Waveform diagnostics: Ignition diagnostics you will actually use

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Waveform diagnostics: Ignition diagnostics you will actually use

Alex Portillo is the head technician of Car Clinic, a state-of-the-art automotive repair facility in Mahopac, N.Y. He is a protégé of technician Jerry Truglia and has been trained by Automotive Technician Training Service and is Technicians Service Training (TST) certified. Portillo’s real-world, in-depth diagnostic articles will appear in future issues of Auto Service Professional magazine on a regular basis.

Research for this article was conducted by G. “Jerry” Truglia, Craig Truglia and Kevin Quinlan. This is the second in a series of articles begun by Craig Truglia, owner of Car Clinic (see the July/August 2012 issue). Here, Portillo continues with an advanced look on how to diagnose ignition problems. Some of the artwork has been done with the help of Ralph Birnbaum.

We are not going to get into the old stuff here that you’re already well acquainted with. The basic ignition system principles at work in a distributor are no different from those that make a modern coil-on-plug (COP) ignition work. COP, even without a cap and rotor, still:

• charges and fire a coil with a switched circuit;

• needs good power and ground; and

• needs to know engine speed for ignition timing.

Being that pretty much everything has moved over to COP or waste-spark ignition, we are going to cover the essentials on these systems first.

Ignition coils and primary ignition

The one thing that every ignition system has in common is the ignition coil.

A coil is in effect the “middle” of the ignition system. Every component in the ignition system leading up into the coil is primary ignition. If you want to get real technical, there is a metal coil with a carbon bar in the middle in the first part of the coil. Secondary ignition is every part after the coil. Again, technically speaking there is a second set of winding, in the second half of the coil.

In Figure 1 we can see the infamous 2000’s era Ford ignition coil. Circled is the primary connection section of the coil, which can be diagnosed by probing the ground side of the connector with a labscope.

Swapping coils and just looking at misfire counter on Mode 6 or checking for misfire DTCs is a common, but less precise, practice. Personally, we sell the customer on all new ignition coils or threaten them with diagnostic costs to pick out the bad ones. That seems to do the trick.

Every coil, no matter the vehicle, needs power. So anything that inhibits the coil getting power will compromise its performance. Be sure to check for the following:

• Correct voltage — A voltage drop to the coil can make it ineffective.

• Good switching — If the “switch” wherever it’s location (points, ICM, D.I.S, PCM these days, etc.) contact has high resistance, or if the ground connection is bad, power to the coil is reduced, weakening the spark.


Secondary ignition

On old vehicles, after the ignition coil was an ignition rotor, distributor cap, spark plug wires and spark plugs. The ignition coil provided the high voltage power, and this power was distributed and transferred to the plugs through the rotor, cap and wires. All of this is secondary ignition.

Modern ignition systems still have what we call “secondary,” but they use fewer parts.

• Waste-spark ignition gets rid of the distributor cap and just has spark plug wires and spark plugs.

• COP gets rid of everything but the plugs! As we can see in Figure 2, the secondary boot and spring on these coils essentially connects right to the plugs, making everything else unnecessary. The secondary has more windings than the primary. The windings increase the voltage, but decrease current. This voltage increase enables power to jump the spark plug gap.

• NOTE: Replace ignition coil boots when doing tune-ups whenever possible, especially if they are on GMs or you see any evidence of arcing (white crust).

Coil-on-plug ignition in detail

COP is here to stay until vehicles become all like diesels and ignite using compression. Because COPs do not use plug wires, this reduces the amount of ignition parts and makes them better suited for high kilovolts (KV) demand during high engine loads. Figure 3 shows a close-up view of the inside of a COP ignition coil.

With one coil per plug, a dead coil affects only one cylinder. Each coil has a two-pin connector. One pin receives system voltage and the other is grounded to the PCM, allowing the coil to charge and discharge. This makes them very easy to diagnose via coil swapping or the “comparison game” with the scope. As a side note, leaking coil boots should be replaced, not only to stop misfires but also to protect the PCM.

Testing COP system voltage

Unplug the coil and probe the hot side of the coil connector with your DMM. Watch the voltmeter as you turn the key ON, or set the meter to volts DC in Min/Max mode so it can record the maximum voltage for you. Our meter in the photo above has captured a 13.36 volt reading indicating that our circuit is intact. This is a good specification for Chrysler COP (see Figure 4).

