Auto Service Professional

FEB 2018

Magazine for the auto service professional

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14 | ASP February 2018 Technical density conditions the primary spark duration periods will be too short and lean cylinder conditions will abruptly bend the spark line voltage up during a power brake condition. On the Chrysler and Ford COP units, a secondary KV wand can be used to pick up a secondary waveform by simply laying the probe on top of the coil. Keep in mind the attenuation factor of the KV probes are 1,000 to 1. is means that if your scope is set to 1 volt per division the attenuation is now 1 KV per division. A time base of 1 ms. per division is usually ideal. e secondary KV wand can also be used on DIS secondary leads, as well. Half the cylinders are fired at a negative polarity and the other half are fired positively. On the cylinders that are fired with negative polarity, you must use the invert function on your scope while turning it off while viewing the positively fired cylinders. By the way, all distributor-equipped engines and COP-type ignition systems fire secondary at a negative polarity, meaning that you must use the invert function of the scope. e secondary KV wand will not work on most Asian COP units because they are heavily potted causing the magnetic field to be too weak to be sensed by the COP wand. is will require a secondary lead between the coil and spark plug and using the conventional KV probe around the lead. Remember that we refer to the spark line characteristics as our electronic window inside the combustion chamber. ere are several versions of the secondary KV wand available at On the GM coil near plug units found on the V-8 Vortec engines, there is a short 8.5-inch secondary lead between the coil and spark plug. e COP wand works very nicely on these systems by laying it next to the plug wire, or you can simply use the conventional secondary KV probe clamped around the plug wire. Again, the invert function must be used. ere are three different vendors that supply the coils for GM. ey are Delphi, Melco and Denso. e coils are not interchangeable, but the secondary leads look identical and are not interchangeable because the resistance values vary greatly. As we stated earlier, access to the primary side of a Ford or Chrysler can be done by back probing the negative terminal of these coils and thus viewing the primary side of the coil. at test is not possible on the GM coil near plug units because the ignitor is integrated into each individual coil. e PCM uses low current drivers to bias (turn on) the coil current. e signal from the PCM is a 5 volt / 0 volt toggle. e rising edge to 5 volts turns primary on while the falling edge to 0 volts turns primary off causing the primary magnetic field to collapse, which is mutually inducted into secondary and multiplied to create the high secondary firing voltages needed. A tech tip here may be needed. During KOEO we can bias these coils with a standard 12 volt test light. By picking up 12 volts with the alligator end of the test light and by pierc- ing the ignitor control wire and momentarily touching the ignitor control wire, we will fire a 25 KV spark tester. While we are on the subject of using a spark Figure 4 Figure 5

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