General Description Serial Data Communication Network

CLASS II
The Class II link is used for communication between all module. This link is compliant with SAE J1850 specifications for serial data diagnostics in order to meet the OBD II legislation.

Class II link is used for normal vehicle operation by transferring information between modules. For example, the PCM uses the crank sensor input as an indication of engine rpm. The PCM will generate a message on the Class II link, which can be read by any other controller. In order to display rpm on the tachometer, the I/P cluster reads the serial data message and commands the tachometer to indicate actual engine rpm. The block diagram shown the block diagram on this page illustrates the inputs that are connected to each controller. The information from these inputs are transmitted out on the Class II link to be used by the other control modules.

There is also information transmitted through serial data that is based on internal calculations such as certain inputs being in a given state, or on-board diagnostics. An example of this type of message is the command to turn on the Service Engine Soon (SES) telltale. When the PCM's on-board diagnostics detects a fault with an emission related component, a message is sent to the I/P cluster to illuminate the SES (MIL) lamp.