COP testers

There are several COP testers that give us valuable information such as firing KV and burn time without any backprobing. In Figure 5, we show a COP tester, here an E-COP from Automotive Test Solutions (ATS), which can make it easy to get an ignition waveform.

The TA500 from GTC does a very good job of this, but it does not give you a waveform. As you see in Figure 6, the GTC TA500 COP tester can be used by simply playing the comparison game between coils. If you want to interpret a waveform, the E-COP from ATS allows us to do this easily. All you have to do is place the E-COP on top of the coil, connect the E-COP to your scope and instantly you are capturing waveforms.


Waste-spark ignition in detail

A few stragglers (Subarus for example) still use waste-spark. Waste-spark ignitions all work the same. They fire a pair of spark plugs directly connected to a single coil via ignition wires. Because they are connected right to the coil, both plugs in a given pair fire simultaneously.

How does the engine run properly? Well, one cylinder is in its compression stroke while the other is in its exhaust stroke. So, one plug fires during the compression stroke allowing the power stroke to subsequently take place. As for the cylinder with the exhaust stroke, the plug fires but nothing happens. The spark is just wasted, hence the term “waste-spark.”

If you see one spark plug with normal wear and the other one wearing inside out don’t worry, that is normal. When diagnosing these systems, just apply standard diagnostic principles, but use your common sense. Two cylinders misfiring that share the same coil likely have an issue whose source stems to the coil.

Instead of a distributor, all new systems use the CKP or CMP sensors. The rpm signal from these sensors is sent to the PCM through the ignition module. Using primarily engine speed, the PCM controls the primary circuit switching on/off.

Waste-spark ignition waveforms

The required firing voltage for the spark plug on the power stroke should be higher than firing voltage for the plug on the exhaust stroke. (In Figure 7, you’ll see ignition waveforms of a waste-spark system.)

Why do we care that waste-spark waveforms are smaller than regular ignition waveforms captured during the combustion event? If waste-spark firing voltage ever equals power stroke firing voltage, look for an open plug wire with an air gap greater than either plug gap. This open becomes the greatest gap and it forces coil energy to rush through it. This need for increased voltage will push up the waveform to a level it should not be at.

Real-world ignition tests

The following list of standard ignition tests is not as long as you might think. The reason? As we’ve already pointed out, the fundamentals of ignition system operation are universal.

Here are common ignition tests that ought to be in your troubleshooting arsenal:

• available voltage test;

• secondary insulation tests;

• ignition coil primary voltage drop test; and

• low amp probe current ramping test.

Available voltage test

The available voltage test is a good test if it is performed properly. It tells us if the ignition system can generate enough spark voltage to jump the spark plug gap.

To perform the test, you’ll need a dedicated spark tester that stresses the coil to output at least 40,000 volts. Do not use any other spark tester, such as the one with a light or the old clip/no ground spark plug tester, it won’t place a high enough demand on the ignition to truly test it.

1. Remove the plug wire and connect it to the spark tester.

2. Make sure the tester is properly grounded.

3. Crank the engine and look for a strong blue spark at the tester.

A strong blue spark tells you that everything leading up to the wire (including the coil) is good and that your problem lies elsewhere. If the spark is any other color, you will want to test further up the secondary or even the primary side of ignition to see where the power loss originates. (In Figure 8, you’ll see the available voltage test being conducted with a spark tester.)

Insulation tests

Why do engines that idle with no misfire begin to misfire under load?

The main reason is that spark plugs require increasingly higher KV to fire when engine load increases. Secondary insulation that can contain 10 KV at idle may leak when asked to contain 20 KV or more under load.

So how do you find a leaking plug boot, especially in engines where the plug and plug boot are out of sight inside a tube in the valve cover? You can either eliminate the insulation leak altogether, or make it worse.

You can make the condition worse by wrapping the plug boot in aluminum foil and reinserting it. The foil makes a more conductive path between the plug wire and metal tube. This should make a damaged plug boot leak at lower KV and misfire at idle.

If you locate a likely suspect with the first test, verify that the boot is indeed a problem by wrapping it with high quality electrical tape. Put the engine under load again and see if the misfire is gone.

In the real world you might decide to sell a tune-up or plan to replace COP boots the moment you see anything fishy. But just in case the front office guy isn’t able to do that, these tests might be a real help to get him to sell the parts.

Ignition coil primary voltage tests

Perfectly good ignition coils, plugs and wires are often replaced because the technician observed poor secondary spark. However, low primary voltage might be the real culprit! Be sure to do a primary voltage drop test before condemning the coil!

