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Automotive Application 1.
Ignition System |
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Connecting the DIS90
ScopeMeter for Measurement
The DIS90 consists of
three main components. The ignition adapter box connects to the
ScopeMeter. Two sets of secondary pickup sets allow the DIS90 to be used
on engines up to eight cylinders.
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An
easy-to-use menu-driven interface allows fast selection of the desired
pattern. |
Use the
secondary single mode of the Fluke 98 to examine each coil
output. |
Use the
secondary parade mode of the Fluke 98 to quickly compare all
cylinders. |
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1.
Cylinder in the
parade picture. |
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Connect the DIS90 adapter to the Fluke 98. Use a ground lead to
connect the COM input on the small connection unit to a good ground.
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Connect the round cable plug of the red secondary pickup set and
the black secondary pickup set to the ignition adapter box.
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For the moment, clamp the capacitive secondary pickup (CAP90-2 or
PM 9096/101, included with the Fluke 98) on the spark plug lead of
cylinder #1, as close as possible to the spark plug.
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Connect the black secondary pickup clamps to the spark leads with
negative polarity and the red secondary pickup clamps to the spark
plug leads with positive polarity. With the engine running, you can
identify the polarities as follows:
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Use the black or red secondary pickup and clamp it to a spark
plug lead
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If the Fluke 98 displays upward spark voltage, this pickup is
connected to the correct polarity. If a downward spark voltage
is displayed, the pickup is connected to the wrong polarity. Use
a pickup with the other color for this cylinder.
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Use the next black or red secondary pickup and repeat until
all steps are connected
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Clamp the remaining pickups on the connecting cable to the
ignition adapter unit to insure reliable results (see above).
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To get a stable picture on the Fluke 98 display, the capacitive
secondary pickup (CAP90-2 or PM 9096/101) must be clamped on a spark
lead with a negative polarity. If cylinder #1 has a positive
polarity, clamp the capacitive secondary pickup on the spark plug
lead of a cylinder having negative polarity. This cylinder is then
displayed as first cylinder in the parade picture.
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How the
system works |
| To properly
diagnose a malfunction in a distributorless ignition system it helps to
understand how the system works.
As the name DIS implies, there is no distributor used. Each cylinder
has only one spark plug, but shares a coil pack with its companion
cylinder (cylinders that reach top dead center -- TDC -- at the same
time). The cylinder that is at TDC on the compression stroke is referred
to as the "event" cylinder, while the cylinder at TDC on the
exhaust stroke is the "waste" cylinder. When the coil pack
charges, both plugs fire at the same time to complete a series circuit.
This is where the term "waste spark" comes from.
To determine which cylinder is the companion, divide the engine's
firing order in half. As an example, a firing order of 1 8 4 3 6 5 7 2
would be 1 8 4 3 / 6 5 7 2. 1 and 6 would be paired together, 8 and 5, 4
and 7 and 3 and 2. When one of the cylinders is on its compression
stroke, the other is on exhaust.
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Current
flow, through the primary side of the coil pack, is controlled by a
transistor in the ignition module, thus creating a magnetic saturation of
the primary. A high-voltage discharge is created, through the secondary
coil output terminals, when this transistor opens and drops the induced
magnetic field.
Since both plugs fire at the same time, it is not necessary for the
module to recognize which cylinder is on the exhaust stroke. Because of
lower pressure in the exhaust cylinder (exhaust valve open), the plug on
the waste cylinder requires less voltage to produce an arc across the plug
gap. Therefore, most of the available voltage is used to fire the plug on
the compression cylinder.
In most cases, the current that flows through the ignition module to
the primary side of the coil pack is limited to 6 to 9 amps. This is done
by modifying the base current of the driver transistor. The module also
monitors the last magnetic saturation buildup to determine if the maximum
current was attained. If maximum current is attained, the dwell time is
shortened to reduce the overall power consumed by the system.
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