (8 kph), move the gear lever across the H-Gate and wait in neutral for the light to stop flashing and the audible warning to stop. When the change has been completed, select a gear and drive away. If the vehicle is driving too fast or the lever is moved into gear before the range change has taken place, the range change will not occur and a message will be displayed on the Message Centre. By following the instructions displayed (e.g., "Slow Down" or "Select Neutral") the change will then take place.

Normally a shift will be completed in approximately 11/4 seconds with a current draw of 11/2 amps. If the shift is blocked or the motor is stalled against resistance, the TCU will make three attempts before timing out. The first attempt will last for two seconds, followed by a half second pause, followed by a one second attempt and a four second pause, then a final two second attempt. The TCU will not attempt any further range change until the ignition is cycled or the request is repeated by moving the shift lever across the H-Gate and back again.

Because one of the components in the system can affect another, it is important that the technician confirm that all the circuits between components are in good condition without any short or open circuits. Failed communication between ECUs can cause problems seemingly unrelated to the transfer gearbox control system (e.g., poor shifting and inoperative cruise control).