Description of On-Board Diagnostics
OBD-II OVERVIEW1. OVERVIEW
The California Air Resources Board (CARB) began 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 Recirculation (EGR) system and additional emission related components. The Malfunction Indicator Lamp (MIL) was required to light and alert the driver of the fault and the need for repair of the emission control system. 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 CARB to improve air quality by identifying vehicle exceeding emission standards. Passage of the Federal Clean Air Act Amendments 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 be used.
The OBD-II system meets government regulations by monitoring the emission control system. When a system or component exceeds emission threshold or a component operates outside tolerance, a DTC will be stored and the MIL illuminated.
The diagnostic executive is computer program in the Engine Control Module (ECM) 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 status at the point the first fault is detected. Previously stored conditions will be replace only if a fuel or misfire fault is detected. This data is accessible with the scan tool to assist in repairing the vehicle.
The center of the OBD-II system is a microprocessor called the Engine Control Module (ECM) which has two connector (MT:81 pin,AT:121 pin).
The ECM receives input from sensors and other electronic components (switches, relays, and others) based on information received and programmed into its memory (keep alive random access memory, and others), the ECM generates output signals to control various relays, solenoids and actuators.
2. CONFIGURATION OF HARDWARE AND RELATED TERMS
1. GST (GENERIC SCAN TOOL)
2. MIL (MALFUNCTION INDICATION LAMP) MIL ACTIVITY BY TRANSISTOR
The Malfunction Indicator Lamp (MIL) is connected between ECM-terminal Malfunction Indicator Lamp and battery supply (open collector amplifier).
In most of car, the MIL will be installed in the instrument panel The lamp amplifier can not be damaged by a short circuit.
Lamps with a power dissipation much greater than total dissipation of the MIL and lamp in the tester may cause a fault indication.
At ignition ON and engine revolution (RPM) < MIN RPM, the MIL is switched ON for an optical check by the driver.
3. MIL ILLUMINATION
When the ECM detects a malfunction related emission during the first driving cycle, the DTC and engine data are stored in the freeze frame memory. The MIL is illuminated only when the ECM detects the same malfunction related the DTC in two consecutive driving cycles.
4. MIL ELIMINATION
Misfire and Fuel System Malfunctions:
For misfire or fuel system malfunctions, the MIL may be eliminated if the same fault does not reoccur during monitoring in three subsequent sequential driving cycles in which conditions are similar to those under which the malfunction was first detected.
All Other Malfunctions:
For all other faults, the MIL may be extinguished after three subsequent sequential driving cycles during which the monitoring system responsible for illuminating the MIL functions without detecting the malfunction and if no other malfunction has been identified that would independently illuminate the MIL according to the requirements outlined above.