On-Board Diagnosis, Function - GF07.10-P-1020MMG

ENGINE 276.8/9 in MODEL 166 up to model year 2016 

Function requirements for European On-Board Diagnosis (EOBD), general points 

• Circuit 87M (engine management ON)

European OBD (OBD), general 

An OBD system of the second generation (OBD II) is used. In Europe, the OBD II, with appropriate adaptation for the European market, is called European On-Board Diagnosis (EOBD).

The OBD system is integrated in the ME-SFI [ME] control unit (N3/10) and constantly monitors all emissions-relevant components and systems of the vehicle.

The EOBD has the following tasks:

• Monitor emissions-relevant components and systems while driving
• Establish malfunctions and save them
• Display of malfunctions via the engine diagnosis indicator lamp (A1e58)
• Transmit errors via a uniform interface (diagnostic connector (X11/4)) to a diagnostic unit (e.g. Xentry Diagnostics)

EOBD pursues the follow objectives:

• Achieving permanently low exhaust emissions
• Protecting endangered components (such as CATs) against backfires

The following components and systems are monitored:

• Oxygen sensors
• NOx sensors control units on the left and right (N37/5, N37/6) (for code 920 (Gasoline direct injection with stratified charge))
• Efficiency of catalytic converters (catalytic converter function)
• Catalytic converter heating
• Purging
• Smooth running analysis (recognition of combustion misfires)
• EGR (for code 920 (Gasoline direct injection with stratified charge))
• Other emissions-relevant components or such components a malfunction of which prevents diagnosis of other components

Function sequence for European OBD 

The EOBD is described in the following steps:

• Function sequence for fault detection 
• Function sequence for test procedure 
• Function sequence for cyclic monitoring 
• Function sequence for continuous monitoring 
• Function sequence for readiness code 
• Function sequence for fault storage 
• Function sequence for avoiding consequential faults 
• Function sequence for saving the fault freeze frame data 
• Function sequence for fault message 
• Function sequence for reading out fault memory 
• Function sequence for erasing faults 

Function sequence for fault detection 

The ME-SFI [ME] control unit checks its input and output signals for plausibility and detects possible faults.

Malfunctions and the way they are stored are classified as follows:

• The fault is always there
• Loose contact which occurs during a drive

The following malfunctions are recognized in their frequency and duration:

• Signals above or below a limit value (e.g. short circuit open circuit, defective sensor)
• An illogical combination of various signals
• Closed-loop control circuit (e.g. lambda control) at the lower or upper limit of the controlling interval
• Faults in function chain (faulty test sequences, e g. for purging)
• Fault messages via the CAN buses

Function sequence for test procedure 

A differentiation is made during the test procedure between component testing and function chain testing.

Component testing 

Component testing is direct testing of a component. It includes:

• Monitoring the power supply and electric circuit
• Comparison of sensor signals with other sensor signals and stored comparative values

The following three test results can occur:

• Signal present (checking passed)
• Signal not present (a fault)
• Signal present, but implausible (a fault)

Function chain test 

The function chain test is indirect testing of the effect of a controlled change.

In this process individual components and systems are checked which cannot be tested by means of component testing.

The function chain is a controlled process studying cause and effect. The ME-SFI [ME] control unit controls one or more components (cause) and evaluates the resulting sensor signals (effect). *In the process the ME-SFI [ME] control unit compares the sensor signals with stored comparative values and thus recognizes trouble-free or not trouble-free functioning of components and systems.

The following are monitored by means of function chain tests:

• Self adaptation of mixture formation
• Smooth running analysis (recognition of combustion misfires)
• Catalytic converter function
• Oxygen sensors (aging and controlling)
• NOx sensors control units on the left and right (for code 920 (Gasoline direct injection with stratified charge))
• Oxygen sensor heater
• Purging
• EGR (for code 920 (Gasoline direct injection with stratified charge))

Function sequence for cyclic monitoring 

Cyclic monitoring takes place for components and system which are not permanently active. Purging only takes place, for example, for driving in partial-load range and can therefore also on be monitored in this operating phase.

