410 Section 4 Computer Control Technology
which wires, connections, and components are
in the circuit leading to the potential trouble
source.
• How many components are affected? If several
components are not working, something close
to a common power source or ground point is
at fault. If only one or two sections of the circuit
are faulty, begin your tests at those sections of
the circuit.
• Is the problem always present or is it intermittent
(only occurs under some conditions)? If the prob-
lem is intermittent, the conditions causing the
problem will have to be simulated. For exam-
ple, a loose electrical connection could open
and close with vibration or movement. You
might simulate driving conditions by wiggling
wires and connectors in the circuit to make the
problem occur.
• Is the problem’s occurrence related to heat or
cold? If it occurs only on a hot day or when the
engine is warmed to full operating tempera-
ture, heat is related to the problem’s occurrence.
Electronic circuits (transistors in particular) are
greatly affected by heat. In fact, too much heat
can ruin an electronic component. You can use
a heat gun to simulate the heat in an engine
compartment.
• Is the problem’s occurrence affected by mois-
ture? If the trouble occurs only on wet or humid
days, you have information to use when ana-
lyzing the source of a problem. In most cases,
moisture cannot enter a sealed electronic com-
ponent, but it may enter and affect the wire
connections and any components exposed to
the environment.
Stress testing refers to the use of heat, cold, or
moisture to simulate extreme operating conditions
of components, like spark plug wires, explained in a
later chapter under spark plug wire leakage.
Sensor and Actuator Problems
As with other electrical and electronic compo-
nents, sensors, actuators, and their circuits can
develop opens, shorts, or abnormal resistance or
voltage values. When your pinpoint tests fi nd a
problem, the sensor or actuator should be replaced
or the circuit repaired.
In most cases, a scan tool is used to fi nd the prob-
lem circuit and component. A digital multimeter
is sometimes used to measure the electrical signal
from the component or circuit to verify problems
indicated by the scan tool. Then, this value (voltage,
resistance, or current) is compared to factory speci-
fi cations. If the test value is too high or too low, you
would know that the sensor or actuator is faulty and
must be replaced.
The shop manual will also have a wiring diagram,
or schematic, for the computer system. The diagram
will show the color codes of the wires and the num-
ber of connectors that are used to feed signals from
the sensors to the computer and from the computer
to the actuators. This can be very helpful when ser-
vicing any computer system. The following para-
graphs discuss the most frequent circuit problems.
Poor Electrical Connections
Poor electrical connections are the most common
cause of electrical-related problems in a computer
system. Discussed previously, a wiggle test will help
fi nd poor connections and intermittent problems.
Always check electrical connections when diagnos-
ing sensors and other electronic components. OBD II
scan tools will help you fi nd poor electrical connec-
tions, as shown in Figure 25-2.
Poor electrical connections can be due to corroded
terminals, loose terminal ends, burned terminals,
chafed wires, and other problems. Dirt and moisture
can get into connectors, causing high resistance. Any
of these conditions can prevent a normal sensor sig-
nal from returning to the computer. They can also
prevent the control current from reaching an actuator.
Vacuum Leaks
Vacuum leaks are frequently caused by deterio-
rated, broken, or loose vacuum hoses. Vacuum leaks
often make a hissing sound. Some vacuum leaks can
upset the operation of a computer system and cause a
wide range of symptoms. Also, some engine sensors
and actuators rely on engine vacuum for operation.
Always check for vacuum leaks when they could
be causing a performance problem, Figure 25-3. For
example, if the trouble code indicates a problem with
the MAP (manifold absolute pressure) sensor, check
the vacuum lines leading to the sensor. If there is a
vacuum leak, the sensor cannot function normally.
Also check the intake manifold gasket area, as this is
a common location for vacuum leaks.
Air leaks after a mass airfl ow sensor can also
cause problems. The sensor cannot measure the
actual amount of air being drawn into the engine,
and an incorrect air-fuel mixture will result.