1. Connect a meter between the battery positive post and the coil primary positive terminal.

2. Test KOEO and KOER. (If a no start, check engine cranking.)

3. Only 0.2 V is acceptable.

If the test results fail, you need to isolate where the voltage drop is, starting between the batter post and cable end, then working your way up.

Don’t try this with a test light, because it won’t tell you anything useful. You need to find out if there is significant electrical resistance, not whether there is any power there whatsoever.

Using the low amp probe on DIS

Not sure where to clamp on with your low amp probe to measure coil current in DIS?

Most DIS coil pack electrical connectors have a single power feed, regardless of the number of coils in the pack. Connect here to get a good overall look of ignition coil amperage. To get a look at the spark plug pair, connect to the one coil primary control wire leaving the coil pack.

Interpreting ignition waveforms

There are two different ways to measure the spark firing event: hooking up an amp clamp on the primary side of the ignition coil for current and backprobing the voltage primary side of the coil with a labscope lead to see the waveform. In Figure 9, we can see how the amperage (above) and voltage (below) ignition waveforms differ.


 The current waveform

1. The PCM (or points, or ICM, or etcetera) closes the ignition circuit and the coil begins to charge up. That steady increase in amperage indicates that the coil is charging up.

2. The PCM opens the circuit just when amperage reaches its peak, causing current to plummet. Instantly, voltage skyrockets allowing spark at a low current to jump the gap.

This current waveform shows a lower than normal rise and less of an angle. The coil is obviously defective, notice the burn in the circle?

Figure 10 shows a real-world example that helps us understand the difference between a good and bad ignition waveform.

The voltage waveform

First, let’s begin with a good overview of what makes up an ignition waveform (see Figure 11 for the different parts of an ignition waveform.)

1. The switch internal to the PCM (or ICM/points) closes. Current rushes into the coil and begins to build, which is why voltage drops close to ground and essentially remains there until the firing spark.

2. The coil is now saturated with electricity, as indicated by the jump in voltage. The coil is no longer charging up thanks to the ICM/PCM.

3. The PCM switch opens, unleashing all the built-up current. Amps drop like a rock and voltage skyrockets.

4. The spark line indicates the length of the spark event at the plug.

5. When not enough power is left for the spark, remaining power is rung out and the event begins all over again.

To understand the voltage waveform, you need to isolate each part of it to know what’s going on.

1. Firing voltage is the voltage in KV required to jump the largest single gap in the secondary (most likely the spark plug gap). The gap between the rotor and distributor cap sometimes may be larger, and this will affect what you see on your waveform. (See Figure 12 for the firing voltage section of the waveform.)

Expect spark plugs with plug gaps of .045-.060 inch to require 8 to 12 KV to jump the gap at no-load idle.

Sometimes an issue with weak spark, incorrect spark timing, fuel supply/delivery, low engine compression, or something else that can cause a misfire can affect firing KV.

In fact, the waveform in Figure 13 is an example of how we can pick out an obvious misfire simply by looking at ignition voltage.

Something is definitely making ignition in that cylinder work harder than it needs to!

2. Spark voltage is the voltage in KV required to maintain the spark across the plug gap for the period of time required to ignite the gas.

If firing voltage is higher than 2 or 3 KV, this decreases the duration of the spark, known as firing time. Firing time should be about 1 or 2 mS. (See Figure 14 for an example of the spark voltage part of the ignition waveform.)


• Shorter spark times often indicate a weak ignition coil.

• In times where spark time is short and firing voltage is atypically high, this indicates that something is forcing the ignition to expend all its energy on the initial spark, leaving nothing left for its duration.

• The slight upward spike in the end of the spark line is normal.

• Lean systems spike up right after the firing spike.

3. Oscillations occur when the spark has ended because the energy necessary for the spark has been extended. There is still some energy left in the ignition coil, but not enough to continue the spark event. So, the remaining power oscillates up and down, effectively ringing itself out. (In Figure 15, you’ll see that oscillations can be seen in most ignition waveforms.)


• Sometimes, some ignition systems have one coil oscillation.

• A fouled spark plug has neither a definitive spark line nor oscillations.


Why look at ignition waveforms?

Spark line voltage, shape and time tell us more about the spark than the firing voltage KV does. Because of this, try stacking (Raster) the waveforms on your scope. It will make comparing the spark line much easier to do. Take a look at the spark line on number 6 cylinder in Figure 16 (ignition waveforms on the ATS EScope) that had a compression problem.

