P1391 after replacing crank position sensor - Page 2 - JeepForum.com
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post #16 of 29 Old 08-23-2010, 01:08 PM
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When changing topics, it is best to start your own thread.

The most likely cause of it cranks and cranks but won't start up is the Crankshaft Position Sensor (CPS) located on the transmission bell housing. Often this part is also referred to as the CranKshaft Position Sensor (CKP) CPS/CKP failure is very common. The CPS/CKP can stop working with no warning or symptoms and the engine will not run or the engine may randomly stall for no apparent reason.

Crank Position Sensors can have intermittent “thermal failure”. This means that the CPS/CKP fails when the engine gets hot, but works again (and will test as “good”) when it cools back down.

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Do the same for the grounding wires from the starter to engine block, and from the battery and engine to the Jeep's frame/body. Jeeps do not tolerate low voltage or poor grounds and the ECM/ECU may behave oddly until you remedy this.

Symptoms –
- Starter cranks and cranks but engine won't start up and run.
- Fuel gauge and voltage gauge may not work or display properly.
- You sometimes will have No Bus on the odometer after 30-60 seconds.
- A failed CPS/CKP may or may not throw a CEL trouble code.
- No spark at the spark plugs.
- Fuel pump should run and prime for 3-5 seconds.

If the CPS/CKP is failed sometimes the OBD-II trouble code reader cannot make a connection to the ECU/computer or cannot read Check Engine Light/MIL codes because the CPS/CKP has failed. Disconnect the code reader, disconnect the CPS/CKP wire connector, and reconnect the code reader. If the code reader establishes contact with the ECU and scans, your CPS/CKP is failed and needs to be replaced.

Diagnostic steps to confirm the CPS is the cause of your no-start

You should be able to verify a bad cps, by unplugging it, and turning the ignition key to on. If the voltage gauge and/or the fuel gauge now displays correctly and/or the No-Bus is gone, replace the CPS.

Unplugging and reconnecting the CPS sensor where it connect to the main harness near the back of the intake manifold usually resets the ECU and if the jeep fires right up after doing this you can bet that the CPS is faulty and needs to be replaced.

Exchange the fuel pump relay and the ASD relay with one of the other similar ones in the PDC to eliminate the relays as the cause of the no-start. Check the ASD relay fuse.

Confirm that the fuel pump to runs for 3-5 seconds when you turn the ignition key to ON.

Eliminate the NSS as a cause of no start. Wiggle the shift lever at the same time you try to start. Put the transmission in Neutral and do the same. Do the reverse lights come on when the shifter is in Reverse?

Inspect the wires and wire connectors at the O2 sensors on the exhausts pipe. A short circuit from melted insulation or from broken O2 sensor wires can blow a fuse and the ECU/ECM will loose communication. Don’t get tunnel vision and assume the sensor is bad (unless it tests bad with a meter). Damaged wiring or a dirty connector can inhibit the signal from making it to the computer. Inspect/test/clean/repair wiring or connectors as necessary.

CPS Testing

Crankshaft Position Sensor Connector (CPS/CKP)


TESTING PROCEDURE 1991 – 2001 4.0L H.O. engines

1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C (See Image). Ohmmeter should be set to 1K-to-1OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

TESTING PROCECURE for 1987 – 1990 4.0 L engines

Test # 1 - Get a volt/ohm meter and set it to read 0 - 500 ohms. Unplug the CPS and measure across the CPS connector's A & B leads. Your meter should show a CPS resistance of between 125 – 275 Ohms. . If the CPS is out of that range by much, replace it.

Test # 2 - You'll need a helper for this one. Set the volt/ohm meter to read 0 - 5 AC volts or the closest AC Volts scale your meter has to this range. Measure across the CPS leads for voltage generated as your helper cranks the engine. (The engine can't fire up without the CPS connected but watch for moving parts just the same!) The meter should show .5 - .8 VAC when cranking. (That's between 1/2 and 1 volt AC.) If it's below .5vac, replace it.

The 2000 and 2001 will have the CPS in the same location on the bell housing, but the wire connector may be on the passenger side, not as shown in the diagram below. Simply follow the wire from the sensor to the connector.

If this information has been helpful, please click on the "thumbs up" icon on the bottom right side of this section.



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post #17 of 29 Old 08-23-2010, 01:16 PM
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Thanks for the large amount of info Tim. That should get me started nicely. I thought my question was someone related to the discussion but next time I won't be lazy and will start my own thread. My apologies to the OP.

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post #18 of 29 Old 08-25-2010, 02:13 PM
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Thanks guys good info
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post #19 of 29 Old 12-21-2012, 06:38 PM
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Hi, 2001 PT Cruiser here. I found this forum. Pretty much everything everyone else has done was done to try to get the P1391 Code to clear. I finally found the problem to be the wiring harness that runs around the back of the engine. The wire tie broke and let the harness drop and run the axle. The harness has a mesh sleeve over in. When sitting in the garage jacked up, the harness was a couple of inches away from the axle. It must have dropped when driving the car. It missed very badly under load. When I pulled the harness down to look for problems, it rolled over and I didn't see that one of the crank position sensor wires was worn in two. I pulled the harness up out from behind the motor and spliced the wire back together. It now has two layers of split loom taped around it and 10 wire ties holding it in position. It runs great again.

