DIS: Using An Automotive Scope
by Corey W. Glassman Note: Some of the terms and procedures outlined in this article might differ depending on the type of automotive scope used.
To properly diagnose a malfunction in a Distributorless Ignition System (DIS), it is important to understand how the system works.
As the name implies, there is no distributor used. Each cylinder has only one spark plug, but shares a coil pack with its companion cylinder, which reaches 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 discharges, both plugs fire at the same time to complete a series circuit. This is where the term "waste spark" comes from.
To determine which cylinders are companions, divide the engine's firing order in half. As an example, 1 8 4 3 6 5 7 2 would be 1 8 4 3 / 6 5 7 2. One and 6 would be paired together; 8 would be with 5; 4 with 7; and 3 with 2. When one of the cylinders is on its compression stroke, the other is on exhaust.
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 which stroke. Because of lower pressure in the waste cylinder (exhaust valve open), the plug on that 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.
Remember that current always flows in one direction. That is why half of the plugs wear the ground side electrode while the other half wear the center electrode.
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 build-up 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. Parade display of all cylinders
For an accurate diagnosis, a multiple connection to all coil outputs is not necessary. The lead set can be costly and takes additional time and effort to make all the connections.
You should use an automotive scope that allows you to examine each coil output in a type of "single cylinder" mode.
The scope should contain readings on the display indicating peak kV, RPM, burn time and burn voltage.
The peak kV voltage should be similar for each event firing. Simply move the secondary pickup from wire to wire while monitoring the recorded peak kV values. Remember that the firing line will be inverted on some of the cylinders. If the pattern is inverted, your scope should have a function key to invert the waveform.
There should be a function on the scope to record the ignition signal over time.
If a low kV spark voltage is recorded, look for a shorted spark plug wire associated with that coil pack. If a high kV spark voltage value is recorded, look for high resistance on that spark plug wire. If there were an open plug wire, then neither plug would fire due to the series circuit design of the distributorless ignition system.
Plug Wire Resistance Test
Where applicable, configure your scope to measure resistance in the "Multimeter Mode." With the engine not running, disconnect the questionable spark plug wire and connect the red and black leads (again, depending on your scope) from your scope to each end of the wire. Resistance values should be about 10K½ per foot (30 K½ per meter), depending on the type of wire being tested; some may be considerably less.It is important to compare the results with other spark plug wire values to ensure accuracy of the test.
System Analysis
The secondary side of the ignition system is affected by the primary control side of the ignition system. For this reason, start your diagnosis on the secondary side of the ignition system. If all of the peak kV measurements are equal, and there are no intermittents, then focus your diagnostics in another area.If an intermittent condition is noted, then work back in the system to determine the cause. After testing the plug wires, test the coil packs for problems.
Coil Pack Short Circuit
A shorted coil pack will require a greater amount of current to build the magnetic field and electrical charge required by the ignition system.A shorted coil or intermittent misfire can be detected by monitoring the voltage drop across the driver transistor ground or the current required by the driver transistor. This same "current hump" may be viewed on the ground circuit for the crank sensor as well.
Test Procedure
Part 1 - Voltage Drop
Back probe the "Sensor Ground" circuit with the red test lead to the positive lead on the scope and connect the ground lead to a good engine ground.Start the engine and configure the scope to display four to six patterns. A normal coil pack should drop only 100mV and the waveform signatures should appear identical. If an excessive drop is noted on all coils, look for a bad ground on the ignition module.
Part 2 - Current Test
Connect the current clamp around the feed wire for the primary side of the coil. Start the engine and adjust the time base to view four to six patterns on the display. The humps display the current increase in the coil packs during magnetic saturation. The patterns should look similar in rise time and slope. Duration may change as the module adjusts dwell.Evaluating The Results
An open coil primary winding will be identified by a missing pulse in the current pattern.A shorted coil will have a square shaped current ramp (due to reduced primary coil resistance).
Whether you use an automotive scope or another tool when troubleshooting DIS, it's still much easier to diagnosis the problem when you understand how the entire system works.
Corey Glassman is the senior automotive product specialist for Fluke Corporation, and is ASE Triple Master (World Class) certified. He is a member of SAE, STS, and is the second vice president of the Automotive Training Managers Council.
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AutoInc. Magazine ®, Vol. XLV No. 2, February 1997