It’s tough when using labscopes. When are we being too critical and when are we not looking close enough? Sometimes we only know in retrospect. Here the firing time reflects the cylinder with the problem, but the firing lines of all the cylinders are hardly identical. That’s why we need many tools in our boxes to pinpoint vehicle problems, ignition diagnostics is just one of them.

Comparing voltage and current in primary waveforms

Comparing voltage and current waveforms is a great way to locate the problem and the reason for the problem. The ICM or PCM is responsible for switching on/off the primary circuit. When the switch opens, it must open quickly to properly create spark. Slow switching reduces secondary spark intensity. Have trouble visualizing this? Take a look at how the current drops instantly when the switch opens compared to the opposite in the waveform below (Figures 17 and 18).

Why does the spark event look so bad in the below waveform? The current waveform tells us here. The amperage does not drop straight down during the firing event. It instead steeply slopes down. This means that the switch internal to the PCM/ICM is opening too slow. We just diagnosed a bad module. (In Figures 17 and 18, we can diagnose a bad computer by looking at voltage waveforms against amperage waveforms.)

Secondary ignition waveform analysis

Now, you won’t be needing to scope too much secondary ignition anymore, but some vehicles still use ignition wires. And when the mood strikes you to scope secondary ignition, keep in mind that secondary waveforms look a lot like primary waveforms. (In Figure 19, can you tell the difference between the secondary and primary waveform? Neither can I.)

The reason this is so is because primary ignition directly affects secondary. In the below waveform Channel A is Primary Ignition and Channel B is Secondary Ignition. They both look very similar except for the dwell section.

For this reason if an ignition primary originates on the primary, it will askew your secondary waveforms. Be sure to scope secondary ignition AFTER confirming primary ignition is good.

This is how secondary waveforms should look.

1. The coil begins charging up full of current. The oscillations are magnetic interference from a working ignition coil.

2. During the dwell period, voltage builds very slowly.

3. The ignition coil unleashes all of its current into the secondary side of that same coil. Voltage skyrockets. Instantaneously the spark jumps the spark plug gap.

4. Spark line should be relatively high in voltage. Its duration is the firing time.

5. Oscillations happen when there is not enough power to continue the spark and any remaining power is squeezed out.

Real world secondary ignition diagnostic tips

• High voltage at point 3 in Figure 20 is usually the result of a wide plug gap (or some other gap on the secondary side depending upon the system) or high cylinder pressure. This lessens firing time (point 4).

• A voltage drop on the secondary side will not affect firing KV, but will increase the spark line (4) and lessen firing time.

• With COP you probably won’t be scoping the secondary. This is a tip we covered in part one of this article. If you really want to, connect ignition wires between the coil and the plugs if possible and measure the old fashioned way. In the real world just play the comparison game. You have at least four coils to choose from. Chances are you can just scope the primary and pick out the coil that’s bad. Simply change the coil along with the plug, or better yet sell the whole tune-up after that.


Secondary waveform comparison game

See Figure 21. We have our ignition scope hooked up to secondary ignition, using a parade pattern presentation.

Assume the vehicle has a misfire. Which cylinder is misfiring? Obviously cylinder 4! As you can see, the comparison game is an effective diagnostic method.

Sometimes you won’t be able to pick up an issue with the secondary ignition under normal conditions (such as misfiring at high speeds).

So, a snap throttle test places more demand on the secondary ignition, making the comparison game easier to play. As you can see in Figure 22, snapping the throttle can accentuate a misfiring cylinder so you can catch it.

In cylinder 1 it required more voltage to fire the plug, noticeably shortening the spark duration.


Yes, understanding ignition diagnostics is not quite as crucial as it used to be 25 years ago when vehicles needed tune-ups so much that my teacher, G “Jerry” Truglia, ran three repair shops all named “Car Tune.” The tune-up these days is a money maker, but in many cars you might only do one tune-up its whole life.

Now, with auto repair much more expensive and many problems much more technical, our reputations rely upon our ability to diagnose these vehicles.

So, if you don’t want to lose business to the supposedly all-knowing dealership, correctly diagnosing the vehicle is critical.

My goal in writing this article is this: When you’re posed with a vehicle misfire situation, you now will have more tools in your arsenal.

Understanding ignition is just one piece in the puzzle in diagnosing the customer’s car correctly and being successful in today’s automotive world.   ●

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