On-Board Diagnostics System
OVERVIEWThe California Air Resources Board (CARB) initiated regulation of On Board Diagnostics (OBD) for vehicles sold in California beginning with the 1988 model year. The first phase, OBD I, required monitoring of the fuel metering system, Exhaust Gas Recirculating (EGR) system, and additional emission related components. The Malfunction Indicator Lamp (MIL) was required to light and alert the driver of the malfunction and the need for service of the emission control system. The MIL was required to be labeled "CHECK ENGINE" or "SERVICE ENGINE SOON." Associated with the MIL was a fault code or Diagnostic Trouble Code (DTC) identifying the specific area of the fault.
The OBD system was proposed by the CARB to improve air quality by identifying vehicles exceeding emission standards. Passage of the federal Clean Air Act in 1990 has also prompted the Environmental Protection Agency (EPA) to develop on board diagnostic requirements. CARB OBD II regulations will be followed until 1999 when the federal regulations will take precedence.
The OBD II system meets government regulations by monitoring the emission control system. When a system or component exceeds emission thresholds or a component operates outside of a design tolerance, a DTC will be stored and the MIL will be illuminated.
Fault detection strategy and MIL operation are associated with trips and drive cycles. Each monitor has requirements for setting and clearing DTCs and for controlling the MIL.
The diagnostic executive is the computer program in the Powertrain Control Module (PCM) that coordinates the OBD II self-monitoring system. This program controls all the monitors and interactions, DTC and MIL operation, Freeze Frame data, and scan tool interface.
Freeze Frame Data describes stored engine conditions, such as state of the engine, state of fuel control, spark, rpm, load, and warm-up status at the point the first malfunction is detected. Previously stored conditions will be replaced only if a fuel or misfire malfunction is detected. This data is accessible with the scan tool to assist in repairing the vehicle.
OBD II Inspection Maintenance (IM) readiness code P1000 indicates that not all of the OBD II monitors have been completed since the computer memory was last cleared. In certain states, it may be necessary to operate the vehicle (perform a drive cycle) until the code P1000 is erased from the PCM in order to purchase a vehicle license.
POWERTRAIN CONTROL MODULE
The center of the OBD II system is the microprocessor based Powertrain Control Module (PCM). The PCM receives input from sensors and other electronic components (switches, relays, etc.). Based on information received, and information programmed into its memory, the PCM generates output signals to control various relays, solenoids, and actuators.
Keep Alive Memory (KAM)
The PCM stores information in a special area of memory called Keep Alive Memory (KAM). Information such as fuel tables and continuous memory diagnostic trouble codes, that have been learned by the PCM during engine operation, are stored in KAM.
Power is supplied to KAM at all times so that the information is not lost with the ignition key OFF. If power is lost to KAM due to removing of the battery cables, discharged battery, or disconnecting the PCM, all the information that is currently stored in KAM will be erased. In the event that this happens, it is necessary to relearn the fuel tables by performing a Drive Cycle. KAM can also be cleared using the Rotunda(r) New Generation STAR tester 007-00500. If this method is used to clear KAM, it will not erase the fuel tables that are stored in KAM so the fuel tables will not have to be relearned.
Hardware Limited Operation Strategy (HLOS)
This is a system of special circuitry which provides for a minimum level of engine operation should the PCM stop functioning correctly. All modes of self-test are not functional at this time. Electronic hardware is in control of the system while in HLOS.
Adaptive Fuel Control Strategy
The adaptive fuel control strategy is designed to compensate for variability in the fuel system components. If, during normal vehicle operation, the fuel system is detected to be biased rich or lean, the adaptive fuel control will make a corresponding shift in the fuel delivery calculation.
Whenever an injector or fuel pressure regulator is replaced, KAM should be cleared and the fuel tables relearned. This is necessary so the fuel strategy does not use previously learned adaptive values.
