Universal Tests for Oscilloscope Ignition Systems

16-07 Use an oscilloscope to test and examine ignition systems.

The following tests will help find ignition faults in any system or application by adding secondary resistance. Most ignition failures are intermittent and difficult to find. Typically, they are not noticeable during at-idle testing, where minimal firing kV is needed to overcome secondary resistance. These tests raise the firing kV demand by increasing in-cylinder resistance. Before starting any ignition system test, perform a visual inspection first. After the system passes the visual inspection, begin testing. If the scope in use has ignition system testing software, use the parade pattern for the best results. This test produces the best visual information for rapid and easy comparison of all the cylinders at once. Even if the scope in use does not have the parade pattern option, the tests will still be extremely beneficial. They just require more effort and time to check and record the results of each cylinder individually.

The following tests are used for oscilloscope ignition systems:

Using a Scope to Test Cranking and At-Idle Coil Voltage

The cranking kV test checks the coil’s reserve energy and checks for low cylinder compression. There are different methods to perform the test using an oscilloscope. The first procedure is to set a parade pattern on the oscilloscope. Then, disable the fuel by removing the fuel pump relay or fuse or, on some vehicles, by holding the accelerator at WOT (clear flood mode). Next, crank the engine while holding the throttle wide open, increasing the amount of air entering the cylinders.

While cranking, monitor or record the firing line and spark line results. Most modern COP and waste spark coils should be able to output well over 30 kV, a high firing line, and still show about 1.0 ms spark line. As an additional verification of a failing coil in a DIS or COP application, compare each coil’s firing voltage to each other, looking for the coil with low output. If a coil cannot meet the firing demand or if the spark line is too short, replace the questionable coil and retest.

The additional air raises combustion chamber pressures and resistance, stressing the ignition coil’s (or coils’) output to near maximum. The air acts as an insulator (resistance), requiring more of the coil’s energy (higher firing kV) to fire the spark plug. Adding additional fuel serves as a conductor, lowering resistance and coil demand.

Because the spark plug is subjected to cylinder pressures, compression in each cylinder also directly affects the firing kV value. The cranking compression test can find low compression using the law of ionization. Any drop in compression will lower the kV demand, while high compression requires higher firing kV. According to the law of ionization, it takes only 80 volts of energy to overcome a 0.001" air gap under atmospheric pressure. In other words, a cylinder with low compression will reduce the energy demand on the coil, whereas good compression should show kV values that easily surpass 20 kV (FIGURE 16-33). Remember that if the kV output is lower on a single cylinder of a COP system, this could indicate either low compression or a faulty coil.

FIGURE 16-33 The coil output test measures the coil’s maximum kV output by using the parade pattern. This test checks coil output and relative cylinder compression. Depending on the system, this test also checks the distributor cap and rotor (if present), coil wire (if present), spark plug wires (if present), and the high-voltage spring on a COP system.

Another method for the kV output test is to remove the spark plug wire or COP. Attach a spark tester to the disconnected wire or coil, and set the gap to require a minimum of 30 kV. The spark tester simulates WOT driving, substantially increasing the stress on the ignition system. With the engine running, check the firing kV and spark line. Increase the spark tester gap to 40 kV and retest. Compare cylinders against each other on multi-coil applications. All HEI, DIS, and COP coils should easily meet the 30 kV demand, and many will surpass 40 kV (FIGURE 16-34).

FIGURE 16-34 A single-cylinder coil output test of a DIS coil, using an adjustable spark tester set at 0.75". The top horizontal cursor shows that the coil’s maximum output with the tester installed is about 44.5 kV. At least 30 kV should be seen on the scope from a good DIS or COP coil. Some waste spark coils can output over 60 kV with an open secondary circuit. The measured output is not the coil’s maximum voltage output; it is the amount required to jump the tester’s gap.

Notice the spark line starts at about 33 kV and drops down almost vertically to about 5.0 kV. The burn time is only 732 microseconds (µs). The extremely short burn time is due to the coil having used most of its stored energy to jump the large gap of the tester. The missing coil oscillations at the end of the spark line are another indication that the coil has used all its energy and is lacking reserve power. Caution: Do not allow the coil to arc to the atmosphere during this test. Doing so can damage the coil, ICM, or both.

TECHNICIAN TIP

Power braking the vehicle can happen only if the vehicle has an automatic transmission. When power braking, assign any helpers stand to the side of the vehicle and pay close attention to the temperature gauge to make sure the engine does not overheat.

