On-Board Diagnostics (OBD) Monitors
ON-BOARD DIAGNOSTICS (OBD) MONITORSOBD Overview
The OBD system is used on vehicles under 6,350 kg (14,000 lb) gross vehicle weight rating (GVWR). The engine manufacturer diagnostics (EMD) system is used on vehicles over 6,350 kg (14,000 lb) GVWR.
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 concern 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 concern.
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 concerns and initiate DTC setting and MIL activation. Concern detection strategy and MIL operation are associated with drive cycles. The MIL is turned on after 2 consecutive drive cycles with concerns. The DTC is cleared after 40 engine warm-up cycles without the concern being detected once the MIL is turned off. Once a monitor turns on the MIL, it requires 3 consecutive drive cycles without a concern for the MIL to turn off. The operation of each of the OBD monitors is discussed in detail.
The OBD computer program in the 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 scan tool interface.
Freeze frame data describes stored engine conditions such as the state of the engine RPM and load at the point the first concern is detected. This data is accessible with the scan tool to assist in repairing the vehicle.
The OBD system is comprised of the comprehensive component monitor (CCM), the glow plug monitor, the misfire detection monitor, the exhaust gas recirculation (EGR) monitor, the EGR cooler monitor, the diesel particulate filter monitor and the thermostat monitor.
Starting with the 2007 model year, vehicles not required to comply with current OBD requirements use an EMD system. EMD systems are used on all California and Federal gas and diesel on-road heavy duty engines used in vehicles over 6,350 kg (14,000 lb) GVWR. EMD systems are required to functionally monitor the fuel delivery system, EGR system, diesel particulate filter, as well as emission-related PCM inputs for circuit continuity and rationality, and emission-related outputs for circuit continuity and functionality, EMD requirements are very similar to previous OBD system requirements. As such, previous OBD system principles are employed, with the addition of some CCM rationality and functionality checks. EMD vehicles use the same PCM, controller area network (CAN), data link connector (DLC) and PCM software as the corresponding OBD vehicles.
The following list indicates what monitors and functions have been altered from OBD-II for diesel engine EMD calibrations:
