416 Section 4 Computer Control Technology
An integrated junction block encloses most or all
of the vehicle’s mechanical relays in a single housing.
Some of the relays found in this block are the fuel pump
relay, cooling fan relay, wide open throttle relay, air con-
ditioning relay, and a host of others. The junction block
is usually mounted in the engine compartment.
When testing a relay, refer to the service manual
wiring diagrams for pin numbers and wire color
codes. Special relay testers can be used to quickly
test relay operation. They plug into the relay and
test the unit automatically.
For information on actuator service, refer to the
index in this textbook. You can fi nd instructions on
testing fuel injectors, glow plugs, and more special-
ized actuators.
Replacing Actuators
Actuator replacement will vary with the type and
location of the unit. Here are a few rules to follow
when replacing an actuator.
• Do not damage the wire connectors when
releasing them. They are made of plastic and
will break if forced open improperly.
• Do not drop the actuator mounting screws dur-
ing removal or installation. If a screws falls into
an engine, major problems can occur.
• Make sure you have the correct replacement
actuator before attempting installation. Two
actuators may look the same, but they will
often have different internal construction.
• Check that the actuator is fully seated before
tightening the mounting screws.
• Double check actuator operation after replacement.
Computer Service
Computer service involves replacing or repro-
gramming the computer. The computer is the last
component to be suspected of being the prob-
lem source, only after all other potential sources of
trouble have been eliminated. It is sometimes possible
for an integrated circuit, transistor, or other electronic
part in the computer to fail and upset system operation.
Measuring Computer Output
If the computer is not tested, a defective computer
is often identifi ed through the process of eliminating
the sensors, actuators, and related wiring as the cause
of the problem. In the process of sensor and actuator
testing, it was necessary for you to measure the com-
puter’s output. A computer output can be a reference
voltage to a sensor or a supply voltage to an actuator.
Measure voltage
to relay
Voltage
source
Feed wires
disconnected
Servo motor
actuator should
operate
Reverse test leads
to check motor
reversing action
Solenoid actuator
should energize
Jumper
wires
Check plunger
movement
Power
in wires
Test light
should
glow
to check
power
from relay
Wires
to load
Relay type
actuator
Volts
A
Voltage
source
B
C
Test
leads
Wires to solenoid
disconnected
Figure 25-9. Actuator testing is straightforward. It is similar
to testing a conventional motor, relay, or solenoid. A—Voltage
can be jumped to a servo motor. The motor should function
when energized by the power source. B—A solenoid can also
be tested in the same manner as a servo motor. C—A relay is
slightly more complex to test. You must make sure there is an
output when voltage is supplied to its input terminals. This can
be done using a voltmeter or a high-impedance test light.