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Saturday, January 31, 2009
Scoping DIS Ignition
A well known and respected instructor made one day the following statement: "With the introduction of DIS ignition, scope analysis has lost its flavor"!
Sure, it is true that the KV spikes are unstable and jump all over the place and the firing time duration is a product of two cylinders, and not one.
Sure, it is true that the KV spikes are unstable and jump all over the place and the firing time duration is a product of two cylinders, and not one.
In addition, when we have positive and negative, waste and compression, different time base, capacitive and inductive coupled signals all mixed together in one adapter box, it is not easy to decipher a paraded pattern.
The solution is described in detail at the DATEC Website
However, we have to consider that there are also many features that makes DIS a class by itself. Compared to any other ignition system, with DIS you have the unique opportunity to look at a spark plug firing before it fires under compression, and before fuel or the absence of fuel effects the reading. The waste spark has so long been overlooked as a diagnostic feature.
This is where we can determine a fouled plug, a worn plug or wide plug gap and get a far more accurate concept of a defective plug wire. When a plug wire is defective, you don't have to wait until it is bad enough to exceed the KV demand under compression. Thanks to the waste spark, the evidence of even the smallest break in the wire is present at its infant beginning. You also don't have to wonder if that high KV could be caused by a wide plug gap or a lean fuel-mixture, because in waste there is no fuel to consider. Even a worn spark plug or fouled plug can be conclusively identified at the waste rather than under compression. In fact, we can make the dicstinction between carbon foul and fuel foul.
So then, we have to ask the question, is there any reason why we want to measure the voltage required to fire the plug on a KV scale? If you have a lab-scope, without KV scaling, you have the perfect tool to do total diagnosis on engine performance. All you want to establish is that at idle the KV on the power stroke is about three times more than waste. Any voltage scale on a lab-scope provides that information. The only benefit that jumping KV spike offers is to verify compression and timing advance when we increase engine speed (in park) to about 2000 RPM. The compression KV becomes almost equal to waste, evidence that there was compression and that the timing advance is working. Everything else can be diagnosed in the firing time.
Like voltage is required to make current flow, and current is the objective to do the work, so KV is required to ionize the gap to start the spark, and the firing time is the objective to complete combustion. Firing time is all about combustion efficiency. Any resistance will not only affects the duration, but also limits current flow and reduces the temperature of the spark. The objective of ignition is to keep the spark firing for as long as there is fuel mixture present in the combustion chamber. A short firing time could be caused by several abnormalities, like excessive KV demand, low coil output, or high resistance due to absence of hydro-carbon during firing. Now then, if the waste KV shows nothing abnormal about spark gap, and the firing time is smooth and not disturbed by resistance, we can conclude low coil output when the firing time is short. How short is short? Well, DIS has more than one coil – just compare! That leaves us with resistance! Hydrocarbon is the conductor in the combustion chamber. At 2000 RPM, picture the firing line as a path for electrons to travel, and if all the hydrocarbons combine with all the oxygen, and there are no free floating high resistance O2 molecules left over, the electrical discharge makes good conductivity until the combustion chamber runs out of oxygen. The computer gets the signal and reduces the fuel accordingly. This reduced HC results in an increase in resistance and the firing line slopes up into a nose, which verifies that the computer is in control. At this point, there is still some energy left in the coil, but not enough to maintain the spark. This residual energy dissipates into oscillation. This is an ideal capture.
Like voltage is required to make current flow, and current is the objective to do the work, so KV is required to ionize the gap to start the spark, and the firing time is the objective to complete combustion. Firing time is all about combustion efficiency. Any resistance will not only affects the duration, but also limits current flow and reduces the temperature of the spark. The objective of ignition is to keep the spark firing for as long as there is fuel mixture present in the combustion chamber. A short firing time could be caused by several abnormalities, like excessive KV demand, low coil output, or high resistance due to absence of hydro-carbon during firing. Now then, if the waste KV shows nothing abnormal about spark gap, and the firing time is smooth and not disturbed by resistance, we can conclude low coil output when the firing time is short. How short is short? Well, DIS has more than one coil – just compare! That leaves us with resistance! Hydrocarbon is the conductor in the combustion chamber. At 2000 RPM, picture the firing line as a path for electrons to travel, and if all the hydrocarbons combine with all the oxygen, and there are no free floating high resistance O2 molecules left over, the electrical discharge makes good conductivity until the combustion chamber runs out of oxygen. The computer gets the signal and reduces the fuel accordingly. This reduced HC results in an increase in resistance and the firing line slopes up into a nose, which verifies that the computer is in control. At this point, there is still some energy left in the coil, but not enough to maintain the spark. This residual energy dissipates into oscillation. This is an ideal capture.
There is another opportunity that DIS offers, and that is the ability to peek at a complete same ignition system next door to compare the difference under the same input conditions.
The first step to logical analysis is to understand the above description of what a good or normal pattern should look like. With that knowledge we quickly skim through all the cylinders in pairs of two per coil from IDLE in Drive, to 2000 RPM in Park, to Full load in Drive, while comparing cylinders from behind the wheel.
The whole process takes less than four minutes, hunting for any odd ball that does not match the “good profile”.
If no deviation from the normal is detected, we can give the engine a clean bill of health and we are done. From experience I have learned that before getting focused on what is bad, spend first a few minutes to verify and determine what is good. By performing this routine we may observe and record at what condition the malfunction occurred. In many cases it is a misfire, but in order to do logical analysis we need to know:
Is it LOAD related – SPEED related - or is it at low RPM in drive.
The second observation is:
Does it effect ONE cylinder, ONE bank, MULTIPLE or ALL cylinders.
The third important factor is:
Is the occurrence “ONCE IN A WHILE”, comes and goes, or SWITCHES
from cylinder to cylinder.
All these observations play a role in conclusive diagnosis and are easily pinpointed taking the time to follow this procedure. But most importantly, by doing the total test sequence we can determine what cylinder is next in line, or is close to becoming a future misfire. It may prevent an unhappy customer when the same problem occurs a month or so later. If you have to do the hookup, you might as well spend the extra few minutes.
All these observations play a role in conclusive diagnosis and are easily pinpointed taking the time to follow this procedure. But most importantly, by doing the total test sequence we can determine what cylinder is next in line, or is close to becoming a future misfire. It may prevent an unhappy customer when the same problem occurs a month or so later. If you have to do the hookup, you might as well spend the extra few minutes.
Al it takes is an adapter to convert a 2-channel scope into an 8 cylinder diagnostic capability. But the most important mind set is "ANALYZE, DON'T MEMORIZE"
Go to the above website for an illustrated PowerPoint presentation
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