On-board diagnosis, function - GF07.10-P-1020MMW

ENGINE 276.9 in MODEL 212.095 as of model year 2014 

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

• Circuit 87M (engine timing ON)

On-board diagnosis, general points 

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 OBD has the following tasks:

• Monitor exhaust-related components and systems while traveling
• Identify and store malfunctions
• Indicate malfunctions by means of a warning lamp (engine diagnosis indicator lamp (A1e58))
• Detected faults are transmitted over a uniform interface (diagnostic connector (X11/4)) to a diagnostic unit (e.g. Xentry Diagnostics))

OBD pursues the follow objectives:

• Ensuring permanently low exhaust emissions
• Protecting components at risk (such as catalytic converters) against backfires

The following components and systems are monitored:

• Oxygen sensors
• Cooler sensor for engine diagnosis (B11/15) (for code (494) USA version and code (917) Partial zero emission vehicle emission USA)
• Efficiency of catalytic converters (catalytic converter function)
• Catalytic converter heating
• Purging
• Smooth running analysis (recognition of combustion misfire)
• Other emission-relevant components or components whose malfunction prevents the diagnosis of another component

Function sequence for on-board diagnosis 

The OBD 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 display 
• 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:

• Malfunction permanently present
• Loose contact that occurred while driving

The following malfunctions are recognized in their frequency and duration:

• Signals above or below limit value (e.g. short circuit, open circuit, defective sensor)
• Illogical combination of various signals
• Closed loop (e.g. lambda control) at the lower or upper limit of the regulating interval
• Faults in function chains (faulty test runs, e.g. for purging)
• Malfunction messages via the CAN data buses

Function sequence for test procedure 

For test procedures one differentiates between component testing and function chain testing.

Component testing 

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

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

The following three test results can occur:

• Signal present (test passed)
• Signal not present (malfunction)
• Signal present, but implausible (malfunction)

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-adjustment of the mixture formation
• Smooth running analysis (recognition of combustion misfire)
• Catalytic converter function
• Oxygen sensors (aging and regulation)
• Oxygen sensor heater
• Purging

Function sequence for cyclic monitoring 

Cyclic monitoring takes place for components and system which are not permanently active. Purging takes place, for example only for driving operations in the partial-load range and can only then be monitored in this operating phase. The following components and systems are monitored cyclically:

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

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 misfire)
• Self-adjustment of the mixture formation
• Automatic transmission (equipped with its own OBD with fault memory)
• All other emission-relevant components

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 regulation)
• Oxygen sensor heater
• Purging

If test readiness does not exist for individual systems or components, it can be established using the diagnostic tester. To do this the function chain process is started manually using a menu item in the diagnostic software.

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

Function sequence for fault storage 

Exhaust gas-relevant malfunctions just found from the current and previous driving cycle are temporarily stored in the OBD 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 a fault which has been established occurs in two driving cycles, one after the other, then the fault code is stored after ending the second driving cycle in the fault memory of the ME-SFI [ME] control unit.

A driving cycle consists of an engine start, vehicle journey and stopping the engine, whereby an increase in coolant temperature by at least 22°C up to at least 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 include:

• Vehicle speed
• Engine speed
• Coolant temperature
• Intake manifold air pressure
• Intake air temperature
• Supply voltage
• Engine load status
• Mixture formation adaptation value
• Status of lambda control

Function sequence for fault display 

The ME-SFI [ME] control unit actuates via chassis CAN 1 (CAN E1) the front SAM control unit with fuse and relay module (N10/1). The front SAM control unit with fuse and relay module (N10/1) actuates via the chassis CAN 2 (CAN E2) the engine diagnosis indicator lamp in the IC (A1). 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 indication by means of the engine diagnosis indicator lamp ceases 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. Stored fault codes and their fault freeze frame data as well as the readiness codes can be read out with the ignition ON or engine running via the diagnostic connector using a commercially available diagnostic unit or Xentry Diagnostics.

Function sequence for erasing faults 

The system will automatically erase any stored malfunctions from the fault memory only after 40 consecutive trouble-free driving cycles have occurred. They can, however, also be erased (after repair work has been done) using commercially available diagnostic equipment or Xentry Diagnostics.

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