Auto Service Professional

FEB 2018

Magazine for the auto service professional

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Page 17 of 61

tester, keep in mind that all good COP units can easily fire an ST125 spark tester. For those of you who use the adjustable spark testers, a 3/4-inch gap is very close to a 25 KV demand. One important note here on the GM coil near plug units: e coils are powered up from a single main ignition relay and all the coils and ignitors units get their ground at one location, so don't forget about power and grounds (see Figure 6). Your scan tool may have the ability to turn on this relay during KOEO. In addition, the PCM monitors this voltage from the ignition relay so you will have a scan tool parameter to monitor the ignition feed values from this relay. I recently had a 4.8L come in with an inter- mittent miss and a P0300 MIL. e owner had previously paid a shop $1,500 to replace the plugs, coils and secondary leads — to no avail. e freeze frame data indicated a 45% addition to the short- and long-term fuel trim values. Initially my first suspicions were from a lean density misfire, since all the cylinders on bank 1 indicated multiple live and history misfires. On my diagnostics using my secondary KV probe, I found no secondary events happening on the bank 1 cylinders. By probing the pink power supply wire to the bank 1 coils I found no voltage. An open circuit at the connector on top of the valve cover was the fault. A current probe clamped around this power supply pink wire would have also pinpointed the problem. On most all Asian COP units the coils also have the ignitors integrated into them and are forward biased (turned on) by a 5 volts square waveform. As with the GM coil near plug units, the falling 5 volt to 0 volt toggle turns primary off and fires the coils. Using a COP wand in the Asian COP units does not yield a good secondary ignition waveform because the coils are so heavily potted the magnetic field is too weak to pick up. e website offers the secondary leads to marry the coils to the spark plugs, meaning you can use a secondary KV probe to view secondary in the conventional way. Again, access to the primary side is not possible due to the ignitor integrated inside the coil. However, an amp probe clamped around the coil positive feed wire can verify good or insufficient coil saturation values. e point here is most critical. A single cylinder misfire can easily be a bad coil, but looking at the design of the system, a control circuit issue or a bad PCM driver could also cause a loss of spark. In the real world of diagnostics, whenever a misfire problem occurs we as technicians usually rely on scan data initially. Having said that, the cardinal rule is that a single cylinder misfire from no spark will create very minor and very brief single digit fuel trim corrections while a lean density misfire from, say, a bad injector, low fuel pressure, vacuum leak or a bad MAF sensor will create double digit addition to fuel trim. Conversely speaking, a rich density misfire will create double digit negative fuel trim corrections. Also keep in mind that in most modern day systems, whenever the misfire is severe enough the PCM 16 | ASP February 2018 Technical Figure 6

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