Page 350 - Understanding Automotive Electronics

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DIAGNOSTICS 10

Figure 10.3
Timing Marks

FPO

between the pointer and these marks corresponds to specific crankshaft angular
positions relative to top dead center (TDC). The relationship of the pointer
and crankshaft pulley marks at the time of ignition can be seen by viewing the
pulley using the light from the flash lamp. When the ignition timing is correct,
the pointer will align with the correct pulley mark.

ENGINE ANALYZER
The timing light and The engine analyzer is an instrument that has existed for many years and
traditional engine ana- continues to be used in garages for several tuneup tasks. It provides, for
lyzer are not enough for example, a means for optimally setting the gap for ignition points (in cars that
diagnosis of engine still have them, of course) by measuring the so-called dwell. Dwell is essentially
problems. the fractional duration that the ignition points are closed. However, an engine
analyzer’s role in diagnosing cars that have digital engine control systems is
markedly different than its role in the diagnosis of cars that have analog or
mechanical control systems. For example, ignition points have virtually been
eliminated in digitally controlled cars. Nevertheless, the concept of dwell is still
applicable, only today it represents the amount of time current flows through
the coil primary circuit before this circuit is interrupted (see Chapter 7). In
addition, in certain cases ignition timing can be measured by measuring the
relative strength (peak voltage) of ignition pulses. Although the engine analyzer
continues to be a useful instrument, it is not adequate for diagnosing problems
in electronically controlled systems.
While it is true that both the timing light and the engine analyzer will
continue to be used because there are many cars still on the roads whose
engines are controlled either by analog or mechanical systems, it is probable

UNDERSTANDING AUTOMOTIVE ELECTRONICS 337