Testing Idle Firing Voltage Demand

When using a scope to test idle firing voltage demand, set the screen for a parade pattern if that setting is included in the scope’s software. Measure the firing line voltage levels of all the cylinders, and compare them for uniformity. Normal firing voltage under low demand at idle is usually between 8 and 12 kV. First, compare all the cylinders against each other, looking for any obvious differences. A firing voltage that is too high or low compared to the other cylinders requires additional testing to determine the cause. Next, check and compare the spark voltage and spark duration of each cylinder. Remember that there is a direct relationship between the firing line, spark line, and spark duration. Whatever affects one of the three measurements will affect the other two equally.

Testing Mid-Range Firing Voltage Demand

With the vehicle still in park or neutral, raise the engine speed to between 2500 and 3000 rpm. The higher rpm places additional stress on the ignition system due to the shorter coil saturation time. Again, using the parade pattern, compare the cylinder’s firing voltage, spark voltage, and spark duration against each other, looking for any major differences in any of the three measurements. For most systems, expect the firing voltage to increase slightly, due to the higher rpm and a marginally leaner air-fuel ratio.

Testing In-Bay Power Brake/Pre-Load Voltage Demand

This test simulates normal driving conditions in an automatic transmission–equipped vehicle. Block the wheels and apply the parking brake. Verify that no person or object is directly behind the vehicle. During the test, keep one foot firmly on the brake while pressing the accelerator. Have an assistant put the transmission in reverse gear. Load the engine by opening the throttle. Monitor the scope, comparing the cylinder’s firing voltage and spark voltage. Most faults found during this test are seen by an increase in firing voltage or the required spark line voltage (kV). If the spark line voltage moves significantly higher than at idle, with an upward slope immediately after the firing line voltage, then suspect a lean cylinder or secondary ignition resistance fault. Typical causes include a bad spark plug, high-resistance spark plug wire, COP spring, or failing coil in a DIS or COP system.

Snap Throttle Test

A misfire that occurs only under load or at highway speeds without setting a diagnostic trouble code (DTC) can be difficult to diagnose. Determining which cylinder(s) are causing the concern can prove to be a challenge if the miss occurs too fast for the PCM to recognize or assign a cylinder-specific code. A PCM scan may fail to produce any codes. Other times, the PCM may set a P0300 for a random misfire. This DTC means the PCM knows that there is a misfire, but it is unsure of which cylinder(s) are responsible. Using a scan tool to check a misfire counter or Mode 6 (Mode $06) data may help determine which cylinder is missing, but even this information is not foolproof. Misfire data, regardless of the vehicle manufacturer, have been known to “flag” the wrong cylinder for a variety of reasons and can be cleared when checking codes. On older vehicles, this information may not be present.

To reproduce the misfire, perform a test that stresses the ignition system for secondary leakage. Do not assume that testing the ignition system at idle will show a fault. Checking the required firing voltage (kV demand) at idle usually fails to identify a spark issue, since the kV demand is relatively low, often under 10 kV. Another test is needed to duplicate cylinder conditions while driving. When a load is applied to the engine, the voltage needed to fire the spark plugs rises. A snap throttle test is used to put more demand on the secondary ignition and fuel systems. The snap throttle test creates the highest possible in-bay ignition stress because it simulates a rapid acceleration event during a road test.

A snap throttle test consists of nothing more than momentarily cracking the throttle open as fast as possible and then releasing it just as quickly. Suddenly opening the throttle provides the engine with a massive increase of air, creating an incredibly lean condition and limited timing advance while increasing cylinder pressure and temperature. This sudden escalation of cylinder conditions requires a comparable increase in the firing voltage. By moving the throttle quickly enough, these conditions occur before the PCM can adjust the fuel or timing curves, increasing the stress on the coil(s). Slamming the throttle closed as quickly as possible limits an increase in rpm, which is essential for reliable test results.

Use the parade pattern on the oscilloscope if available. A bad cylinder will be evident in a parade pattern since all the cylinders are displayed at the same time. Even if the parade pattern is not an option, the test is still relevant; it is just slightly harder to perform. On applications where the parade pattern is not an option, test and evaluate each cylinder individually. Saving a screenshot or snapping a picture of the screen for each cylinder may aid in analysis.

