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2000 Jeep Wrangler 4.0L Code P1391

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Chrysler P1391 Intermittent Loss of Crank or Cam Sensor Signal
This code can pop up if the PCM loses an input signal from the crankshaft (CKP) or camshaft (CMP) position sensors when the engine is running or cranking — but only if it caches 20 failures in two consecutive trips. The cause may be a bad sensor, a loose or corroded sensor connector, a defect in the tone wheel or flexplate, or a fault in the PCM itself.

Inspect the wires for any obvious problems, then connect a scan tool and look for an rpm signal, and/or the CKP and CMP PIDS. Or, you can use a scope to backprobe the sensors to look for a good output signal. If you are not getting a good sensor signal, remove the sensor and inspect the tone ring underneath it for debris or damage. If the sensors are producing good signals, the fault is in the wiring or PCM.


Synchronization Roulette
The next morning, the Jeep started perfectly. As the day progressed, the Jeep would produce a CE light and a P1391 code each time the engine was cranked. Obviously, the mechanical relationship between the CMP and the CKP was changing — sometimes a little, sometimes a lot. Further TSB and archival research indicated that the 4.0L historically has had a problem with the bearing seizing in the cam position sensor and twisting the housing out of synchronization with the crankshaft sensor and producing the DTC P1391 and the accompanying P0352, 353 and 351 DTCs.

20720agif_00000013820.gif Fortunately, many aftermarket scan tools can detect CMP synchronization problems in the 4.0L through a synch mode feature included in the scan tool menu, see Photo 2. Simply put, the CMP can be adjusted in the synch mode to within +/- 1 or 2 degrees tolerance.

In this case, the CMP synch was nearly perfect, which dismissed any problems with a seized cam sensor bearing. I might say at this point that I normally don’t start a diagnosis by disassembling components because a faulty connection might be re-established or a cracked circuit board might be reactivated. But, in this case, I removed the cam sensor only to discover the magnet stuck to the CMP’s shutter! See Photo 3.

20720bgif_00000013821.gif Clearly, I had taken the “scenic route” on this diagnosis by not inspecting the CMP for loose components in the first place, see Photo 4. But, the loose magnet clearly was causing an intermittent synchronization problem that had created a roulette wheel effect that resulted in the occasional slow-start or cranking/no-start complaint that cropped up after the vehicle came into the shop. The diagnostic curve ball in this case was clearly the unexpected issue of a magnet coming loose in the camshaft position sensor without causing a catastrophic failure.

Edited by HarrytheCarGeek
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I used to have a TJ, and had this exact same issue, except my problem was excessive crankshaft end play. In my case, like yours I would only set the code during cranking. Sometimes, it would have an extended crank time before starting. I chased the problem on my free time for a couple of weekends, checking all sensors, and data pids. Found nothing out of the ordinary The refresh rate on many scantools is just not quick enough to capture any glitches during cranking. These systems are old and slow. You can also check the cam and crank sensors manually, by passing a flat bade screwdriver (without magnetic tip) across the tip of the sensors. I did this, while hooked up to a scope. I heated and cooled the sensor, nothing abnormal. eventually I got to thinking and said "ok, this only happens during cranking." I disable fuel delivery, hooked up a remote starter and just would crank it over and over while observing data. Nothing abnormal. I eventually caught the glitch inadvertently, when i jumped in and stepped on the clutch and cranked it with the key. Occasionally, I would lose an RPM signal. I said hmm. Cranked it with my remote starter, all is well. Step on the clutch? Intermittent loss of RPM signal. Checked crankshaft endplay, and it was out of spec by several thousands.

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