Principles of Operation: Notes

WARNING: This page is about a different car, the 2008 Mercury Sable, 2008 Ford Taurus X, and 2008 Ford Taurus. However, it is still accessible from the selected car via links, so may be relevant.

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

NOTE: Whenever a network message is suspected as missing and confirmed by a missing message DTC (U-code), it is important to look for other symptoms that may also be present in the instrument cluster (IC) and throughout the vehicle. Once a DTC is set in the instrument cluster (IC), it may be helpful to review the complete message list available in MODULE COMMUNICATIONS NETWORK article 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 (IC) utilizes a microprocessor to control the gauge, indicator, message center and warning chime functions. Data is sent to the instrument cluster (IC) over the medium speed controller area network (MS-CAN) and the high speed controller area network (HS-CAN) bus lines and through hardwired circuitry from individual components. The instrument cluster (IC) uses each input to output an action to the gauges or indicators. 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 brake status message is missing for less than 5 seconds and the brake warning 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 (IC) 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 the normal operation descriptions located before each individual pinpoint test for further description 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.

It is very important to understand:

where the input originates.
all the information necessary in order for a feature to operate.
which module(s) receive(s) the input or command message.
does the module which received the input control the output of the feature, or does it output a message over the communication network to another module.
which module controls the output of the feature.