476 Auto Electricity and Electronics
You may also need to check the circuit feeding current
to the glow plugs. The glow plug controller, for example,
might not be feeding current to the glow plugs.
An ammeter can also be used to measure current fl ow
into the glow plugs. This is an easy way to check their
operation because you can simply clip the inductive pickup
over the wires. Nothing has to be disconnected.
Servicing Other Actuators
The service of other actuators is similar to those just
discussed. A solenoid for a door latch is constructed like a
solenoid in a fuel injector or other solenoid-type actuator.
Servo motor actuators are also similar to other motors, as
in fuel pumps. Remember to refer to the service manual for
testing and replacement details.
Computer Service
Computer service usually involves a few tests and
computer replacement, if needed. The computer is usually
one of the last components to be tested and suspected.
Only after all other potential sources of trouble have been
eliminated is the computer suspected of being the problem
source. It is sometimes possible for an integrated circuit,
transistor, or other electronic part in the computer to fail
and upset system operation, Figure 24-30.
If your scan tool does not work and cannot read
vehicle electrical values, suspect a problem with the on-
board computer. When the vehicle computer goes bad, it
will often not send data to the scan tool.
Measuring Computer Output
Refer to the vehicle’s service manual to get an illustra-
tion of the computer wiring. This will let you probe the
diagnostic connector to check for proper signals and volt-
ages at the connector and coming out of the computer.
A computer output can also be a reference voltage
to a sensor, current fl ow or supply voltage to operate an
actuator, or dwell signal to open and close a mixture con-
trol solenoid or injector.
Use a voltmeter to make sure the correct reference
voltage is being sent to a sensor. Most computers produce
a reference voltage of about 5 volts. If not correct, check
the wiring before condemning the computer. You can also
measure voltage to make sure the correct voltage output is
being fed to operate actuators.
An oscilloscope can also be used to check the
computer output signal. In addition to checking refer-
ence voltage, you can measure and observe pulses going
to the fuel injectors, solenoids, and servo motors. See
Figure 24-31.
Again, always refer to the service manual for exact pro-
cedures for testing a computer system. One wrong electri-
cal connection can “fry” delicate electronic components.
Figure 24-30. Computers are quite dependable. However, they
can sometimes fail to operate properly because of problems with
integrated circuits, transistors, and other electronic components.
Make sure other problems, such as radiation interference, sen-
sors, and wiring, are not the trouble before replacing a computer.
If your scan tool will not turn on or read vehicle operating values,
suspect a bad on-board vehicle computer.
Figure 24-31. A—An oscilloscope will also check the reference volt-
age going to the sensors and the control pulses from the ECM to
the actuators. Compare readings and waveforms to service manual
specifi cations. B—Reference voltages should meet specifi cations
and the waveforms should refl ect smooth DC voltage. (Fluke)
A
B
Computer
or ECM
Probe to
Service Manual
Recommended
Wire
Scope