Chapter 25 Computer System Service 415
• Use thread sealant sparingly. If the sensor seals
a coolant, oil, or vacuum passage, do not use
too much sealant.
• Use thread and engine sealants that are safe for
oxygen sensors. Some sealants can poison the
oxygen sensors.
• Tighten the sensor properly. Overtightening a
sensor could damage it. Undertightening could
cause leaks.
• Adjust the sensor, if needed. Some throttle posi-
tion sensors require adjustment after installation.
• Scan for trouble codes after sensor replacement.
Actuator Service
Actuator service involves testing the actuators
for possible electrical or mechanical problems and
replacing them, if necessary. If an actuator fails, the
computer cannot control the engine and vehicle sys-
tems properly.
Testing Actuators
Since actuators are simply relays, solenoids, and
motors, they are fairly easy to test. Figure 25-9 shows
several ways to test actuators.
Testing Servo Motors
In Figure 25-9A, a voltage source has been con-
nected to a servo motor. The wiring harness to the
motor has been disconnected. Jumper wires feed
current directly to the motor. This is a simple way to
check the operation of an electric or servo motor. If
the motor begins to function with an external volt-
age source applied, you should test the wire harness
leading to the motor.
Testing Solenoids
In Figure 25-9B, a voltage source is being used to
check a solenoid. When jumper wires are connected
to the vehicle’s battery and the solenoid, the sole-
noid should operate. If the solenoid tests good, you
should check the voltage coming to the solenoid
through its harness.
An ohmmeter can also be used to test a solenoid.
You can use the meter to measure the internal resis-
tance of the unit. By comparing resistance readings
to specifi cations in the service manual, you can fi nd
out if the actuator must be replaced.
Testing Relays
Figure 25-9C shows how to test a relay. Check the
voltage entering the relay and the voltage leaving
the relay. It is possible that voltage is applied to the
relay but the relay points are not sending voltage
out to the controlled device.
Since relays contain movable contact points, they
are a common source of computer system problems.
The scan tool may indicate a problem with the cir-
cuit containing the relay. However, you must test the
relay to pinpoint the problem source.
Relays can be located almost anywhere on a
vehicle: in the engine compartment, under the dash,
under the seat, or in the trunk. The service manual
will give exact locations. See Figure 25-10.
MAP
sensor
Jumper
wires
Harness
connector
To
vacuum
pump
22°C 45°C
Figure 25-7. Small jumper wires are often used when
measuring sensor circuit values. Make sure you do not short
the jumper wires together or to ground. This test is used to
measure the frequency signal in hertz from a manifold absolute
pressure sensor. (Fluke)
Figure 25-8. Always make sure the replacement sensor is
same as the old one. These two temperature sensors look the
same but have different temperature and resistance values.
If the wrong one is installed, the vehicle may not function
correctly. (Snap-on Tool Corp.)