Engine management system - Td5 (Operation): Description and operation (page 3 of 3)

Controller Area Network (CAN) system The CAN system is a high speed serial interface between the ECM and the Electronic Automatic Transmission (EAT) ECU. The CAN system uses a data bus to transmit information messages between the ECM and the EAT ECU. Because there are only two components in this CAN system, one will transmit information messages and the other will receive information messages, and vice-versa.

The CAN system is used by the EAT ECU and the ECM for the following: l Gearshift torque control information. l EAT OBD information. l MIL request. l Vehicle speed signal. l Engine temperature. l Engine torque and speed. l Gear selected. l Gear change information.

The CAN system uses a twisted pair of wires to form the data bus to minimise electrical interference. This method of serial interface is very reliable and very fast. The information messages are structured so that each of the receivers (ECM or EAT ECU) is able to interpret and react to the messages sent.

The CAN data bus is connected directly between pin 32 of connector C0158 of the ECM and pin 44 of connector C0193 at the EAT ECU, and pin 35 of connector C0158 of the ECM and pin 16 of connector C0193 at the EAT ECU.

The CAN system can fail in the following ways: l CAN data bus wiring open circuit. l CAN data bus wiring short circuit.

In the event of a CAN data bus failure any of the following symptoms may be observed: l EAT defaults to reverse and 4th gear if the vehicle is moving, 3rd gear if the vehicle is stationary. l Harsh gearshifts. l Sport and manual warning lamps flash alternately.

Vehicle Speed Signal (VSS) The VSS is an integral part of the ECM's overall adaptive strategy. The ECM receives the signal direct from the SLABS ECU. The SLABS ECU is not connected to the controller area network (CAN) so therefore is hard wired.

Vehicles fitted with automatic transmission have two vehicle speed input signals to the ECM. One signal is from the SLABS ECU and the other is from the automatic transmission ECU. The ECU compares these speed signals.

The ECM also receives transfer gearbox information. This allows the ECM to take in to account the vehicle being driven using low range gearing and compensate as necessary. The signals generated by the SLABS ECU for manual transmission, and by the EAT ECU for automatic transmission are received by the ECM in the form of a PWM signal. The frequency of this signal changes in accordance with road speed.

The input signal for the SLABS is measured via pin 13 of connector C0658 of the ECM. The SLABS ECU generates a PWM signal switching between 0 and 12 volts at a frequency of 8000 pulses per mile.

For vehicles with automatic transmission the input signal for the EAT ECU is measured via pins 32 and 35 of connector C0158 of the ECM. These pin numbers provide a bi-directional communications link using the CAN data bus.