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Chapter A: General Diagnosis
9
right in firing order sequence, figure 1-6. This allows
easy comparison of voltage levels between cylinders. A
parade display is useful for diagnosing problems in the
secondary circuit.
A raster pattern shows the voltage traces for all cyliders
stacked one above another in firing order sequence, fig-
ure 1-7. This display allows you to compare the time pe-
riods of the three sections of a voltage trace.
Primary Ignition Patterns
Primary scope patterns are displayed as a low-voltage
trace moving from left to right across the screen. Elec-
tronic ignition patterns vary by system, so it is important
to know what is normal for the system being tested.
Scope manufacturers generally provide sample patterns
for comparison.
Firing Section
The firing section of the pattern corresponds to the
amount of time the spark plug is firing. The voltage trace
records induced voltage surges in the primary circuit as
secondary voltage is being dissipated from the coil. A
typical pattern begins with a vertical rise as spark is es-
tablished, which is followed by a series of diminishing
oscillations until the spark is extinguished, figure 1-8.
With some electronic ignition systems, there are no os-
cillations. Firing is displayed as a vertical spike followed
by a relatively flat line until the spark ends. In parade dis-
play, the height of the oscillations should not vary be-
tween cylinders.
FIRING
SECTION INTERMEDIATE
SECTION DWELL
SECTION
ALL CYLINDERS
90
60
80
50
70 60
40
50
30
40 30
20
20
10
4 CYL
6 CYL
8 CYL
0
0
0 45 40 35 30 25 20 15 10
FIRING ORDER
4
2
6
3
5
1
25
20
15
10
Fig. 1-7. The raster pattern stacks the cylinder traces on top of one
another.
Fig. 1-5. All cylinder traces are displayed one on top of another in a
superimposed pattern.
Fig. 1-6. Cylinder traces are displayed one after another in a parade
pattern.
Intermediate Section
The intermediate section begins when the spark stops
and continues until the primary circuit is switched on by
an electronic signal, figure 1-9. Some ignition systems
do not have an intermediate section, and the primary
circuit is switched on the moment the spark is extin-
guished.
The voltage trace is created by the dissipation of ener-
gy remaining in the coil after firing. A series of diminish-
ing oscillations, similar to the firing section but consid-
erably smaller, is normal.
Dwell Section
The dwell section is the time the primary circuit is com-
plete and a low-voltage current flow is building up the
magnetic field of the coil. The trace should drop sharply