Many technicians incorrectly assume that the snap throttle test in only a check of the ignition system. However, paying attention to only the firing kV demand neglects additional in-cylinder operating information. A nearly equal increase in firing kV during the snap is only part of the analysis. Although the snap throttle test substantially stresses the ignition system, it also reveals some fuel system concerns. Poor spark conductivity can be a result of secondary ignition leakage or a lean air-fuel mixture (FIGURE 16-35).

FIGURE 16-35 An intermittent secondary ignition misfire caught during a snap throttle test. Instead of showing the intersect point where the burn line jumps out from the firing line, there is excessive hash. The hash where the spark line is supposed to be is a result of a coil failing internally. Under load (snap throttle), the coil does not have enough energy to overcome the temporary sudden lean condition created when the throttle is opened quickly, filling the cylinder with extra air, before the PCM can adjust the fuel curve. Besides a weak coil, an internally carbon-tracked coil, a faulty spark plug wire, or a faulty spark plug can result in the same basic pattern. To determine which is at fault, remove the spark plug wire from the coil and test the coil’s output directly at the secondary tower. If the spark cannot overcome the gap, the coil is weak and needs to be replaced. If the spark at the coil is satisfactory, disconnect the plug wire from the spark plug. Retest at the end of the spark plug wire. If the spark cannot jump the gap, the plug wire has failed. If the spark at the wire is OK, replace the spark plug and retest.

During the test, the firing kV for all the cylinders should increase, and the spark lines should rise uniformly. Depending on the ignition system, a snap throttle test should drive the firing voltage up to approximately 15 to 22 kV on average. Some systems may be slightly more. More important than an arbitrary number, however, is making comparisons between cylinders. Correctly performing the snap throttle test can also show a clogged injector or fuel system concern. A spark line that moves up from the firing line instead of horizontally indicates a high-resistance concern. Compare the results of the spark firing line from each cylinder. They should all be relatively equal, within a few kV of each other.

To perform the snap throttle test, use the following procedure:

Snap throttle tests can produce different results:

FIGURE 16-36 To check the ignition system under load, perform a snap throttle test. After snapping the throttle open and closed, quickly note the rise in firing voltage. Check the firing voltages for uniformity. The normal increase varies depending on the engine. Expect to see an increase of 1 to 10 kV. The red dashed line shows the expected firing kV range at idle. The blue dashed line shows a comparison between cylinders during snap throttle. Note the height of the spark firing kV lines. Cylinder 1 is showing an obvious ignition system fault due to a worn spark plug or high-resistance spark plug wire.

FIGURE 16-37 Using a parade pattern for cylinder comparison to identify a misfiring cylinder. When using a scope, always look for the signal that sticks out from the rest. The black dotted line was added to the waveform to show the differences in firing height. The red trace is the secondary ignition from a capacitive probe. The blue trace is the sync trigger for Cylinder 1. The firing order is 1 8 4 3 6 5 7 2. A high spark kV demand with a missing spark line shows that the firing line for Cylinder 3 has expended all its energy to attempt to jump the additional, unwanted air gap caused by an open secondary spark plug wire or COP high-voltage spring.

Note: Before attempting a snap throttle test with the engine, check for smooth operation of the throttle linkage. Also, limit rpm to prevent engine damage. If the engine rpm increases too much, close the throttle and try the test again. With practice, a technician will be able to perform the test without much of an increase in rpm. On vehicles using ETC, this test is performed using the accelerator pedal instead of the throttle linkage, making it more difficult to close the throttle rapidly and control the increase in rpm. On these vehicles, pay close attention to the sudden increase when the throttle is initially cracked open.

TECHNICIAN TIP

If a no-code random misfire that is not trackable to a specific coil, spark plug, or fuel injector failure is the concern at hand, suspect a failing alternator. An alternator producing excessive alternating current (AC) voltage due to bad diodes can create electromagnetic interference (EMI). The EMI can enter the crankshaft position (CKP) or camshaft position (CMP) wiring, causing erratic signal inputs that confuse the PCM. The PCM cannot process the AC voltage correctly, leading to improper fuel and ignition timing decisions. To diagnose this alternator issue, disconnect the alternator and retest it to determine whether the misfire disappears. If it does, replace the alternator. Note: Other electrical motors, such as blower motors, cooling fans, or electric air pumps, can cause similar issues, but the alternator is the most common failure.