If the engine management (MCM) control unit (A4) detects a malfunction or a fault in the system or on a system component, it proceeds in accordance with a precisely specified method depending on the severity of the actual fault.
1 Response in the event of drive train CAN (CAN 4) malfunction
In order to achieve the greatest possible security against breakdowns, the signal line of the drive train CAN (CAN 4) is a double line. The individual lines are designated as CAN Low and CAN High and are fed with reverse poled signals. Thus faults which effect both lines simultaneously cancel each other out.
Faults which only occur on one CAN line also do not create confusion for data transmission since the message can be reconstructed on the basis of the signal of the undisturbed line, i.e. the drive train CAN (CAN 4) is capable of operating over one wire.
The drive train CAN (CAN 4) only fails when both lines are interrupted or there is a short-circuit with the battery voltage or to ground.
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A failure of the drive train CAN (CAN 4) is detected by the engine management (MCM) control unit (A4), and it switches to limp-home mode. If the engine speed at this stage is below the limp-home mode speed, the engine speed is then maintained. In all other cases, including after restarting the engine, the engine management (MCM) control unit (A4) adjusts the limp-home mode speed, irrespective of the load.
It is thus possible for the driver to at least bring the vehicle to the nearest workshop.
The limp-home mode is notified to the other systems and displayed in the instrument cluster control unit (ICUC) (A1).
There is also entry of the condition in the fault memory.
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2 Response when camshaft position sensor (B601) fails
If the camshaft position sensor (B601) fails when driving, then the engine continues to run. Nevertheless, the maximum torque of the engine will be limited to protect the engine from damage. The engine management (MCM) control unit (A4) can also regulate the limp-home speed.
If the camshaft position sensor (B601) fails to return any pulses during the startup phase, then in each case a portion of the double firing, with which the engine is started, is cutoff, until the dip in speed, which then occurs, when a piston is not in ignition TDC, enables ignition TDC to be recognized. Once ignition TDC is recognized, the engine continues to run in unchanged state. Nevertheless, the maximum torque of the engine will be limited to protect the engine from damage. The engine management (MCM) control unit (A4) can also regulate the limp-home speed.
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3 Response when crankshaft position sensor (B600) fails
If the crankshaft position sensor (B600) fails, then the engine management (MCM) control unit (A4) still receives the rpm signals from the crankshaft position sensor (B601).
Its pulses do not come so closely together as those from the crankshaft position sensor (B600) but they are still adequate for calculating control time of the respective fuel injector. The time determination will be somewhat less exact so optimum fuel consumption or pollutant emission levels will not be achievable. Furthermore, for engine protection reasons, the maximum torque of the engine is limited to about a half. The engine management (MCM) control unit (A4) can also regulate the limp-home speed.
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4 Response when a temperature sensor fails
If the values from a temperature sensor are implausible, or if a temperature sensor has an open circuit or a short circuit, the engine management (MCM) control unit (A4) continues to operate with substitute values.
Own replacement values are stored for each temperature sensor since the measurement values of the individual temperature sensors can deviate from each other. It is not feasible for these to be suitable for all operating conditions, however, which is why a mild reduction in the maximum possible torque available takes place.
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5 Response when an exhaust gas recirculation (EGR) component fails
If the values of the charge air pressure and temperature sensor in the charge air pipe (B608), the charge air temperature sensor in the charge air housing (B617), the exhaust gas recirculation (EGR) differential pressure sensor (B621) or the exhaust gas recirculation positioner (Y621) are implausible, or if one of them suffers an open circuit or a short circuit, the engine management (MCM) control unit (A4) adjusts the limp-home mode speed and the engine's maximum torque is limited.
The fact that the limp-home strategy is alterable by the manufacturer means that it is possible that the engine speed remains unchanged and only the maximum torque of the engine is limited.
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6 Response when atmospheric pressure sensor in engine management (MCM) control unit (A4) fails
If the atmospheric pressure sensor in the engine management (MCM) control unit (A4) has a malfunction or a fault, then - depending on the particular situation - it uses substitute values:
If the pressure sensor in the charge air pressure and temperature sensor in the charge air pipe (B608) is intact, the engine management (MCM) control unit (A4) uses the air pressure in the charge air housing during the start procedure and the values stored in a substitute characteristic when the engine is running.
The reason why the engine management (MCM) control unit (A4) falls back on values in a substitute characteristic while the engine is running is because the charge pressure in the charge air housing can differ according to the engine loading and cannot therefore be used as the atmospheric pressure.
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If the pressure sensor in the charge air pressure and temperature sensor in the charge air pipe (B608) is also defective, the engine management (MCM) control unit (A4) then uses the values of the previously-mentioned substitute characteristic for all operating conditions.
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7 Response when engine management (MCM) control unit (A4) fails
The engine management (MCM) control unit (A4) has only one host CPU, and no limp-home mode CPU.
If the host CPU fails the engine stops and cannot be started anymore.
The drive control (CPC) control unit (A3) however is capable of diagnosing the failure of the engine management (MCM) control unit (A4) by reading out the fault memory, so that diagnosis is still possible.
While the engine management (MCM) control unit (A4) switches to the limp-home mode when a severe fault occurs or if a less serious fault occurs it operates using substitute values, it saves every fault that occurs and notifies the driver of them by means of an appropriate display in the instrument cluster (ICUC) control unit (A1).
In addition to information about the type of fault and possibly also a measurement value, the engine control (MR) also stores information about whether the fault is current or just transient or occurs sporadically. The fault memory can also be automatically deleted again, of course, for less serious faults if the stored fault does not occur again within the next 50 hours. In order to avoid coming to wrong conclusions when troubleshooting, please observe that certain operating conditions can lead to limitation of the maximum torque of the engine.
e. example, a radiator contaminated with dirt and dust will have a reduced cooling output. The engine management (MCM) control unit (A4) determines an increase in coolant temperature and limits the engine's maximum torque so that the coolant temperature limit is not exceeded.