Auto Service Professional

OCT 2018

Magazine for the auto service professional

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18 A S P O c t o b e r 2 018 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 inch secondary leads found on General Motors' coil-near-plug equipped engines. e time base on the scope should be set to 1 millisecond per division. e time base can be reduced to 500 micro-seconds per division to get a better look at the spark line characteristics. Another option would be to insert a second- ary lead between the coil and spark plug and clamp the conventional KV probe around the secondary lead. e Pico scope people make these leads available. Now let's begin on a conventional distribu- tor-equipped engine with a misfire under light load. We use a conventional KV probe clamped around the coil tower of the distributor cap. is distributor has a built-in coil so there is no coil wire. Now notice the secondary ignition wave- form in Figure 4. is was captured during idle no-load conditions. Notice the very short spark duration period of the spark line. It is actually below 1 millisecond. e tech has previously replaced the spark plugs with no improvement. Since this is a cap and rotor-equipped engine he proceeded to replace the cap and rotor and plug wires. Remember that on distributor-equipped engines there are two authorized air gaps — the rotor air gap and the spark plug air gap. Notice now Figure 5. Do you see a much longer spark duration period? e misfire is now gone. Let's look at another example from a dis- tributor-equipped engine from a Nissan with a dead miss with no codes. Figure 6 shows a raster pattern from this 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. e waveform was captured at a light load 1,100 rpm power brake condition. e resistance specifica- tion for the injectors on this engine is 14 to 16 ohms. On this vehicle, the number 3 injector re- Figure 8: Dodge Caravan 3.8L (secondary super-imposed) All cylinder firing events are stacked on top of one another. Notice that two cylinders' firing events show an elevated spark KV point. This occurs from the carbon fouled spark plug electrodes. In addition, notice the sloping downward spark line voltage from a rich condition. Remember, fuel molecules are conductive, thus drastically reducing the spark line voltage indicated by the sloping down spark line.

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