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

ENGINE 651.9 in MODEL 207, 212 (except 212.098/298) as of model year 2014 

ENGINE 651.9 in MODEL 218 as of model year 2014 

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

On-board diagnosis (OBD), general 

An OBD system of the second generation is used (on-board diagnosis II (OBD II)). 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 CDI control unit (N3/9) and has the following tasks:

OBD pursues the follow objectives:

The following components and systems are monitored:

Function sequence for on-board diagnosis (OBD) 

The OBD is described in the following steps:

Function sequence for fault detection 

The CDI control unit checks itself and its input and output signals for plausibility and detects possible malfunctions.

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

The following malfunctions are recognized in their frequency and duration:

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:

The following three test results can occur:

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 CDI control unit controls one or more components (cause) and evaluates the resulting sensor signals (effect). In the process the CDI control unit compares the sensor signals with stored comparative values and thus recognizes fault-free or faulty functioning of components and systems.

The following are monitored by means of function chain tests:

Function sequence for cyclic monitoring 

Cyclic monitoring takes place for components and system which are not permanently active. Regeneration, for example, only takes place when the vehicle is being operated in partial-load range and can therefore only be monitored during this operating phase.

The following components and systems are monitored cyclically:

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:

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 systems once they have been tested:

If test readiness does not exist for individual systems or components, it can be established using the diagnostic unit.

To do this the function chain process is started manually using a menu item in the diagnostic software.

IMPORTANT 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 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 an established fault occurs in two consecutive driving cycles the fault code is stored in the fault memory of the CDI control unit after ending the second driving cycle.

IMPORTANT Driving cycle

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 during travel.

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 

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

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:

Function sequence for fault display 

The CDI control unit sends the request for the engine diagnosis indicator lamp (A1e58) to the front SAM control unit with fuse and relay module (N10/1) via chassis CAN 1 (CAN E1).

The front SAM control unit with fuse and relay module actuates the engine diagnosis indicator lamp in the instrument cluster (A1) via 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.

The fault display indicated by the engine diagnosis indicator lamp goes out automatically after 3 consecutive trouble-free driving cycles.

Function sequence for reading out fault memory 

The CDI control unit is connected to the diagnostic connector via chassis CAN 1, the front SAM control unit with fuse and relay module and the diagnostic CAN (CAN D). 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 clearing 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, however, also be deleted (after repair work has been done) using commercially available diagnostic equipment or the Xentry Diagnostics system.

  Electrical function schematic for on-board diagnosis Model 207 PE07.10-P-2720-97EAN
  Electrical function schematic for on-board diagnosis Model 212 PE07.10-P-2720-97DAR
  Electrical function schematic for on-board diagnosis Model 218 PE07.10-P-2720-97XAH
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