460 Auto Electricity and Electronics
In previous chapters, you learned how sensors, actua-
tors, computers, and other electronic components operate.
You also learned how to use computer self-diagnosis and a
scan tool to help fi nd electric/electronic problems.
This chapter concentrates on how to do specifi c tests
on computer system components. Most sensors and actua-
tors can be checked with a high-impedance digital VOM.
The wiring leading from the computer to these devices can
also be checked for opens, shorts, and a reference voltage,
if applicable.
The information in this chapter will help you develop
the skills needed to verify where problems are after reading
trouble codes and using scanners or analyzers. Remember
that trouble codes only indicate the area of a trouble, not
what part is at fault. It is, therefore, imperative that you
know how to do pinpoint tests on individual components.
Preliminary Inspection
A preliminary inspection involves looking for signs
of obvious trouble, such as loose wires, leaking vacuum
hoses, and part damage. For example, if the trouble code
says there is something wrong in the engine coolant tem-
perature sensor circuit, you could check the sensor resis-
tance and the wiring going to that sensor.
When there is a malfunction in a system, always
remember the cause is usually something basic. It is easy
for the untrained technician to instantly think “computer
problems” when an engine misses, runs rough, fails to start
properly, or exhibits some other performance problem.
Cipet Principle
The abbreviation cipet refers to an easy way to remem-
ber a logical thought process for fi nding performance prob-
lems. Cipet stands for:
❑
Compression. (Do any of the cylinders have low
compression?)
❑
Ignition. (Is the ignition system operating properly?)
❑
Power. (Is each cylinder producing equal and suffi -
cient power?)
❑
Exhaust. (What is the chemical content of the engine
exhaust?)
❑
Timing. (Is the timing set, and is it advancing properly?)
If you use conventional methods to answer these
questions, you will fi nd the source of most engine per-
formance problems. Studies have shown that 80% of all
engine performance problems are caused by something
other than the computer system electronics. Only 20% of
all performance problems are due to the computer, sen-
sors, or actuators themselves. Therefore, before suspecting
these components, you must fi rst fi nd out if the engine is
in good mechanical condition, if the ignition is fi ring the
plugs normally, if timing is correct, and if the fuel system is
up to specs.
For example, even extremely dirty engine oil can trig-
ger a computer trouble code. Contaminated oil fumes can
be drawn into the engine intake manifold from the crank-
case. The PCV system is designed to remove these fumes
from the lower engine area and burn them in the engine. If
these fumes are excessively strong, the oxygen sensor could
be fooled into signaling a rich fuel mixture. The computer
would then lean the fuel mixture to compensate for the
crankcase fumes. An oxygen sensor trouble code could be
produced and engine performance problems could result.
As this points out, it is critical that you check or
troubleshoot conventional or basic problems fi rst. Start
checking for computer or electronic problems only after all
basic system troubles have been eliminated.
Kiss Principle
“Kiss” is another abbreviation that could help you
fi nd the source of performance problems on a computer-
controlled car. Kiss stands for “keep it simple stupid.” This
means that you should start your troubleshooting with the
simple checks and tests. Then, as the simpler, more common
problems are eliminated, you can move to more complex
tests of sensors and actuators. The kiss principle will help
you become a more competent auto electronic technician.
Scan Tool Information
As you learned from the previous chapter, your scan
tool will help you fi nd the component and circuit causing
system problems. A scan tool will not always tell you which
part is at fault, but it will point you in the right direction for
doing further tests to verify the source of the problem.
For example, look at Figure 24-1. By retrieving stored
trouble codes, the scan tool will tell you if the vehicle has
stored, pending (not fully out of spec), or no trouble codes.
In an OBD II vehicle, the scan tool will also tell you if the
engine is misfi ring and, if so, to check the operation of the
fuel system, catalytic converters, heated catalyst, evapora-
tive emission control system, air conditioning system, O2
sensors, exhaust gas recirculation, and other systems.
Electrical Component
Damage Precautions
Semiconductor devices, such as transistors and inte-
grated circuits, are very easy to damage. They can be dam-
aged by static electricity, voltage spikes, heat, and impact
shocks. Here are some things to remember when working
with semiconductor devices and their wiring:
❑
Arc welding can damage the on-board computers. If
welding on the vehicle is necessary, remove all of the
on-board computers. If this is not possible or too time-
consuming, unplug their connectors and make sure
the welder’s lead is securely grounded.