Expert: Roland Finston - 2/4/2012

Question
I have a no start situation here on this Chrysler and I checked the signal at the ckp sensor with a vantage and got a square wave from the pcm output. So i checked the cmp sensor by disconnecting the plug from the sensor and tried cranking it and the car starts up. and when i turn off the car and plug it back in there is the no start problem. Can you please help me by guiding me where to go from here with my diagnostic. Thank you very much.

Answer
Hi Gabriel,
The 725 says that the engine speed sensor circuit is unrelialble. It may point toward the crank sensor but it is also possible that trans input speed sensor is the cause, I can't be certain. The 1391 means an intermittent loss of cam or crank sensor output. Based upon the engine running without the cmp and that it won't start with the cmp I would believe that you probably have a bad cmp or cmp wiring. The two sesors share a 5V supply (orange) and ground wire (black/light blue), so if the issue were wiring it would be the signal wire at fault. The crank signal wire goes to pin 32 (gray/black) while the cam signal goes to pin 33 (tan/yellow). If the wiring checks out, the cam sensor would be the one I would replace first, but you might try to check out its voltage pulse signal as well as you did the crank sensor's.
Roland
PS You can check the cmp by measuring the signal with a voltmeter while you rotate the engine by hand via a socket on the crank pulley bolt and with the key in the 'run' position. It should oscillate between 5 and 0.3V frequently. Measure between the signal and the ground wire.