Diagnostic functions.

Design and Function

Applies to: 2008–2012; B4184S11, B4184S8, B4204S3, B4204S4

Overview

General

The engine control module (ECM) diagnoses both internal signals and functions as well as signals and functions from connected components.

Conditions for diagnosis

For the diagnosis of a component or function to start, certain specific conditions must be met. The conditions for diagnostics vary depending on the component or function being diagnosed.

The "operating cycle" for the diagnostic must have been run before the diagnostic can be completed. An operating cycle varies depending on the component or function being diagnosed. Certain diagnostics only require the ignition to be switched on and off for an operating cycle to be run. Other diagnostic require that several different conditions are met regarding for example:

• vehicle speed
• engine coolant temperature (ECT)
• time since start
• different load and engine speed relationships in the same trip
• a certain event (for example the evaporative emission system (EVAP) valve is operating).

When the engine control module (ECM) has run all the implemented diagnostics, the control module has completed a "trip". An extensive driving schedule in various conditions is required to complete a trip. The engine may also need to be shut off for a certain amount of time and then switched on again to complete a trip.

Emissions related diagnostic functions

The engine control module (ECM) checks that the emissions related systems are working. These systems are checked by running a diagnostic function. The diagnostic function checks the components and that the system is functioning.

Diagnostic trouble code (DTC) memory

When the engine control module (ECM) detects a fault, the diagnostic trouble code (DTC) is stored with a qualifier and status in the DTC memory in the control module. For certain types of diagnostic trouble codes (DTCs), the missing signal is replaced with a substitute value so that the system can continue functioning.

If the fault disappears, the diagnostic trouble code (DTC) will remain in the DTC memory for some time, but its status will change.

Lighting the malfunction indicator lamp (MIL)

In the event of emissions related diagnostic trouble codes (DTCs), a counter counts down to determine when to light the malfunction indicator lamp (MIL). The conditions for lighting the malfunction indicator lamp (MIL) vary depending on which diagnostic trouble code (DTC) is stored.

Misfire diagnostic

If the fuel/air mixture does not ignite completely in the ignition stroke the engine is misfiring. The engine control module (ECM) detects the misfire by registering deviations in the rotation of the flywheel.

Every two hundred revolutions the engine control module (ECM) calculates the number of misfires which can cause catalytic converter damage (Type A) and compares this with a threshold value in the engine control module (ECM).

Every one thousand revolutions the number of misfires which can result in excessive emissions is also calculated and this is compared with threshold values in the engine control module (ECM).

If any value is exceeded then a diagnostic trouble code (DTC) is generated in the engine control module (ECM).

Deviation in flywheel rotation is due to:

• driveline oscillations
• normal variations caused by uneven combustion
• flywheel mechanical tolerances
• misfiring, which is caused by:
  • incorrect air/fuel ratios
  • poor ignition sparking
  • insufficient compression.

Diagnostics

The mechanical tolerances and oscillations in the drive line disrupt the signal. This makes it difficult to determine if a misfire has occurred or if the detection is incorrect.

The signal from the flywheel is adapted to filter out the irregular rotation. This eliminates the mechanical variations in the flywheel.

The adaptation is performed in different load and engine speed ranges. For the value to be set in the different ranges the rotation deviation must be within certain stipulated limits.

Oscillations in the driveline, e.g. on a very rough road, can cause the engine to run unevenly.

The misfire diagnostics are closed when:

• the accelerator pedal is fully released and the vehicle is using the engine brake
• the engine control module (ECM) limits the torque on the output shaft
• the engine control module (ECM) requests A/C on or the engine is loaded by the servo pump
• the engine control module (ECM) activates the EGR valve flow test (internal).

Diagnostic trouble code (DTC) management

The engine control module (ECM) determines how much the engine is misfiring by counting the number of misfires during a certain number of engine revolutions. If the engine control module (ECM) detects a certain number of misfires during 1000 engine revolutions, this is interpreted as misfiring which affects emissions. If the misfires exceed a certain value during 200 engine revolutions, this is interpreted as misfiring which damages the three-way catalytic converter (TWC).

Diagnostic trouble codes (DTC) for misfiring will light the malfunction indicator lamp (MIL). If there is risk of damage to the three way catalytic converter, the malfunction indicator lamp (MIL) will flash during misfiring and then switch to a constant light.

