Chapter
B
: Ignition System Diagnosis and Repair
50
Internal PCM functions
Battery, or system, voltage is supplied to the PCM at
two points, pins 12 and 13. To understand the need for
two power sources, you must be familiar with the basic
functions of a computer system, figure 2-29.
All automotive computers perform four functions:
1.
Input
- Information on engine operating conditions
provided to the computer, or PCM, as voltage sig-
nals from system sensors.
2.
Processing
- The internal function that converts
input voltage signals into combinations of binary
numbers (1's and 0's). The PCM then compares
the binary information to instructions and other
programmed data to make logical decisions and
send output commands to control other devices in
the system.
3.
Storage
- All computers have some kind of infor-
mation storage capacity. This electronic memory
stores the computer operating instructions, or pro-
gram. In addition, some computer systems also
store certain input signals for later reference. These
computers "remember" previous operating condi-
tions and adapt output commands to the "learned"
characteristics of vehicle operation. This feature,
known as adaptive strategy, allows the system to
compensate for wear, climatic conditions, driving
style, and other variables.
4.
Output
- After receiving an input signal, processing
it, and comparing it to information stored in mem-
ory, the computer transmits a command to output
devices that control engine operation. Output de-
vices, such as solenoids and relays, are actuators
that convert electrical signals to mechanical mo-
tion to obtain a predetermined response and regu-
late engine operation. The computer may also send
output information to display devices such as elec-
tronic instrument panels.
On the composite vehicle, pin 13 provides the power
supply for the PCM storage circuits. Internal memory
circuits can be further broken down into two categories:
volatile and non-volatile. Volatile memory is temporary
and requires a constant power supply to retain stored
information, information stored here is lost if the battery
is disconnected. Information that changes during vehi-
cle operation, such as adaptive strategies and diagnos-
Fig. 2-28.
Sample vehicle electrical symbols.
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