Auto Service Professional

OCT 2018

Magazine for the auto service professional

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16 A S P O c t o b e r 2 018 that the attenuation factor on these probes is 1,000 to 1. is means that if you set the scope at 1 volt per divi- sion, the voltage scale is now 1 KV per division. ere are three examples of KV probes in Figure 3. When using a KV probe on a lab scope al- ways use the peak detect mode. e one in the middle is the standard secondary KV probe from Snap-on. e one on the right is a universal KV probe. e unit on the le 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 in- vert 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. e early model Ford and Chrysler COP units were not heavily potted, so getting a good stable secondary igni- tion waveform was easy. e Asian and European COP units and the late model Ford COP units are heavily potted and with the scope setting at 1 V or 1 KV the magnetic field is too weak for the scope to sense. e key here is to drop the volt- age per division down to .5 V per division which means with the 1,000 to 1 attenu- ation factor we now have a setting of 500 volts per divi- sion. e lower voltage set- ting allows the scope to pick up the collapsing magnetic field from the coils' firing event. In addition, the COP wand can be used on DIS plug wires as well as the 8 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 7: Secondary raster (lean cylinder) Another example of a lean density condition from a GM V6 4.3L Vortec engine. In this raster pattern notice the spark line duration, angle and presence of turbulence during a light power brake condition. This is another example how lean density conditions caused by bad or restricted injectors can be detected by reading a secondary ignition waveform. COURTESY OF VETRONIX CORP. Figure 6: 1999 Nissan Quest secondary (intermittent misfire, no codes). Symptom system separation logic? This shows a raster pattern from a distributor-equipped engine. Notice the spark duration period of the #3 spark line. In addition, notice the abrupt upward angle of the spark line indicating a lean cylinder. The waveform was captured at a light load 1,100 rpm power brake condition. The resistance specification for the injectors on this engine is 14 to 16 ohms. On this vehicle, the number 3 injector resistance measured 145 ohms, thus causing a lean density misfire. COURTESY OF ROBERT BOSCH LLC

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