The following components and systems are monitored cyclically:

• Catalytic converter function
• Catalytic converter heating
• Oxygen sensors (aging and controlling)
• NOx sensors control units on the left and right (for code 920 (Gasoline direct injection with stratified charge))
• Oxygen sensor heater
• Purging
• EGR (for code 920 (Gasoline direct injection with stratified charge))

Function sequence for continuous monitoring 

Continuous monitoring means constant monitoring from engine start up to "ignition OFF".

The following components and systems are monitored continuously:

• Smooth running analysis (recognition of combustion misfires)
• Self-adjustment of mixture formation
• A/T (A/T is fitted with its own EOBD with a fault memory)

Function sequence for readiness code 

In order to gain reliable information as to the trouble-free status of cyclically monitored components and systems when reading out the fault memory, these components and systems must be test ready.

The test readiness of a component or a system is shown by the readiness code. The readiness code tells you whether malfunction detection tests have been run at least once, indicating that the component or the system is active.

Test readiness is checked at least once per driving cycle. If test readiness exists, the readiness code will be set. In order to set the readiness code it is sufficient if the vehicle has checked all of the components belonging to a system at least once.

The test result is not significant in setting the readiness code. This means that it is also set if a fault in the system or the component is found.

The readiness code is set for the following components and system if their testing has occurred:

• Catalytic converter function
• Catalytic converter heating
• Oxygen sensors (aging and controlling)
• NOx sensors control units on the left and right (for code 920 (Gasoline direct injection with stratified charge))
• Oxygen sensor heater
• Purging
• EGR (for code 920 (Gasoline direct injection with stratified charge))

If the test readiness of individual systems or components is not given then these can be created using the XENTRY diagnostic unit.

To do this the function chain sequence is started manually over a menu item of the software.

All readiness codes are reset automatically when the fault code is deleted.

Function sequence for fault storage 

Emissions-relevant malfunctions just found from the current and previous driving cycle are temporarily stored in the EOBD until confirmed (through occurrence in two consecutive driving cycles) in the form of a fault code, also called a diagnostic trouble code or DTC.

If an established fault occurs in two driving cycles one after the other, the fault code is stored in the ME-SFI [ME] control unit fault memory after ending the second driving cycle.

Driving cycle

A driving cycle consists of engine start, vehicle driving and stopping the engine whereby an increase in the coolant temperature by at least 22°C to a maximum of 70°C must occur.

Function sequence for avoiding consequential faults 

If a faulty signal is detected and stored, all tests where this signal is required as a reference parameter are aborted (interlock). This prevents consequential faults from being stored.

Function sequence for saving the fault freeze frame data 

The faults which arose and the operating parameters or conditions, the so-called fault freeze frame data are stored.

If the malfunction occurs a second time, the associated fault freeze frame data will again be stored. If the malfunction continues to occur then the last stored fault freeze frame data will be updated. This means that the fault freeze frame data from the first and last occurrence of a malfunction can be read out.

Fault freeze frame data are, for example:

• Vehicle speed
• Engine speed
• Coolant temperature
• Intake manifold/boost pressure
• Intake air temperature
• Supply voltage
• Engine throttle condition
• Adaptation value of the mixture formation
• Status of the lambda control

Function sequence for fault message 

The engine diagnosis indicator lamp in the IC (A1) is actuated by the ME-SFI [ME] control unit via chassis CAN 1 (CAN E1) and chassis CAN 2 (CAN E2).

If a fault occurs in two driving cycles, one after the other, the indicator lamp engine diagnosis lights up.

In the case of catalytic converter damage caused by ignition misfires the engine diagnosis indicator lamp flashes for as long as the ignition misfires occur and then lights up permanently during the whole (remaining) driving cycle.

Fault message by means of the engine diagnosis indicator lamp goes out automatically after 3 consecutive trouble-free driving cycles.

Function sequence for reading out fault memory 

The diagnostic connector is networked via chassis CAN 1 and diagnostic CAN (CAN D) with the ME-SFI [ME] control unit. With "ignition ON" or with the engine running, stored fault codes and their fault freeze frame data as well as the readiness code can be read out using XD over the diagnostic connector.

Function sequence for erasing faults 

The system will automatically clear any stored malfunctions from the fault memory only after 40 consecutive trouble-free driving cycles have occurred. They can also be cleared again after repair using XD.

	Electrical function schematic for on-board diagnosis		PE07.10-P-2720-97NAE
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