The science of scopes

Jan. 1, 2020
Not every shop can afford more than one scan tool, but a scope might fill the gap by evaluating combustion behavior at all driving conditions. With a scope, you won't be cliff hanging when your scan tool is not compatible with a specific vehicle. You

Why scope ignitions if you have a scan tool? The answers appear before your eyes.

electrical scopes scoping vehicles scan tools vehicle scopes automotive scopes repair shop training technician training A/C training automotive aftermarket

WHY SCOPE IGNITION IF YOU HAVE A SCAN TOOL?

Not every shop can afford more than one scan tool, but a scope might fill the gap by evaluating combustion behavior at all driving conditions. With a scope, you won't be cliff hanging when your scan tool is not compatible with a specific vehicle. You still can do total diagnosis and find the cause of the misfire within minutes. With the proper adapter hookup, you can explore each cylinder from the driver's seat at any rpm or load.

In addition, the scan tool might tell you there is a misfire if it repeats on a regular basis, but the scope does more and shows where it is and under what driving condition it appears. Plus it will indicate what caused the misfire. Furthermore, the scope also points to other cylinders with a potential misfire, and that makes the scope ideal for preventive maintenance.

Hidden DIS Ignition System Secrets

When DIS ignition was first introduced, the hangover from distributor ignition still existed with the belief that you have to have a paraded pattern, because scope analysis is useless without it. At great expense, equipment designers scrambled to come up with a variety of adapters to satisfy the cry of the scope users. It takes a lot of ingenuity and talented engineering to unscramble: positive and negative, waste and compression, different time bases, inverting signals and high voltage radiation in close proximity with each other, all in one box.

Some adapters were fairly successful in doing so, while others missed the mark completely. Even those with somewhat decent outputs worked great on a good running vehicle, but were all over the map on the problem car. This turned even the most enthusiastic scope nerd off with the conclusion that scoping DIS ignition was impossible and too time consuming, having to look up polarity and firing order, etc.

So the question is, "Why do we need parade?" And the next question is, "Why do positive and negative spikes have to point in the same direction?" And finally, "Why do we bother with waste spark when it does not contribute to performance?" It took us two years of exploring with several prototypes to come up with an alternative with a fairly inexpensive tool to perform total drivability testing from the driver's seat.

The solution is simple. DIS has the unique ability to allow looking at the same spark plug without and under combustion. (We will explore later how important that is.) In addition, today's engines use multiple identical ignition systems, using the same fuel supply, under the same load or RPM. This allows us to compare several good performing cylinders against the odd one, and then we know when it happens, where it happens and how many cylinders are affected.

Figure 1 shows a 6 cylinder engine using three separate coils built into one coil pack. The lab scope uses two channels to display one coil at the time. Instead of having Waste and Compression alternate on top of each other, the key to a stable scope pattern is setting the time base at 20 Ms. This separates Compression and Waste so that both, positive and negative firing can be evaluated on one scope capture. Then we compare each coil display by switching from coil No. 1 to coil No. 2 to coil No. 3. If all captures are identical, we move to 2,000 rpm no load and repeat the same under load.

What makes idle the yardstick? Notice the 5:1 ratio of compression KV versus waste KV? This single capture tells us that we have no electrical problem and we are ready to proceed to increase RPM and load test. It is important to include idle in every diagnosis to qualify the electrical part of the ignition system like coil, plugs and plug wires.

COIL ENERGY: The coil output is designed to maintain the sparkplug's firing for as long as fuel is present in the combustion chamber. Because this is determined by a collapsing magnetic field, this available energy is the same at any load or speed. The firing time indicates how well this energy is used. Therefore firing time is our ruler to measure conductance by leaving the time base at 20 Ms and then expanding the pattern for closer observation.

Disqualifying the Electrical System at Idle

Figure 2 illustrates the various malfunctions we can observe at idle. Because timing is near TDC at idle and compression is at its peak, the 5:1 ratio only holds true at idle, and is an excellent alternative to comparing KV reading. The objective of ignition is the firing time, but we need to make sure that coil energy is not wasted in KV demand. Any of the failures in this illustration will reduce firing time and needs to be corrected before proceeding with further testing, or at least taken into account with diagnosing at higher speed and load.

OPEN PLUG WIRE: This is identified when Waste is out of proportion. The voltage required to fire the plug is determined by the greatest gap in the circuit, or rather called the "dominating gap." Because waste is not under compression, it is the smallest gap and a break in the plug wire appears there first with an increased KV demand. Then, when the plug wire continues to burn over time, the inner core of the plug wire will eventually become the dominating gap of both Waste and Compression KV.