For everyone that has had the P1391 Code, I feel your pain. It has been raining for days here and every time I took it out and found it still wasnt fixed, then pulled back into the garage to work on it and crawled under the car, it dripped on me making it harder to find the problem. I was a couple of hours away from setting the car on fire and pushing it over the hill.
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post #20 of 29 Old 12-22-2012, 08:00 AM
Real Jeeps have dents
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While the OBD diagnostics system and the P trouble codes are helpful for narrowing ones search for problems, having specific codes does not always equal having a specific failure. If the ECU engine computer cannot see data from a sensor, it reacts the same as if the sensor itself has failed.

A wise troubleshooter should always start with the simple, cheap, and obvious solutions and then work their up to more complex fault possibilities. First you should always inspect the wires, and wire plugs to the systems/sensor covered by trouble codes. As suggested, look for chafed or melted wire insulation, corrosion, missing/bent pins in the wire plugs, and damaged or pinched wires. Next, test the sensors for proper function.

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post #21 of 29 Old 12-22-2012, 09:31 AM
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post #22 of 29 Old 12-22-2012, 10:15 AM
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the one reason I carry a space in my glove box in the last 15 years these have left me stranded 2 or 3 times..

Gotten pretty good at replacing them too.
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post #23 of 29 Old 12-23-2012, 09:08 AM
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My aftermarket sensors have been fine but I still carry spare CPS and CKP in the toolbox.

My build thread has pics again. https://www.jeepforum.com/forum/f177/kevin108s-2000-xj-build-867428/
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post #24 of 29 Old 03-11-2014, 10:56 PM
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I know this is an old thread but im just now having a problem...... so

Tim, thank you for all your help and your detailed info
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post #25 of 29 Old 01-22-2016, 06:34 PM
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First time posting.
Had the same problems (CPS and CPK Codes) rough idle or no start. Read posts, here and other forums. Was going to re-replace (distributor style) Cam Position sensor but decided to garb hold of the tone spinney thingy that the sensor senses. (Don’t do this while it’s spinning). I thought to myself “Hmmm that’s odd…. this spinney thing rotates freely. This is supposed to be, directly, geared to the oil pump / crankshaft” I then decided to pull it out….”Clink clank clunk” The small gear that was on the Cam position sensor shaft dropped into the oil pan.

The spring pin that holds the gear to the Cam position sensor shaft had sheared off. Once I got the gear out of the oil pan and replaced the whole assembly the engine worked fine.

Hope this helps.
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post #26 of 29 Old 03-26-2016, 09:26 AM
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Cool P1391 code

Originally Posted by nousernamesleft View Post
Hey guys (n gals)..new member here with a problem. I'm trying to fix a friends 2001 cherokee classic. He was having cylinder misfires and multiple cylinder misfires, occasionally the car would shake badly on idle. a new coil pack was fitted, new plugs, oil change, new idle sensor.
He just got back from a 200 mile drive and complained that when he got there the a/c wasn't working (blowing warm air i think) the car was very lumpy at idle, and when it stalled would not restart until 1 hour later. i checked the OBD and it had a multiple cylinder misfire code and cylinder 1 & 3 misfire codes.
I just got replaced (20 mins ago) the crank position sensor and now there is a P1391 code and it feels like the engine is misfiring or something, when it had misfire codes before the engine idled very nicely ( except for the few lumpy occasions)..I don't get it...any idea what could be going on.
now you need to download a timing sink program in the pcm and it will run like new
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post #27 of 29 Old 12-15-2016, 07:06 PM
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Yes, it will cause your jeep to die as you're going down the road. I was going about 65 mph and lost ALL power, put it in nuetral and start her right up. I bought a new crankshaft sensor from the Jeep dealership yesterday for $117. I took out the old one and put in the new one last night. Not sure if the swap fixed my problem yet but just writing to explain how difficult it was to get the old one out and put the new one in. That sensor is way up in there. I needed a two foot long extension, a swivel socket connector, and a 7/16 socket (I think an 11mm works as well). Another tip, take out the small bracket that holds the wiring for the crankshaft sensor and the oxygen sensor out of the way. That made it much easier to get the long extension up in there. Getting the new one in was very difficult for me. Off to reset the PCM. Good luck.
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post #28 of 29 Old 01-12-2017, 09:18 PM
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Well it's been about a month and still no stalling. Looks like it was in fact the crankshaft position sensor. Was fairly difficult to replace. Well worth the $117 at the dealer.
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post #29 of 29 Old 08-03-2017, 01:01 PM
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Mine was the Cam sensor. The Hall effect magnet actually broke off of its weld in the sensor. Said mad in mexico. Supposedly changed with the engine 30k ago. Somebody returned a used one to the auto parts store and it was the only one they had. I figured what the hell, lets give it a try and it worked. Hooray for me. will see if it lasts.

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cam position , ckp , cps , crank position , p1391

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