The engine control module (ECM) registers and stores the engine speed and load parameters within which the misfire was detected. For the diagnostic trouble code (DTC) to be stored, the misfire must occur twice more within the same engine speed and load parameters.

In the event of emissions related misfiring, the malfunction indicator lamp (MIL) lights during the second operating cycle and a diagnostic trouble code (DTC) is stored.

If the misfire stops, the requirements for the engine speed and load parameters must be met without misfiring before the engine control module (ECM) will begin counting down to extinguish the warning lamp and erase stored diagnostic trouble codes (DTCs) for misfiring.

Heated oxygen sensor (HO2S) diagnostic

The heated oxygen sensors are the binary type and work with voltage control. Therefore, the signals from the heated oxygen sensors cannot be measured with a multimeter.

The engine control module (ECM) checks the heated oxygen sensors (HO2S) when the conditions for the diagnostic have been met.

The following faults can be registered by the control module:

• Electrical faults in the circuits for the heated oxygen sensors (HO2S) and for oxygen sensor preheating.
• Heated oxygen sensor (HO2S) dynamics. The control module adds a calculated control signal and then checks that the transition period between rich and lean mixture is not too long.
• Transition period. The control module continuously measures the transition period during the control, and if the period is too long or no transition takes place then the fault is registered and the corresponding diagnostic trouble code (DTC) is generated.

For further information about diagnostics for the heated oxygen sensor, see "Diagnostic trouble code information" in VIDA, information manager.

Three-way catalytic converter (TWC) diagnostics

The catalytic converter stores oxygen from the exhaust gases and uses it to make toxic gases more environmentally friendly. The catalytic converter is a three-way catalytic converter (TWC) in which HC (hydrocarbons) and CO (carbon monoxide) are oxidized and NO_x (nitrous oxide) is reduced. As the catalytic converter ages, its ability to store oxygen is reduced. The conversion capacity of the catalytic converter is reduced and unburned residue which is harmful to environment is released. To reduce the environmentally damaging emissions, the engine control module (ECM) checks the efficiency of the catalytic converter. In brief, this check is carried out as follows.

Two heated oxygen sensors (HO2S) are used to check the catalytic converter, one heated oxygen sensor (HO2S) in front of the catalytic converter and one heated oxygen sensor (HO2S) located in the center of the catalytic converter, i.e. the rear heated oxygen sensor (HO2S). The main function of the heated oxygen sensors (HO2S) is to measure the oxygen content in the exhaust gases so that the control module can keep the engine's fuel and air mixture around λ=1. With this mixture the optimum stage for conversion in the catalytic converter is achieved.

When the efficiency of the catalytic converter is good and the fuel and air mixture is normal then the front heated oxygen sensor (HO2S) sends a signal between rich and lean while the rear heated oxygen sensor (HO2S) is on an even level. When the efficiency of the catalytic converter is poor but the fuel and air mixture is normal then the rear heated oxygen sensor (HO2S) will also send a signal between rich and lean. The reason for this is that the exhaust gases then flow directly through the catalytic converter without being affected.

If the frequency of the rear heated oxygen sensor (HO2S) is too great in relation to the frequency of the front heated oxygen sensor (HO2S) during a number of checks then a diagnostic trouble code (DTC) for catalytic converter efficiency will be generated.

The catalytic converter diagnostics are shut down when the emission related diagnostic trouble codes (DTC) are stored in the engine control module (ECM), for example for the EGR, heated oxygen sensor (HO2S) or the throttle unit.

EGR diagnostics

To reduce the nitrous oxide (NO_x) discharges the engine is equipped with exhaust gas recirculation (EGR).

Coolant encloses the pipes through which the exhaust gases passes. In certain situations this lowers the temperature of the exhaust gases by up to approximately 180 °C. The lower temperature results in a further reduction of NO _x.

The volume of recirculated exhaust gases is controlled by the engine control module (ECM) via the EGR valve stepper motor and its four coils.

The volume of returned gases is dependent on factors such as:

• coolant temperature
• intake temperature
• engine speed
• the engine load

The engine control module (ECM) continuously monitors the EGR valve with respect to electrical faults, i.e. short circuits and open circuits.

In addition, a flow test is performed by opening and closing the valve and then measuring the pressure in the intake manifold. This is done 10 times to obtain a greater selection of measurement values.

The average value of the 10 measurements is then used to calculate the pressure difference in the intake manifold, between fully open and fully closed valve.

If the difference is too small compared with the calculation by the engine control module (ECM) then a flow error is registered.