Savana 1/2 Ton Van V8-305 5.0L VIN M SFI (1998)
Information Bus: Description and Operation
Serial Data Communications
Class II Communications
U.S. Federal regulations require that all automobile manufacturers establish a common communications system. This vehicle utilizes the Class II
communications system. Each bit of information can have one of two lengths: long or short. This allows the vehicle wiring to be reduced by the
transmission and reception of the multiple signals over a single wire. The messages which are carried on Class II data streams are also prioritized. In
other words, if two messages attempt to establish communications on the data line at the same time, only the message with the higher priority will
continue. The device with the lower priority message must wait. The most significant result of this regulation is that the regulation provides the scan
tool manufacturers with the capability of accessing the data from any make or model vehicle sold in the United States.
The Diagnostic Executive
The Diagnostic Executive is a unique segment of the software which is designed to coordinate and prioritize the diagnostic procedures as well as define
the protocol for recording and displaying their results. The main responsibilities of the Diagnostic Executive are:
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Monitoring the Diagnostic Test Enabling Conditions
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Requesting the Malfunction Indicator Light (MIL)
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Illuminating the MIL
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Recording Pending, Current, and History DTCs
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Storing and Erasing Freeze Frame Data
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Monitoring and Recording Test Status information
Test Enabling Conditions
A diagnostic test is a series of steps which has a beginning and an end. The result of which is a pass or fail reported to the Diagnostic Executive. When
a diagnostic test reports a pass result, the Diagnostic Executive records the following data:
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The diagnostic test has completed since the last ignition cycle
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The diagnostic test has passed during the current ignition cycle
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The fault identified by the diagnostic test is not currently active
When a diagnostic test reports a fail result, the Diagnostic Executive records the following data:
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The diagnostic test has completed since the last ignition
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The fault identified by the diagnostic test is currently active
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The fault has been active during this ignition cycle
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The operating conditions at the time of the failure
Trip
The ability for a diagnostic test to run depends largely upon whether or not a Trip has been completed. A Trip for a particular diagnostic is defined as
a key ON and key OFF cycle in which all the enabling criteria for a given diagnostics as been met allowing the diagnostic to run vehicle operation,
followed by an engine OFF period of duration and driving mode such that any particular diagnostic test has had sufficient time to complete at least
once. The requirements for trips vary as they may involve items of an unrelated nature;
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driving style
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length of trip
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ambient temperature, etc.
Some diagnostic tests run only once per trip (e.g. catalyst monitor) while others run continuously (e.g. misfire and fuel system monitors). If the proper
enabling conditions are not met during that ignition cycle, the tests may not be complete or the test may not have run.
Warm-Up Cycle
A Warm-up cycle consists of an engine start-up and vehicle operation such that the coolant temperature has risen greater than 40°F from the start-up
temperature and reached a minimum engine coolant temperature of 160°F. If this condition is not met during the ignition cycle, the diagnostic may not
