Engine management system - MEMS (Diagnostics): Description and operation

Diagnostics

A diagnostic socket allows the exchange of information between the ECM and TestBook or a diagnostic tool using Keyword 2000 protocol.

The diagnostic socket is located in the driver's footwell behind the centre console.

A dedicated diagnostic (ISO 9141 K Line) bus is connected between the ECM and the diagnostic socket and allows the retrieval of diagnostic information and the programming of certain functions using TestBook.

The ECM uses a 'P' code diagnostic strategy and can record faults relating to the engine management system. P codes are accessed via the ECM when TestBook is connected.

On-Board Diagnostics (OBD)

The MEMS 3 ECM software is programmed to meet current emission standard ECD 3. This regulation is being introduced throughout Europe from the year 2000 and is similar to the OBD (phase II) regulations in place in North America.

OBD is concerned with the monitoring of certain functions, the failure of which would result in an increase of exhaust emissions above legislated thresholds. The OBD is concentrated on the engine management system.

If a fault occurs the ECM will store an applicable 'P' code in its memory and the MIL will be illuminated. The failure codes can be accessed with TestBook and the faults stored by the ECM are normally qualified by one of the following failure types: l Min - the minimum expected value has been exceeded l Max - the maximum expected value has been exceeded. l Signal - the signal is not present. l Plaus - an implausible condition has been detected.

The OBD operates in the background, monitoring the operations controlled by the ECM. The systems are monitored as the driver operates the vehicle, although the driver will be unaware that any monitoring is being performed. Individual system tests take place as the applicable circumstances occur.