Instruments (Diagnostics): Description and operation

Diagnostics

A diagnostic socket allows the exchange of information between the instrument pack and TestBook or other diagnostic tool. The diagnostic socket is located behind the centre console, in the passenger footwell.

The diagnostic socket is connected to the instrument pack on an ISO 9141 K Line. When the diagnostic mode is accessed the LCD displays 'dIAg, which remains until the diagnostic mode is exited.

The instrument pack stores fault flags which can be retrieved using TestBook or any diagnostic tool using Keyword 2000 protocol.

Each fault stores two flags. The first flag (fault log) indicates that a fault has occurred with that function. This flag can only be removed using a diagnostic tool.

The second flag (active fault) indicates that there is currently a fault with that function. This flag is only active while the fault condition exists. Removal of the fault condition will clear the active fault flag.

Controller Area Network (CAN)

The CAN bus is a high speed broadcast network between the instrument pack, the ECM, the EAT ECU and the ABS ECU, allowing fast exchange of data between the components every few microseconds.

The bus comprises two wires which are identified as CAN Low (L) and CAN High (H). The wires are twisted together to minimise the electromagnetic interference (noise) produced from the CAN messages.

To prevent message errors from electrical reflections, 120Ω resistors are incorporated into the CAN wire terminals of the ECM, ABS ECU, EAT ECU and the instrument pack.

CAN messages consist of a signal which is simultaneously transmitted, in opposite phase, on both wires. CAN L switches between 2.5 and 1.5 volts, while CAN H switches between 2.5 and 3.5 volts. This causes a potential difference between the two lines to switch between 0 volt (logic 1) and 2 volts (logic 0) to produce the digital signal message.

In the event of CAN bus failure, any of the following symptoms may be observed: l Tachometer, speedometer and temperature gauge will fail to operate l CAN controlled warning lamps will be inoperative.

CAN Bus Switching

A = CAN High B = CAN Low