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Using the right techniques and test gear should enable you to troubleshoot DIS systems with the best of 'em. Here's the lowdown.

Direct, or distributorless, ignition systems are no longer brandnew, cutting-edge technology. As a matter of fact, they've become the "conventional" ignitions of the 1990s. The first systems appeared on the Buick 3.8-liter V6 in 1983. Today, DIS is mainstream technology, appearing on most new models introduced in the past decade.

Direct (distributorless) ignition systems can be classified as being one of two types-individual-coil systems, with a separate coil for each cylinder (often called "coil-on-plug" designs), or waste-spark systems, which use one coil for every two cylinders.



Although waste-spark DIS has been with us for more than 15 years, diagnostic techniques and test equipment are still in a growth stage. More and more technicians are asking for equipment that will test these systems, preferably oscilloscopes that will provide waveforms in familiar patterns. Professional technicians also are asking for more training and troubleshooting information on waste-spark DIS. You can apply the information presented here to waveform diagnosis using either a traditional ignition scope or one of the latest-generation lab scopes.

System Operation

A waste-spark system fires one coil for each pair of cylinders that are at top dead center (TDC) at the same time. These cylinder pairs are called "running mates." One cylinder is at TDC on the compression stroke, while the other is at TDC on the exhaust stroke. The spark in the cylinder at TDC on the compression stroke ignites the air/fuel mixture to produce power. The spark in the cylinder at TDC on the exhaust stroke is "wasted," hence the name "waste-spark."

Each waste-spark DIS coil is hooked in series with its two spark plugs,. As the coil fires, secondary current creates a high-voltage spark across the gaps of both plugs. One plug fires with the traditional forward polarity of an ignition system: negative (-) to positive (+) The other plug fires with opposite polarity: positive (+) to negative (-) Thus, one plug always fires with what has always been called "reversed polarity." The voltage capacity of a DIS coil is high enough, however, to ensure that the available voltage is always high enough to fire the plug with reversed polarity when it's on the compression stroke.

Ignition oscilloscopes have always inverted the traditional negative-topositive firing voltage to show an upward firing line, or spike. Because both plugs for a waste-spark DIS coil fire simultaneously-one negative and one positive-an ignition scope cannot display the traditional voltage patterns without an electronic adapter. Several adapters are on the market-from scope manufacturers and from independent equipment makers-which invert the negative-firing voltage waveform so your scope can display it. Most of these adapters work with either an ignition scope or a lab scope.

Every Waste-Spark Pattern Is Superimposed

Because both plugs for each coil fire simultaneously, every waste-spark DIS waveform is actually a superimposed pattern of two cylinders . The waste spark in the cylinder on the exhaust stroke fires at a slightly lower voltage than the spark in the cylinder on the compression stroke. With a scope adapter that maintains the "real-time" superimposed firing display, you may see the waste spark as a bright spot three-quarters up the firing line of a parade or superimposed pattern. (Intensity will vary from one scope to another.)

Here some variety enters into the way in which DIS scope patterns can be displayed. Some scope adapterssuch as the DIScope 105 adapter that was a MOTOR Top 20 Tools award winner in 1995-maintain the superimposed firing event and display a parade or raster pattern of half the number of engine cylinders. With this kind of adapter, you must set your scope for half the number of cylinders in the engine. You also must set the scope for two-cycle engine operation because total ignition firings with a waste-spark DIS occur twice as often as a distributor ignition for a four-cycle engine.

Many hand-held digital lab scopes split the superimposed waste-spark DIS firing event and display a waveform for each cylinder. Digital sampling technology makes it possible to select one cylinder from a superimposed pair and display its firing event separately from its running mate. In a parade or raster DIS pattern that displays all six cylinders of a V6, for example, the second half of the pattern is a repeat of the first, but the alternate cylinders are displayed. Then every cylinder is alternately displayed on the power stroke and the exhaust stroke.

It's not really complicated if you concentrate on the secondary pattern and what you've known about ignition waveforms for years. The engine is still a four-cycle gasoline engine, and the ignition system is still an inductive-discharge ignition. Moreover, your scope-old-fashioned analog ignition scope or modern digital lab scope-is still synchronized by the No. 1 cylinder pickup. Therefore, parade and raster patterns still appear in firing order sequence.

All of the pattern examples shown here are based on a scope adapter that maintains the superimposed firing event and displays a V6 parade or raster pattern as three superimposed pairs. This puts DIS troubleshooting in the context of the familiar ignition scopes we've known for decades