Auto Service Professional

OCT 2018

Magazine for the auto service professional

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12 A S P O c t o b e r 2 018 spark KV point and represents the point at which we have established current flow across the spark plug air gap. Notice on the vertical scale the spark KV point comes in at about 1.5 KV. is point is critical to monitor because lean density mis- fires will cause this point to vertically jump up and down. A fouled spark plug will elevate this point, followed by a sloping downward spark line. e most critical point occurs at Point E, and I have always referred to this point as our electronic window into the combustion chamber. is is known as the spark line and represents the spark duration period. Notice the time base on the scope is set at 1 millisecond per division. Look closely at the duration period, which measures 1.3 milliseconds. Normally you should never see this value below 1 millisecond during idle and no load conditions. Lean conditions and high secondary resistances will shorten this duration period. On the contrary, low cylin- der compression problems and rich density fuel problems will drastically lengthen this period. One point that needs to be made here is this waveform example is from a distributor- equipped engine. On a COP type ignition system the spark duration period will measure much longer since we only have the one authorized air gap being the spark plug air gap. Normally you should not see a spark duration period below 1.5 milliseconds on the COP systems. It is normal on these COP systems to maintain a spark duration period between 1.5 to 2.0 milliseconds. When monitoring the spark line there are three areas to always focus on: length (duration) at idle, angle and the presence of turbulence during a light power brake condition. Going back to distributor-equipped engines, we simply clamped our secondary KV probe around the coil wire and clamped our synch probe around the #1 plug wire. We could view the secondary event in a parade pattern, a raster pattern, a super imposed pattern or we could in- dividually look at each cylinder's event. For those techs who have the modern day lab scopes such as the Mastertech 5200, the Snap-on Vantage Pro, the Snap-on Modis, the Verus and the Zeuis, I g n i t i o n W a v e f o r m D i a g n o s t i c s Figure 3: Secondary KV probes When using a KV probe on a lab scope always use the peak detect mode. The one in the middle is the standard secondary KV probe from Snap-on. The one on the right is a universal KV probe. The unit on the left is a COP wand. Keep in mind that secondary ignition is negatively fired on COP and distributor-equipped engines. You will need to use the invert function of your scope to view negative firing events. On DIS systems half of the cylinders are fired negatively while the other half are fired positively. COURTESY OF VETRONIX CORP. Figure 2: Anatomy of a secondary waveform Pont A is the point of primary turn-on and we are starting to saturate the coil. Point B is the point of primary turn-off. So affectively we have charged the coil. At point B as we turn off primary, the magnetic field collapses and gets mutually inducted into the secondary windings of the coil and the voltage potential is multiplied by 100 to give us the high voltage needed to arc across the pressurized plug gap. Notice that the scope is set at 2 KV per division on the vertical scale. Pont C is known as the firing line and measures a good 12 KV demand. Point D is known as the spark KV point and represents the point at which we have established current flow across the spark plug air gap. Notice on the vertical scale the spark KV point comes in at about 1.5 KV. This point is critical to monitor because lean density misfires will cause this point to vertically jump up and down.

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