EMERGENCY MODE, BACK-UP MODE "LIMP-HOME"
When a malfunction has occurred in the system that is confirmed (permanent malfunction) and is registered by the control module, modes are activated for certain systems and functions to handle the malfunction.
The purpose of these actions is to "protect" the control system and at the same time retain as much functionality or driving function as possible.
The minor malfunctions do not activate any back-up modes, there are different programs depending on type of malfunction and which control system to which it applies. All control systems do not have emergency modes or back-up modes.
Sometimes emergency modes must be activated immediately when a malfunction is detected, even if there has been no time to confirm and store any diagnostic trouble code. This is to maintain certain function, e. g., in case of malfunction of the mass air floe sensor for the engine management system, when the emergency program tries to prevent the engine from stopping.
HINT: Limp-home, emergency mode or back-up mode that may appear in case of malfunction is described in diagnostic trouble code information under replacement values.
For, e. g., automatic transmission TF-80SC AWD (XC90, B8444S) there are two back-up modes
- Failsafe action (temporary action)
- Emergency/limp-home mode
Failsafe action is activated at the first detection of the malfunction, if the malfunction disappears the system returns to normal function.
Emergency mode is activated in case of less serious malfunctions Limp-home mode is activated for the most serious malfunctions.
The warning light in the driver information module lights up, and a text message is shown in the text window in the driver information module that emergency/limp-home mode has been activated. When the ignition is turned off and on again, no text is shown until the malfunction is detected again.
Note. If the malfunction disappears (intermittent malfunction) the control module returns to normal function first when the ignition is turned on the next time.
For automatic transmission's emergency mode, the following may take place, e. g.
- Adaption function is blocked.
- Lock-up function is blocked.
- Function slipping lock-up is blocked.
- Function neutral check is blocked.
- The transmission only shifts to 3rd gear and reverse gear. All other shifting is blocked.
The above actions will be noticed by the driver since only one gear can be sued. Some may also notice that lock-up is not engaged. Then the vehicle comes to the workshop with a symptom for some form of lost driving function, probably also with the warning light on.
This symptom shall not be confused with the malfunction itself, instead it is a symptom of the back-up mode. The symptom that the malfunction itself causes (it it causes any) may be noticed briefly or not at all.
Note. If the malfunction is confirmed (permanent) it may be that the control module resumes normal function at ignition off and on again (symptom does not exist any longer). First when the malfunction is detected again (diagnostic trouble code test started and conditions fulfilled), the back-up mode is activated. The malfunction may then first be interpreted as if it was intermittent (non-existent or not active).
MISFIRING
If the fuel / air mix does not completely ignite in the combustion chamber, the engine misfires. The engine control module (ECM) detects the misfire by registering deviations in the rotation of the flywheel.
Deviation in flywheel rotation is due to
- driveline oscillations incorrect fuel / air mixture poor ignition sparking insufficient compression.
- normal variations caused by uneven combustion incorrect fuel / air mixture poor ignition sparking insufficient compression.
- flywheel mechanical tolerances incorrect fuel / air mixture poor ignition sparking insufficient compression.
- misfiring, which is caused by: incorrect fuel / air mixture poor ignition sparking insufficient compression.
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.
Misfires can be detected by registering the time difference between the intervals and comparing this with the filtered time difference in the flywheel. If the flywheel signal has not been adapted, the diagnostic is active. However this cannot identify misfires as accurately as an adapted flywheel signal. The adaptation is carried out at different loads and engine speed ranges. For the values to be set in the different ranges, the deviation in speed must be within certain fixed limits.
The misfire diagnostic is shut off when certain component faults have been detected by the engine control module (ECM).
"Rough Road" detection (drivetrain oscillations caused by, for example, extremely uneven roads) is not activated on this engine.
MISFIRE AFFECTING EMISSION LEVELS
If the Engine control module (ECM) detects a certain number of misfires during 1, 000 engine RPM this is interpreted as misfires that affect emissions. Trouble code is stored and MIL-light is activated. Immediately in connection with engine start, that is, during the engine's first 1, 000 revs after start, the misfire diagnosis is four times as sensitive as later on in the driving cycle.
CATALYTIC CONVERTER DAMAGING MISFIRE
If the engine control module (ECM) detects a certain number of misfires during 200 engine revolutions, this is interpreted as misfiring which damages the three-way catalytic converter (TWC). The number of misfires that the engine control module (ECM) allows before interpreting it as misfiring depends on the load and engine speed range. the malfunction indicator lamp (MIL) flashes when the misfire occurs, and shines constantly once the diagnostic trouble code (DTC) is stored.
