Page 353 - Understanding Automotive Electronics

2735 | CH 10 Page 340 Tuesday, March 10, 1998 1:27 PM

10 DIAGNOSTICS

The on-board diagnostic capabilities can be manually activated by causing
the engine controller to enter a diagnostic mode. The method for
accomplishing this task varies from car to car. For example, in certain older-
model fuel-injected Cadillac cars, the diagnostic mode is entered by first
switching on the ignition, then simultaneously depressing the Off and Warmer
buttons on the climate control system.
A two-digit fault code is The use of on-board diagnostics can easily be illustrated by outlining
assigned to each failure. typical procedures and using the Cadillac system as an example system.
Although display of the fault codes varies from car to car, in each case the
codes must be readable as a numeric code. A typical method involves flashing
the “Check Engine’’ light. The mechanic enters the display mode and then
counts the number of times that this light is flashed, in two-digit groups. For
example, a fault code 24 would be given by 2 flashes then 4 flashes. After a
short time interval the next code would similarly be flashed in two-digit
sequence. At one time Cadillac used the environment controller to display a
two-digit code. Figure 10.4 is a drawing of a Cadillac environmental system
displaying code 88. This particular code is used to check that all display
segments are working correctly and is the first display shown when the
diagnostic mode is entered.
After verifying that all display segments are working, the fault codes for all
component failures are displayed in sequence, beginning with the lowest and
proceeding to the highest. The mechanic notes the fault codes that are
displayed and, using a reference manual, identifies the failed components. Table
10.1 is a summary of the Cadillac fault codes, which we present here simply as

Figure 10.4
Fault Code Display

FPO

340 UNDERSTANDING AUTOMOTIVE ELECTRONICS