Principles of Operation

PRINCIPLES OF OPERATION

NOTE: The smart junction box (SJB) is also known as the generic electronic module (GEM).

The message center is an integral part of the instrument cluster that receives and acts upon much of the same information that is input and used to operate the instrument cluster gauges, informational indicators, and warning indicators. The message center, located in the center of the instrument cluster is a vacuum fluorescent 2-line display. The message center electronic functions use both hardwired, and the controller area network (CAN) circuitry to transmit and receive information.

Whenever conditions are present that require a warning message, the message center replaces the last selected display with the new warning display. Once the message is reset or cleared, the message center returns to the last selected display. If multiple warning messages are present, the message center displays each warning for approximately 4 seconds. Warning messages are also generally associated with other observable instrument cluster indications. For example, when the LH front door is opened, the message center displays the message DRIVER DOOR AJAR along with the door ajar warning indicator. This allows the message center to be a more informative supplement to the instrument cluster gauges and indicators.

It is very important to understand:
- where the input (command) originates.
- all the information (messages) necessary in order for a feature to operate.
- which module(s) receive(s) the input or command message.
- if the module that received the input (message) controls the output of the feature, or if it outputs a message over the CAN to another module.
- which module controls the output of the feature.

Instrument Cluster Network Messages

NOTE: Whenever a network message is suspected as missing, confirmed by a missing message DTC, it is important to look for other symptoms that may also be present in the instrument cluster and throughout the vehicle. Once a DTC is set in the instrument cluster, it may be helpful to review the complete message list available in Information Bus (Module Communications Network) to see what other modules also rely on the same message and run the self-test for those modules. If the message is missing from other modules, the same DTC may also be set in those modules. Confirmation of missing messages common to multiple modules may indicate that the originating module is the source of the concern or the communication network may be experiencing some problems.

The instrument cluster uses input messages from other modules to control the gauges, informational indicators, warning indicators and message center displays over the communication networks. If a required message is missing or invalid for less than 5 seconds, the gauge or indicator that requires the message remains at the last commanded state based upon the last known good message. For example, if the message is missing the driver door ajar off status for less than 5 seconds and the door ajar indicator was ON, the indicator remains in the ON state until the next good message is received. If the message remains missing or invalid for greater than 5 seconds, the instrument cluster sets a U-code DTC and the output becomes a default action for the indicator or gauge. Each indicator or gauge utilizes a different default strategy depending on the nature of the indication. Refer to Instrument Cluster for further descriptions of the default action specific to each indicator or gauge. If the messaged input to the cluster returns at any time, the normal function of the gauge or indicator resumes.