The engine control module (ECM) does not store specific diagnostic trouble codes (DTCs) for emission or catalytic converter damaging related misfires.
The engine control module (ECM) registers and stores the engine speed and load range and whether the engine is warm or cold when misfiring is detected.
If the misfire stops, the requirements for the engine speed, load parameters and engine temperature must be met without misfiring before the engine control module (ECM) will begin counting down to extinguish the malfunction indicator lamp (MIL) and erase the stored diagnostic trouble codes (DTCs) for misfiring
The number of misfires can be read off.
If the fuel / air mix does not completely ignite in the combustion chamber, the engine misfires. The engine control module (ECM) detects the misfire by registering deviations in the rotation of the flywheel.
Deviation in flywheel rotation is due to
- driveline oscillations incorrect fuel / air mixture poor ignition sparking insufficient compression.
- normal variations caused by uneven combustion incorrect fuel / air mixture poor ignition sparking insufficient compression.
- flywheel mechanical tolerances incorrect fuel / air mixture poor ignition sparking insufficient compression.
- misfiring, which is caused by: incorrect fuel / air mixture poor ignition sparking insufficient compression.
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.
Misfires can be detected by registering the time difference between the intervals and comparing this with the filtered time difference in the flywheel. If the flywheel signal has not been adapted, the diagnostic is active. However this cannot identify misfires as accurately as an adapted flywheel signal. The adaptation is carried out at different loads and engine speed ranges. For the values to be set in the different ranges, the deviation in speed must be within certain fixed limits.
The misfire diagnostic is shut off when certain component faults have been detected by the engine control module (ECM).
"Rough Road" detection (drivetrain oscillations caused by, for example, extremely uneven roads) is not activated on this engine.
If the Engine control module (ECM) detects a certain number of misfires during 1, 000 engine RPM this is interpreted as misfires that affect emissions. Trouble code is stored and MIL-light is activated. Immediately in connection with engine start, that is, during the engine's first 1, 000 revs after start, the misfire diagnosis is four times as sensitive as later on in the driving cycle.
If the engine control module (ECM) detects a certain number of misfires during 200 engine revolutions, this is interpreted as misfiring which damages the three-way catalytic converter (TWC). The number of misfires that the engine control module (ECM) allows before interpreting it as misfiring depends on the load and engine speed range. the malfunction indicator lamp (MIL) flashes when the misfire occurs, and shines constantly once the diagnostic trouble code (DTC) is stored.
The engine control module (ECM) does not store specific diagnostic trouble codes (DTCs) for emission or catalytic converter damaging related misfires.
The engine control module (ECM) registers and stores the engine speed and load range and whether the engine is warm or cold when misfiring is detected.
If the misfire stops, the requirements for the engine speed, load parameters and engine temperature must be met without misfiring before the engine control module (ECM) will begin counting down to extinguish the malfunction indicator lamp (MIL) and erase the stored diagnostic trouble codes (DTCs) for misfiring
The number of misfires can be read off.
READINESS MONITOR (CERTAIN MARKETS ONLY)
This is a function in Engine control module (ECM) that controls the control module's emission-related sub-system. With the function you can read out if the control module has run all diagnostics for the following sub-systems
- Misfiring
- Fuel system
- Catalytic converter
- Evaporative emission (EVAP) system
- Heated oxygen sensors (HO2S)
- Other related components
Thus, Readiness refers only to if the diagnostics (tests) have been run or not, not if a malfunction was found or not. If a malfunction is found during the test, a diagnostic trouble code is stored and the malfunction light (MIL-light) is lit.
If the tests are run in the driving cycle without detecting a malfunction, then "Readiness" is immediately generated to OK.
However, if a malfunction is found in the driving cycle, then Readiness is generated to Not OK, and another driving cycle is required where the test is run to generate Readiness to OK.
The following alternatives are possible
- Readiness is run (OK), no malfunction detected, which means that the vehicle can be approved in a check.
- Readiness is run (OK), with malfunction detected, which means that the vehicle cannot be approved in a check.
- Readiness is not run (Not OK), which means that the vehicle cannot be approved in a check.
Readiness was originally a requirement from the American authority, United States Environmental Protection Agency (EPA), and is used there for Inspection and maintenance test (IM-test) (corresponds to annual inspection). Depending on legal requirements, the function may also be found on other markets.