WORN SPARK PLUGS: First observation is an abnormally high KV demand, but the 5:1 ratio is still perfect. It stands to reason that an excessive spark gap is evident at Compression as well as Waste. The immediate negative effect of a wide gap creates more resistance affecting high RPM performance. Reduced current means less spark heat and slower combustion.

However, high KV also is an extra strain on insulation, potentially causing crossfire or creating a carbon track. Once a carbon track is present, it is an easy path for crossfire to occur.

LEAN FUEL MIXTURE: Waste is not affected by fuel, but the Compression KV will increase when fuel mixture is lean and the firing time will also reduce.

This can easily be missed since the computer compensates. Fuel trim is global. It feeds the lean condition enough to keep it from misfiring thereby over feeding those cylinders that do not need it. Look for the nose! (See Figure 3). The lean cylinders have a higher nose, a shorter burn time and a higher KV demand. The rich cylinders may have no nose at all.

Once you have isolated the worst coil, set your sample rate at 200 Ms (Mega-Samples), and verify as follows: Hold the throttle at 2,000 rpm to allow fuel trim to go back to normal. Then quickly decelerate and freeze the capture (hit the space bar). You will accumulate 32 frames to identify the extreme high KV demand and possible a crossfire.

QUALIFYING THE YARDSTICK: Once we know that the KV demand is within reason and is not affecting firing time, we can qualify the coil output as OK, as long as the spark duration is over 1 Ms. Now we can concentrate on evaluating the spark behavior inside the combustion chamber at idle — 2,000 RPM — and testing under load.

For all practical purposes it is not even required to have a KV scale. Every lab-scope has a voltage scale.

Qualifying Firing Time Duration

The pattern in Figure 3 is expanded times 16 to get a better look at the firing time duration. The red arrow on the bottom of the scale shows a slider that allows moving the capture left and right. However, looking at a perfect fuel distribution there no need to scan a previous firing.

Let us explain some of the detail in this capture at A, B, C, D.

A. Hydro-Carbon is the conductor to sustain the spark. Any turbulence or reduced volume of HC will be noticeable on the spark line. In this capture the firing line is undisturbed and we can conclude that combustion is very smooth, which is typical at idle, but not necessarily at high speed. This indicates that fuel was present from ionization to the end of the available electrical energy, or coil output.

B. The spark duration was about 1 Ms. The dotted line squares represent 20 Ms/16=1.25 Ms. This is an ideal setting to compare the coils at high rpm and under load. There is going to be turbulence at higher speed and load, but whatever it is, it should be identical for each coil at each rpm or load selection...

C. Notice that the KV demand of Waste versus Power is a perfect 5:1 ratio.

D. This is where DIS is so unique. We already know that whatever happens in the combustion chamber of the companion cylinder is reflected in the waste, since they are both in series. Look at the waste spark and observe the clean firing line. This observation confirms a clean bill of health to the cylinder under compression.

What Are We Looking For?

Figure 4 illustrates the same capture we saw at idle, except it is now at a higher rpm. The dotted lines are time lines positioned exactly at one revolution. The calculated rpm from Hertz is illustrated in the lower right corner inside the circle and it reads 820 rpm.

This higher rpm increases the number of captures of the same two cylinders and we can immediately notice a decrease in KV demand. The reason is that timing advances as rpm increases and the ignition fires at a lower compression far before TDC.

Also note that we measure KV in voltage. The adapter used in these captures serves as a primary and secondary function, and is attenuated 10: 1 to protect the scope from high voltage arcing over. (If you can kill the engine when spraying water on older plug wires, this protection is needed.) In addition, attenuation is also required when hooked to a primary circuit, which creates an inductive kick of up to 400 volts.

A reading of 5 volts with this adaptor is the equivalent of 15 KV. A normal KV reading for DIS at idle is between 8 and 12 KV. Over the past 10 years, we have seen a tremendous amount of fluctuations on KV readings depending on the plug wires used (especially on imports). Even humidity in the air makes a difference.

Therefore, we recommend relying on the 5:1 ratio and the firing time duration of at least 1 Ms. By the way, firing time is totally independent of the voltage scale used.

The insert in Figure 4 shows a single cylinder with a very small nose. The nose is the residual electrical energy left after the hydrocarbon is no longer present and resistance increases. A small nose is considered a perfect match of the amount of inhaled air and fuel. No nose indicates a richer than normal mixture which is often an indicator that a lean condition of other cylinders is being compensated for by the computer. A high nose indicates a starving of fuel and the coil is saying, "It's not my fault."

