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

GF07.10-P-1020MAW On-board Diagnosis, Function
ENGINES 156.9 in MODEL 204.0 /2 /3, 212.0 /2

Function requirements for On-Board Diagnosis (OBD), general
^ Circuit 87M (engine timing ON)

On-board diagnosis, general points
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 ME-SFI [ME] control unit (N3/10) and constantly monitors all exhaust gas relevant components and systems of the vehicle.
The OBD has the following tasks:

- Monitoring emissions-relevant components and systems during travel
- Determining and storing malfunctions
- Indicating malfunctions by means of a warning lamp (engine diagnosis indicator lamp (A1e58))
- Detected faults are transmitted via a uniform interface (diagnostic connector (X11/4)) to a diagnostic unit (e.g. Xentry Diagnostics)) OBD pursues the follow objectives:
- Ensure permanently low exhaust emissions
- Protect components at risk (such as e.g. catalytic converters) against backfires

The following components and systems are monitored:

- Oxygen sensors
- Efficiency of the catalytic converters (catalytic converter function)
- Catalytic converter heating
- Purge control
- Smooth running control (recognition of combustion misfire)
- Secondary air injection
- Camshaft adjustment (for code (494) USA version)
- Cooling system (vehicles with code (494) USA version)
- Tank leak tightness testing (for code (494) USA version)
- Other emission-relevant components or such components the malfunction of which prevents the diagnosis of another component

Function sequence for on-board diagnosis
The OBD is described in the following Operation 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 which occurred during travel

The following malfunctions are recognized in their frequency and duration:

- Signals above or below limit value (for example, short circuit, open circuit, sensor malfunction)
- 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 air injection or 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 actuates 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 control (recognition of combustion misfire)
- Catalytic converter function
- Oxygen sensor (aging and regulation)
- Oxygen sensor heater
- Purge control
- Secondary air injection
- Tank leak tightness testing (for code (494) USA version)

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 operation in 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 sensor (aging and regulation)
- Oxygen sensor heater
- Purge control
- Secondary air injection
- Tank leak tightness testing (for code (494) USA version)

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 control (recognition of combustion misfire)
- Self adaptation of mixture formation
- Automatic transmission (automatic transmission fitted 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 sensor (aging and regulation)
- Oxygen sensor heater
- Purge control
- Secondary air injection
- Tank leak tightness testing (for code (494) USA version)

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.

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.

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.

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 engine diagnosis indicator lamp in the instrument cluster (A1) is actuated by the ME-SFI [ME] control unit via the chassis CAN (CAN E).
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 the chassis CAN 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.