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

Engine 274 in model 447, 448 

except code XM0 (Facelift) 

Function requirements 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:

• Oxygen sensor downstream of 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 the lower or upper limit of the controlling 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
• A/T (is fitted with its own diagnostic system with fault memory) (with code G42 (7G-TRONIC PLUS))
• 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 an assembly part 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 components or the system is active.

The checking readiness is determined at least once per driving cycle and the readiness code is set for a given checking readiness. To set the readiness code it is sufficient if the vehicle has checked all components associated with the system at least once.

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 and 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 components 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 DTCs.

Function sequence for error saving 

Emissions-relevant faults from the current and the previous driving cycle, which have just been detected, are stored temporarily until confirmation (occurrence in two driving cycles one after the other) in the form of a fault code, the so-called "Diagnostic Trouble Code", in the EOBD.

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

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 so-called 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 are, for example:

• Vehicle speed
• Engine speed
• Coolant temperature
• Intake manifold pressure
• Charge 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 control unit (IC) (A11) is actuated by the ME-SFI [ME] control unit via the chassis CAN (CAN E). If a fault occurs consecutively in two driving cycles, the engine diagnosis indicator lamp symbol lights up. For catalytic converter damaging misfires the symbol of the engine diagnosis indicator lamp flashes for as long as the misfires occur. Then it lights up permanently during the whole remaining driving cycle.

A fault message shown over the engine diagnosis indicator lamp symbol goes out automatically after three consecutive trouble-free driving cycles.

Function sequence for reading out the fault memory 

The ME-SFI [ME] control unit is connected via the chassis CAN, the electronic ignition lock control unit (EZS) (N73) and the diagnostic CAN (CAN D) with the diagnostic connector. Saved DTCs and their fault freeze frame data as well as the readiness codes can be read out over the diagnostic connector.

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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