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I have a 2002 Jeep Wrangler X that has been giving me and my technicians fits. The Wrangler is a 4.0L A/T VIN (1J4FA39S92P730409). Customer brought vehicle to us for a strange intermittent crank no/start condition. Vehicle has never stalled on customer and runs great after it is started, with no stalling issues ever happening. Vehicle currently has no codes stored in it. The condition happens both hot and cold, and does not seem to intensify after a heat soak or test drive. We have been able to duplicate the concern, but after a couple weeks of testing have not had any luck with it. The vehicle is losing injector pulse intermittently, and will crank anywhere from 10 seconds and start or other times crank several times in a row and will not start, but the next time you try will start right up. During the no crank we have confirmed loss of injector pulse, and fuel pressure is maintaining at 45PSI. This vehicle has a large 1-piece coil and is difficult to confirm spark output on, but after our findings listed below we are confident there is no spark as well. During this no crank concern we are losing communication to our scan tool as well. We have traced the concern down to a loss of 5 volt feed from PCM to the crank/cam/MAP and TPS sensors. To get to this point so far, we have a lab scope back probed to crank sensor signal wire at the connector of CKP sensor, volt meter hooked to both pins a17 and b31 at the PCM to monitor our 5-volt output to the sensors, and scan tool hooked up to the vehicle. What we are currently seeing is when the vehicle will not start with the key on, we have 5 volts at the PCM output, but as we crank the vehicle during the crank-no start condition after approximately 1 second of cranking the 5 volts is dropping to .3-.4 volts the entire time we are cranking, and the scan tool loses communication during this time so we are unable to watch RPM signal. As soon as the key is released the 5 volts will come back and the next cycle the vehicle will either start and retain 5 volts or will do the same thing and drop voltage while cranking. When cranking if the condition happens our crank signal on the lab scope goes flat to nearly 0 volts and we have monitored the cam sensor signal with the same result. I have seen several times on these engines the CKP sensor will short internally and take down the 5 volt line, but this is ONLY happening during cranking, never key on loss of 5 volts. We cut both 5 volt supply wires listed above at the PCM to the sensors and put in toggle switches to turn off during the condition to rule out any sensors pulling 5 volts to ground. We were able to duplicate the concern, turn of the switches during cranking and the voltage remains at .3-.4 volts. Like I stated before as soon as we release the vehicle from crank to run the 5 volts comes back. I have load tested all powers and grounds at the PCM and all checked good which led me to believe the PCM internal 5 volt regulator was failing and causing the concern. We replaced the PCM with rebuilt unit, with the same concern. I have also replaced the ASD relay for good measure being that it does feed power back to the ECM. At this point I am a bit lost on which direction to go, as it seems to be a 5 volt supply issue, but after taking all of the sensors out of the equation was confident this was an internal ECM issue. Any input would be great as we are pulling our hair out at this point!
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