Increased Speed

Let's take a look at what happens when you increase speed to 1,500 rpm. Looking at Figure 5, the first reaction is, "Wow, look at that high KV. Someone sprayed water on the plug wires."

Because we deal with scope analysis, we will diagnose without the help of a scan tool. The challenge is to do it strictly by Scope Pattern Interpretation (SPI). The first observation is that the neighboring coils in the coil-pack, in close proximity with the coil under test, are also showing a high KV demand. Next action is to verify this by switching to all three coils to confirm that this phenomenon is indeed common to all cylinders. This test result shows that it is equal to all cylinders. The cause indicates a lean fuel mixture common to all cylinders and logic tells us it is computer controlled. Ruling out the O2 sensors on left and right bank, the only fuel control common to all cylinders

On this car is the Mass Airflow Sensor (MAF). This one is under reporting airflow.

Testing Under Load

We all know from experience that any resistance has a greater effect under load.

For example, a resistance of as low as 0.01 ohms on a battery connection means a lot on a starter motor drawing 200 amps, because it results in 2 volts less at the starter terminal. But that same resistance on headlights drawing 16 amps means only 0.16 volt loss at the bulb.

That is not only true for current flow, but it also holds true for any flow restriction, like restricted exhaust, a dirty injector, an injector electrical as well as mechanical resistance, a worn cam lobe, fuel pump, fuel filter, air filter, etc. Consider an 80 percent restricted fuel filter that has no effect on idle, but won't pass enough fuel to make it up a hill! So here is the rule: When the problem acts up under load, start thinking - restriction – obstruction – resistance. It all means the same.

Let us decipher this scope pattern logically.

The above illustration shows two identical cylinders, under the same load, using the same fuel supply, inhaling the same air, etc. The scope sees poor conductivity on cylinder No. 1. The diagnostician translates this as absence of hydro-carbon and since it appears on only one cylinder, he can conclude it is an injector problem. The number of captures stored in this illustration is 32.

Scanning all 32 captures determines if it is consistent or does it come and go.

If that injector appears inconsistent, it is likely caused by a mechanical resistance in pintle movement and might be cleanable.

Now let us assume that this pattern seems consistent on all cylinders when switched from coil to coil, and the engine is intermittently misfiring under load. This makes it "a common to all" problem and the most logical conclusion is a fuel supply failure, like fuel pump volume or plugged fuel filter.

This again shows that an archive of scope patterns with still pictures is no comparison to the ability to compare live on the car with identical operating conditions.

Food for Thought

Let us ask ourselves these questions: Will the scan tool be able to capture this? If this is not creating a misfire yet, will the customer have a complaint other than poor gas mileage? Does your shop promote preventive maintenance, or is the policy the one of "Just find the misfire, fix it and get it out the door ASAP"?

Let us suppose that you network all your computers in the shop and imagine that this testing under load scope pattern is on your computer. You push the button on the upper right corner that says "Print Screen", then go to "Microsoft Word"; then right click and hit "Paste". Now you have this pattern full size on the top of a blank page, ready to print. Let us also imagine that your preventive maintenance program is printed below that picture and at the bottom of the page is the name and location of your shop in bold letters. You discuss this problem with your customer and give him/her the printout with possible solutions.

This piece of paper is the best advertising you ever created, because it is going to be shown to everyone in the office or to all the buddies in the work place — and your name is on it in bold letters.

Let us assume that the printout showed a picture perfect capture? People love to brag about their car and go all out to show their neighbor and friends. A template of this printout is available from [email protected] with your shop name in bold lettering plus address, if you like it.

Mac VandenBrink is owner of Dynamic Auto Test Engineering Corporation (DATEC), a company focused on designing new training concepts and teaching them to the automotive aftermarket.

Sponsored Recommendations

Best Body Shop and the 360-Degree-Concept

Spanesi ‘360-Degree-Concept’ Enables Kansas Body Shop to Complete High-Quality Repairs

ADAS Applications: What They Are & What They Do

Learn how ADAS utilizes sensors such as radar, sonar, lidar and cameras to perceive the world around the vehicle, and either provide critical information to the driver or take...

Banking on Bigger Profits with a Heavy-Duty Truck Paint Booth

The addition of a heavy-duty paint booth for oversized trucks & vehicles can open the door to new or expanded service opportunities.

Boosting Your Shop's Bottom Line with an Extended Height Paint Booths

Discover how the investment in an extended-height paint booth is a game-changer for most collision shops with this Free Guide.