Page 352 - Understanding Automotive Electronics

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DIAGNOSTICS 10

CAN is an asynchronous system in that each node synchronizes to others’
messages on the first bit leading edge of a message and on subsequent bit
leading edges throughout the remainder of the message. The ability of any node
to synchronize to another node is determined by the maximum differences in
oscillator frequencies (see Chapter 4 for an explanation of microprocessor
clocks). Other issues and critical periods include the duration of a bit, message
duration and composition, and cooperative use of the network between master
and slave, known as handshaking.
It is beyond the scope of this book to give a detailed account of CAN-type
data communication. Rather, we wish to identify the use of data
communication in automotive electronics and point out some critical issues.
One important application for digital communication in automobiles is
the serial digital data link from the power train controller to an off-board
diagnostic system. This data communications link has the capability of
transmitting fault codes that have been stored in memory to the external device
(among other uses). This off-board system can range in complexity from a
computer-based diagnostic workstation (explained later in this chapter) to a
simple portable scanner used to simply read and display fault codes in sequence
as they are retrieved over the data link from the power train controller. Using
these fault codes and following a fault tree diagnosis procedure, the service
technician can normally diagnose power train control system problems. We
consider next on-board and off-board diagnosis in detail, using specific
examples.

On-board Diagnostics
Existing microprocessor- Limited diagnostic capability is provided in any modern microprocessor-
based engine control sys- based electronic control system. These diagnostic functions are performed by
tems incorporate some the microprocessor under the control of stored programs, and are performed
self-diagnosis. only when the microprocessor is not fully committed to performing normal
control calculations. While it is beyond the scope of this book to review the
actual software involved in such diagnostic operations, the diagnostic
procedures that are followed and explanations of on-board diagnostic functions
can be reviewed.
During the normal operation of the car, there are intermittent periods
during which various electrical and electronic components are tested.
Whenever a fault is detected, the data is stored in memory using a specific fault
code. At the same time, the controller generates or activates a warning lamp (or
similar display) on the instrument panel, indicating that service is required.
The on-board diagnostic functions have one major limitation—they
cannot detect intermittent failures reliably. For the system to detect and isolate
a failure, the failure must be nonreversible. In most on-board diagnostics, the
electronic control module stores trouble codes that are automatically cleared by
the microprocessor after a set number of engine cycles have occurred without a
fault reappearing.

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