GF07.10-P-1020MI on-Board Diagnosis, Function

GF07.10-P-1020MI On-Board Diagnosis, Function
- as of model year 09

Function requirements of On-Board Diagnose (OBD), general points
^ Circuit 87M ON (engine control 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 exhaust gas relevant components and systems of the vehicle.
The OBD has the following tasks:
- Monitors emission-related components and systems while driving
- Determining and storing malfunctions
- Shows malfunctions by means of a warning lamp (motor diagnosis indicator lamp (A1e58))
- Determined errors are transmitted via a uniform interface (diagnostic connector (X11/4)) on a diagnostic unit (e.g. Xentry diagnostics))

The follow objectives are monitored by the OBD:
- 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 (for 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 items:
^ 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 storage
^ Function sequence for avoiding consequential faults
^ Function sequence for saving the fault freeze frame data
^ Function sequence for fault message
^ Function sequence for read out fault memory
^ Function sequence for error cancellation

Function sequence for fault detection
The ME-SFI [ME] control unit checks its input and output signals for plausibility and detects possible faults.

The faults and their storage are differentiated between as follows:
- Fault permanently present
- Loose contact which occurred during a drive

The following faults are recognized in their frequency and period:
- 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)
- Fault messages via the CAN data buses

Function sequence for test procedure
For test procedures on differentiates between component checking and the function chain test.

Component checking
The component checking is direct checking 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 (fault)
- Signal present, but implausible (fault)

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 from 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 of the 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 obtain a statement about the lack of faults in cyclically monitored components and systems when reading out the fault memory they must be ready to test.

The readiness to test of a component or a system is shown by the readiness code. Through the readiness code it is possible to recognize whether checks for indication of disturbance have run at least once and therefore the component or the system is active. The readiness to test is determined at least once per driving cycle and sets the readiness code for a given readiness to test. In order to set the readiness code it is therefore sufficient if the vehicle has checked all of the components belonging to a system at least once.

The test result for setting the readiness code is not significant. This means that it is also set of a fault in the system or the components is established.

The readiness code is set for the following components and system if their testing has occurred:
- 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)

If there is no readiness to test for individual systems or components then this can be created using the diagnostic unit.
To do this the function chain process is started manually over a menu item in the diagnostic software.

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

Function sequence for error storage
Exhaust gas-relevant faults from the current and previous driving cycle, which have just been determined, are buffered in the OBD until they are confirmed (occurrence in two driving cycles) in the form of a fault code, the so-called DTC (Diagnostic Trouble Code).

If an established fault occurs in two driving cycles one after the other, the fault code is stored in the ME-SFI [ME] control unit fault memory after ending the second driving cycle.

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, for which this signal is needed as a reference parameter (the so-called cross locking mechanism). 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 fault occurs a second time then also this fault freeze frame data is stored. If the fault continues to occur then the last stored fault freeze frame data is updated. So the fault freeze frame data from the first and last occurrence of a fault can be read out.

Fault freeze frame data are:
- 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 message
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 damaging ignition misfires the indicator lamp engine diagnosis lights up for as long as the ignition misfires occur and then lights up permanently in the whole (remaining) driving cycle.
The fault indicator on the indicator lamp engine diagnosis goes out automatically after 3 consecutively occurring trouble-free driving cycles.

Function sequence for read 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 code can be read out using a commercially available diagnostic unit or the Xentry diagnostic system for "ignition ON" or for a running engine via the diagnostic connector.

Function sequence for error cancellation
Stored faults are only deleted automatically from the fault memory after 40 consecutive trouble-free driving cycles. They can, however, also be deleted (after repair work has been done) using a commercially available diagnostic equipment or the Xentry diagnostic system.