Adaptive Idle Air Control Strategy
The adaptive idle air control strategy is designed to adjust the Idle Air Control (IAC) calibration to correct for wear and aging of components. When engine conditions meet the adaptive strategy learning requirement, the strategy monitors the engine and determines the values required for ideal idle calibration. The adaptive strategy stores these values in a table for reference. This table is used by the PCM as a correction factor when controlling idle speed. The adaptive idle table is stored in KAM and retains the learned values even after the engine is shut off. A DTC is output to indicate that the adaptive IAC strategy has reached its limits.
Whenever an IAC component is replaced or cleaned, or a service affecting idle is performed, it is recommended that KAM be cleared. This is necessary so the idle strategy does not use previously learned adaptive values.
Failure Mode Effects Management
Failure Mode Effects Management (FMEM) is an alternative system strategy in the PCM designed to maintain vehicle operation if one or more sensor inputs fail.
When a sensor input is perceived to be out of limits by the PCM, an alternative strategy is initiated. The PCM substitutes a fixed value or may substitute the value of the faulty sensor with that of another sensor. For example, if a fault is detected in the volume air flow sensor, the value from the throttle position sensor may be substituted for continued engine operation. The PCM continues to monitor the incorrect sensor input. If the suspect sensor operates within limits, the PCM returns to the normal engine running strategy.
Engine RPM/Vehicle Speed Limiter
The PCM will disable some or all of the fuel injectors whenever an engine RPM or vehicle overspeed condition is detected. The purpose of the engine RPM or vehicle speed limiter is to prevent damage to the powertrain. In this strategy, the vehicle will exhibit a rough running condition. Once the driver reduces speed, the vehicle will return to the normal operating strategy.
OBD II MONITORS
The OBD II monitors are:
- Heated Oxygen Sensor (HO2S) monitor
- Catalyst efficiency monitor
- Misfire detection monitor
- Fuel system monitor
- Comprehensive component monitor
Heated Oxygen Sensor Monitor
OBD II regulations require monitoring of the upstream heated oxygen sensor to detect if the deterioration of the sensors has exceeded emission thresholds. An additional sensor is located downstream of the warm-up three way catalytic convertor to determine the efficiency of the catalyst. The downstream sensor is a similar type used for fuel control except that it is only used for monitoring the catalytic convertor. They are monitored to determine if a voltage is generated that would indicate a deterioration of the catalyst material. That voltage is compared to a calibrated acceptable range.
Catalyst Efficiency Monitor
The catalyst efficiency monitor is a self test strategy within the PCM that determines when a catalyst has fallen below the minimum level of effectiveness in its ability to control exhaust emissions.
Misfire Detection Monitor
Misfire is defined as the lack of proper combustion in the cylinder due to the absence of spark, poor fuel metering, or poor compression. Any combustion that does not occur within the cylinder(s) at the proper time is also a misfire. The misfire detection monitor detects fuel, ignition, or mechanically induced misfires. the intent is to protect the catalyst from permanent damage and to alert the driver of an emission failure or an inspection maintainence failure by illuminating the MIL. In the event that catalyst damage is likely to occur, the MIL will flash. When a misfire is detected, special software called "Freeze Frame" data is enabled. The Freeze Frame data captures the operational state of the vehicle at the time of the malfunction. Freeze Frame data is useful to the technician because the precise conditions at the time of the first occurrence are known. This can aid in quicker diagnosis of the problem.
Fuel System Monitor
The fuel system monitor is a self test strategy within the PCM that monitors the adaptive fuel table. The fuel control system uses the adaptive fuel table to compensate for normal variability of the fuel system components caused by wear or aging. During normal vehicle operation, if the fuel system appears "biased" lean or rich, the adaptive fuel table will shift the fuel delivery calculations to remove the bias.
Comprehensive Component Monitor
The comprehensive component monitor is a self test strategy within the PCM. It detects malfunctions of any electronic powertrain component or system that provides input to the PCM and that is not exclusively an input to any other OBD II monitor.