Chapter 7 Computer Control Fundamentals 137
system enters closed loop, provided there are no problems
with any sensor, output device, the engine, or the ECM.
During closed loop, the ECM monitors all sensors for
changes and adjust systems to maintain operation within
set parameters. Of particular importance is the oxygen sen-
sor. The ECM adjusts air-fuel ratio based on oxygen sensor
input.
Data Link Connector
To access the ECM memory, the scan tool’s data link
connector is plugged into the ECM diagnostic connector,
Figure 7-36. Most scan tools display the trouble codes as
numbers and save them for easy recall. This makes it easier
to perform testing based on one or more trouble codes. Scan
tools are able to perform other diagnostic tasks by send-
ing a series of commands to the ECM and output devices
and monitoring the resulting system operation. Many scan
tools can read voltage levels to and from input sensors to
determine whether the sensor is operating properly. Some
scan tools have a freeze frame mode, which can take a
“snapshot” of sensor and output device readings when a
malfunction occurs.
Note: Older systems have provisions for
retrieving trouble codes without a scan tool,
either by flashing a light in the vehicle dashboard or
on the ECM or by producing pulses that can be read
on a multimeter.
Figure 7-32. Schematic of the electronic throttle control system. The pedal and throttle position sensors provide input to the throttle
servo motor. The motor opens and closes the throttle plate as necessary.
ECM
Pedal position
sensor
Accelerator pedal
Throttle position
sensor
Servo
motor
Throttle plate
Figure 7-33. An electronic throttle has a small electric motor
that moves the throttle plate through a set of reduction gears.
(University of Toyota/Toyota Motor Sales USA)
Figure 7-34. The system that regulates oil pressure operates
in a loop similar to the computer control system. Note that each
function is a step in the loop leading back to the beginning.
Oil pressure
too high
Pressure-
relief
valve closes
Pressure-
relief
valve opens
Oil pressure
drops
Figure 7-35. Computer control loop. This is the order of operation
during closed loop. Compare this to the one shown in Figure 7-1.
Sensors monitor
engine
conditions
ECM reads
sensor inputs
and issues
commands
Engine
operates
Output devices
adjust engine
operating
parameters