OBD II Function

OBD-II Function

OBD-I systems verify the normal operation of sensors and actuators by measuring voltage drop at the component. This technique for confirmation of operation is known as a component monitor. This method can be used to determine short circuits to positive, short circuits to Ground, and open circuits.

OBD-II systems monitor inputs and outputs (sensors and actuators) in the same manner as OBD-I systems, but add comprehensive new processes that include:
^ Plausibility of signals and components of emissions-related functions
^ Monitors on functions not completely monitored previously
^ Monitors on systems not monitored previously

Monitors, as used in this context, can take either of two forms:
^ Component monitors: the ECM looks at the operation of individual parts of the system.
^ System monitors: the ECM operates a component (or multiple components) to verify system operation.

Legislation mandating OBD-II systems also requires that the vehicle manufacturer design the diagnostic system in a manner that permits retrieval of OBD data in a standard format using any available generic scan tool. OBD-II data can be retrieved through one of three data transfer protocols:
^ ISO 9141 CARB
^ SAE J185O VPW
^ SAE J183O PWM












Scan tool access to the ECM on Volkswagen vehicles is through ISO 9141 CARB. A generic scan tool can be plugged into the Data Link Connector (DLC), and the required communication initiated with this protocol using the scan tool manufacturer's instructions. The VAG 1551/1552 and VAS 5051 can also operate as a generic scan tool using address word 33.

The OBD-II enabling legislation also allows vehicle manufacturers to supply additional data and functions above and beyond the required data. This is accomplished by means of a proprietary transfer mode which is part of ISO 9141.