Page 348 - Understanding Automotive Electronics

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DIAGNOSTICS 10

Diagnostics

From the earliest days of the commercial sale of the automobile, it has
been obvious that maintenance is required to keep automobiles operating
properly. Of course, automobile dealerships have provided this service for years,
as have independent repair shops and service stations. But until the early 1970s,
a great deal of the routine maintenance and repair was done by car owners
themselves, using inexpensive tools and equipment. However, the Clean Air
Act affected not only the emissions produced by automobiles but also the
complexity of the engine control systems and, as a result, the complexity of
automobile maintenance and repair. Car owners can no longer, as a matter of
course, do their own maintenance and repairs on certain automotive
subsystems (particularly the engine).
The development of The change from traditional fluidic/pneumatic engine controls to
electronic engine control microprocessor-based electronic engine controls was a direct result of the need
has increased the com- to control automobile emissions, and has been chronicled throughout this
plexity of diagnosis and book. However, little has been said thus far about the diagnostic problems
maintenance. involved in electronically controlled engines. This type of diagnostics requires a
fundamentally different approach than that for traditionally controlled engines
because it requires more sophisticated equipment than was required for
diagnostics in pre-emission-control automobiles. In fact, the best diagnostic
methods use special-purpose computers that are themselves microprocessor
based. However, before launching into a discussion of electronic control system
diagnostics, there are two nonmicroprocessor diagnostic instruments that are
still used in garages and repair shops (particularly for older-model cars) that
should be discussed: the timing light and the engine analyzer.
TIMING LIGHT
The timing light (Figure 10.1) is used to measure and set ignition timing.
It is a special stroboscopic light source that generates very short duration light
pulses, the timing of which coincides with ignition pulses. The timing of these
pulses is obtained from a special probe connected to a spark-plug wire. Figure
10.2 is a block diagram of a typical timing light.
The probe generates a very short duration voltage pulse each time the
spark plug fires. The pulse is amplified and then operates a trigger electrode on
a flash lamp that is a gas discharge tube. When triggered, a current pulse flows
through the flash lamp, generating a short burst of light.
In timing the ignition, the light from the flash lamp is directed at the
pulley on the front end of the crankshaft. Adjacent to the pulley is a pointer,
such as seen in Figure 10.3. On the pulley are several marks. The relationship

UNDERSTANDING AUTOMOTIVE ELECTRONICS 335