Magazine for the auto service professional
Issue link: http://asp.epubxp.com/i/636156
57 | February 2016 was OK. The PCM was only detecting a mis- fre on the Number 2 cylinder; at this point I was confdant in saying that the issue wasn't mechanical or a fueling issue: it had to be ignition. But how could it be ignition? All the parts of the cylinder Number 2's ignition system had been changed and were now new genuine Ford parts. But the same problem persisted. In town this truck never misfred, only during hard acceleration or under heavy load did it act up. I went back to something that I hadn't looked at for a while. I visited the Motor- craft website and started to look for the OBD-II information that Ford provides there. Remember that it is the OBD-II system that is turning on the CEL light. I found the information that I was looking for under the header "OBD-II Theory & Opera- tion" (Ford and most manufactures provide this information free on their diagnostic websites). Looking through the document I found the section on misfre monitor, and after reading it carefully, I saw something that I had missed: that an ignition coil's primary circuit failure can also cause fuel injector deactivation (FMEM). The primary ignition circuit is in the PCM, so I hooked up my current probe to the Number 2 ignition coil and went for a road test and fnally saw the problem, it was a bad driver circuit in the PCM. The PCM was not able to supply the ignition The Tech Bench The road test showed that it was the Number 2 cylinder that was causing the fashing check engine light.