Overview
The California Air Resources Board (CARB) began regulation of OBD for diesel vehicles sold in California beginning with the 1997 model year. OBD requires monitoring of emission-related components. The malfunction indicator lamp (MIL) is required to light and alert the driver of a malfunction and the need for repair of the emission control system. A diagnostic trouble code (DTC) is associated with the MIL identifying the specific area of the fault.
Note. The OBD system is used on vehicles under 14,000 GVW.
The OBD system meets government regulations by monitoring the emission control system. When a system or component exceeds emission thresholds or a component operates outside of tolerance, a DTC is stored and the MIL is illuminated.
The OBD monitors detect system faults and initiate DTC setting and MIL activation. Fault detection strategy and MIL operation are associated with drive cycles. An OBD pending DTC is stored in the PCM keep alive memory (KAM) when a fault is first detected. The MIL is turned on after 2 consecutive drive cycles with faults. The DTC is cleared after 40 engine warm-up cycles without the fault being detected once the MIL is turned off. Once a monitor turns on the MIL, it requires 3 consecutive drive cycles without a fault for the MIL to turn off. The operation of each of the OBD monitors is discussed in detail within this article.
The OBD computer program in the electronic powertrain control module (PCM) coordinates the OBD self-monitoring system. This program controls all the monitors and interactions, DTC and MIL operation, freeze frame data, and diagnostic tool interface.
Freeze frame data describes stored engine conditions such as the state of the engine RPM and load at the point the first fault is detected. This data is accessible with the diagnostic tool to assist in repairing the vehicle.
OBD inspection maintenance (IM) readiness DTC P1000 indicates that not all of the OBD monitors have been completed since the PCM KAM was last cleared. In some states, it may not be possible to obtain vehicle registration if P1000 is detected during inspection. To erase DTC P1000 from the PCM, operate the vehicle until the DTC is cleared using the manufacturer's specified drive cycle.
The OBD system is comprised of the comprehensive component monitor, the glow plug monitor, the misfire detection monitor, and the exhaust gas recirculation (EGR) monitor.
The powertrain control module (PCM) contains both engine and transmission microprocessors. Operating information, as well as fault information, is communicated between the 2 processors through controller area network (CAN) communications. Both can be programmed individually. However, the PCM is replaced as an assembly.
Fuel control is accomplished by the PCM and the fuel injection control module (FICM). The 2 modules communicate operational information through private J1939 CAN communications.
The EEC system provides optimum control of the engine and transmission through the enhanced capability of the PCM. The EEC system also has an on-board diagnostics (OBD) monitoring system with features and functions to meet federal regulations on exhaust emissions.
The EEC system has 2 major divisions: hardware and software. The hardware includes the PCM, FICM, sensors, switches, actuators, solenoids, and interconnecting terminals. The software in the PCM provides the strategy control for outputs (engine and transmission hardware) based on the values of the inputs to the PCM. The software in the FICM provides the strategy control for the fuel injectors based on the values of the outputs from the PCM. The EEC hardware and software are discussed in this article.
The PCM receives information from a variety of sensor and switch inputs. Based on the strategy and calibration stored within the memory chip, the PCM generates the appropriate output. The system is designed to minimize emissions and optimize fuel economy and driveability. The software strategy controls the basic operation of the engine and transmission, provides the OBD strategy, controls the malfunction indicator lamp (MIL), communicates to the diagnostic tool through the data link connector (DLC), allows for flash electrically erasable programmable read only memory (EEPROM), and controls failure mode effects management (FMEM).