EFI Engine Management : EFI sensors II >OBD and EOBD

OBD and EOBD

On-Board Diagnostics, or OBD, in an automotive context, is a generic term referring to a vehicle's self-diagnostic capability. If the vehicle's onboard diagnostic system detects a malfunction, a DTC (diagnostic trouble code) corresponding to the malfunction is stored in the vehicle's computer, and in certain cases will illuminate the MIL (malfunction indicator light, or check engine light). A service technician can retrieve the DTC, using a "scan tool", and take appropriate action to resolve the malfunction.
Prior to the advent of digital powertrain control modules which enabled the OBD feature, repairing a vehicle relied solely upon the technician's skill and service literature from the auto manufacturer.

EFI Engine Management : EFI sensors I

• Mass airflow sensor
• Manifold absolute pressure sensor
• Air vortex sensor
• Fuel system sensor
• Temperature sensor
• Throttle position sensor
• Exhaust gas oxygen sensor
• Crank angle sensor
•  Hall effect voltage sensor

Mass airflow sensor

The mass type airflow sensor detects the mass of air flowing into the intake manifold. By measuring the mass of the air, it prevents changes in air density affecting the air-fuel mixture.
The airflow meter has an electrically-heated wire, mounted in the air stream.
A control circuit is linked to the wire, and current is supplied to the wire to keep its temperature constant. The higher the airflow, the more the temperature of the wire falls - and the higher the current needed in the wire to keep its temperature constant. So how this current varies is a measure of what is happening to the air flow.
Current flow variation is then read as an output voltage, and converted by the ECU to an intake air signal. This determines the basic fuel quantity needed for injector pulse duration. The airflow meter can have a self-cleaning function that burns dust and other contaminants from the hot wire. This is done by the control unit heating the wire to 1000°Celsius or 1,832°Fahrenheit for approximately 1 second. This happens 5 to 10 seconds after the ignition is switched off. This function operates only when certain conditions have been met. For this vehicle, the engine must have reached operating temperature, and the vehicle must have been travelling above 10 kilometres an hour or above 6.2 miles per hour.
In some types of sensors, the hot wire is mounted in a sub passage connected to the main passage. This allows maximum airflow through the main passage. The hot wire may be sealed in a glass envelope. This protects the wire, and eliminates the need for burn-off.
In others, the heating element is a ceramic plate.