(European OBD, Function - GF07.10-S-1021TRJ)

Engine 274 in model 447.6/7 

with code XM0 (Facelift) 

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

• Circuit 87M ON (Engine management ON)

EOBD. general 

An EOBD of the second generation is used. The EOBD is integrated in the ME-SFI [ME] control unit (N3/34) and continuously monitors all emissions-relevant components and systems of the vehicle.

The EOBD has the following tasks:

• Monitor emissions-relevant assembly parts and systems while driving.
• Establish malfunctions and save them.
• Display of malfunctions via the engine diagnosis indicator lamp (A1/1e58).
• Detected faults are transmitted via a uniform interface (diagnostic connector (X11)) to a diagnostic unit (e.g. Xentry Diagnostics)

EOBD pursues the follow objectives:

• Achieving permanently low exhaust emissions.
• Protect assembly parts at risk (such as catalytic converters) against backfires.

The following assembly parts and systems are monitored:

• Lambda sensor downstream of the catalytic converter (G3/21)
• Oxygen sensor upstream of catalytic converter (G8/6)
• Efficiency of catalytic converters (catalytic converter function)
• Catalytic converter heating
• Smooth running analysis (recognition of combustion misfires)
• Other emissions-relevant components or such components a malfunction of which prevents diagnosis of other components

Function sequence for the EOBD 

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 error saving
• 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 the fault memory.
• Function sequence for fault clearing

Function sequence for fault detection 

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

The faults and their storage are differentiated between as follows:

• The fault is always there
• Sporadic fault

The following faults are recognized according to their frequency and duration:

• Signals above or below the limit value (for example, short circuit, open circuit, defective sensor)
• An illogical combination of various signals
• Closed-loop control circuit (e.g. lambda control) at lower or upper limit of control interval
• Malfunctions in function chains (faulty test processes, e.g. for the smooth running control)
• Fault messages via the CAN bus

Function sequence for test procedure 

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

Assembly part checking 

The component checking is direct checking of a assembly part. It includes:

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

It can produce the following test results:

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

Function chain test 

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

Individual components and systems are checked which cannot be checked using component testing.

The function chain is a controlled procedure of cause and effect.

The ME-SFI [ME] control unit actuates one or more components (cause) and evaluates the resulting sensor signals (effect). Here the ME-SFI [ME] control unit compares the signals with stored comparative values and thus recognizes the trouble-free or not trouble-free function of components and systems.

The following are monitored over function chain tests, for example:

• Self adaptation of mixture formation
• Smooth running analysis (recognition of combustion misfires)
• Catalytic converter function
• Oxygen sensors (aging and controlling)
• Oxygen sensor heater

Function sequence for cyclic monitoring 

Cyclic monitoring takes place for components and systems which are not permanently active.

The following components and systems are monitored cyclically:

• Catalytic converter function
• Catalytic converter heating
• Oxygen sensors (aging and controlling)
• Oxygen sensor heater

Function sequence for continuous monitoring 

Continuous monitoring means continuous monitoring from engine start to "ignition OFF".

The following components and systems are monitored continuously:

• Smooth running analysis (recognition of combustion misfires)
• Self adaptation of mixture formation
• Automatic transmission (has its own vehicle diagnosis system with fault memory) (with code G42 (7G-TRONIC PLUS) or code G43 (9G-TRONIC))
• All other emissions-relevant components

Function sequence for readiness code 

In order to obtain a statement about freedom from faults of cyclically monitored components and systems during read out of the fault memory, there must be test readiness for this.

The test readiness of a component or a system is shown using the "readiness code". The readiness code allows recognition of whether checks for malfunction detection have run at least once and therefore the component or the system is active.

The check readiness is determined at least once per driving cycle and the "Readiness Code" is set where check readiness is given. To set the "Readiness Code", the vehicle must simply have checked at least once all the components associated with the system.

The test result for setting the "readiness code" is not important. This means that it will also be set if a fault is found in the systems or the component.

The "readiness code" is set for the following components or systems once they have been tested:

• Catalytic converter function
• Catalytic converter heating
• Oxygen sensors (aging and controlling)
• Oxygen sensor heater

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

To do this the function chain test is started manually.

All "readiness codes" are reset automatically when deleting fault codes.

Function sequence for error saving 

Emissions-relevant faults from the current and previous driving cycle are temporarily stored in the EOBD up to their confirmation (occurrence in three successive driving cycles) in the form of a fault code, the so-called "D  iagnostic T  rouble C  ode".

If a found malfunction occurs in three consecutive driving cycles, the fault code is stored in the ME-SFI [ME] control unit (N3/34) fault memory after the second driving cycle is completed.

Driving cycle

A driving cycle consists of the engine start, vehicle trip and engine shutoff, whereby an increase in the coolant temperature by at least 22°C to at least 70°C must take place.

Function sequence for avoiding consequential faults 

If a faulty signal is recognized and stored all tests are broken off for which this signal is need as a comparative value (so-called "transverse locking"). Saving of consequential faults is thereby prevented.

Function sequence for saving the fault freeze frame data 

Besides the malfunctions, the operating conditions under which they occurred are also stored as fault freeze frame data.

If the fault occurs a second time then also these fault freeze frame data are stored. If the fault continues to occur then the last stored fault freeze frame data is updated. The fault freeze frame data can be read out for the first and last occurrence of a fault.

Fault freeze frame data include:

• Vehicle speed
• Engine speed
• Coolant temperature
• Intake manifold pressure
• Charge air temperature
• Supply voltage
• Engine throttle condition
• Mixture formation adaptation value
• Status of the lambda control

Function sequence for fault message 

The engine diagnosis indicator lamp in the instrument cluster control unit (IC) (A1/1) is actuated by the ME-SFI [ME] control unit. If a fault occurs in two driving cycles, one after the other, the indicator lamp engine diagnosis lights up. In the case of catalyst-degrading misfires, the engine diagnosis indicator lamp flashes while the misfires are occurring and is then permanently illuminated during the entire remaining driving cycle.

The fault message via the engine diagnosis indicator lamp goes out automatically after three successive fault-free driving cycles.

Function sequence for reading out the fault memory 

The ME-SFI [ME] control unit is connected via the CAN with the diagnostics connection. Stored fault codes and their fault freeze frame data as well as the "Readiness Codes" can be read out with a diagnostic device via the diagnostics connection.

Function sequence for fault clearing 

Stored faults are only deleted automatically after 40 consecutive trouble-free driving cycles from the fault memory, whereby during driving an increase in the coolant temperature should occur of at least 22°C to at least 70°C. They can also be deleted after a repair using diagnostic equipment.

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