BruceB

1996 Cherokee Classic 4.0L 2WD Auto Trans 181,000 miles

Within last 50000 miles replaced CPS with OEM, replaced Distributor with aftermarket brand, alternator, coil, ignition wires, and many other parts not applicable to this problem, I think.

Drove it this morning and at 45 mph the vehicle bucked like it had loss power just for a split second, enough to make the locker in the rear end thump then started to repeat this every 2 to 4 seconds, unless accelerating or holding in it a lower gear and keeping the rpm's above 2500. At idle it would drop 100 rpms and immediately recover repeatedly. I was about 14 miles from home so I started to head that way. After a few miles the check engine light came on. All gauges read ok. Then the GEN light came on and I watched the battery/charge gauge go from 14v down to the red line by the 9v.

I made it home and checked the voltage in the battery with a tester and it was 12.5v I thought it would be lower after driving it without the alternator charging.

I plugged in the obd and have the code P1391.

Looking for advice on this order or repair.

Reset the ECU to determine if the ECU errored.

If fault code and GEN light is still there, test out of alternator to verify output to determine if the issue is with the altenantor or ECU. If Alternator output is low, then replace alternator. If not, replace ECU?

Prior to replacing alternator or ECU, replace distributor and check for engine code. If code is gone, then recheck for GEN light and gauge output.

If code is still present, replace CPS.

Could code P1391 cause the ECU to send a signal for the GEN light and show low output?

thanks in advance for any advise or comments.

tjwalker

I would put thoughts of replacing that computer out of your mind for now. Computer fails certainly can happen, but they really don't happen all that often. The time to replace the computer is when every other possibility has been fully explored and diagnosed. It is a "diagnosis of exclusion".

Simple stuff first; always. Assuming the engine does start and run now?

First thing on your to-do list is to verify your electrical/charging system. That involves both testing of your alternator AND load testing of your battery. Keep in mind that some parts store bench testers don't fully test an alternator and you can get a "pass" in the store where it truly doesn't cut it installed in the vehicle. A quick and dirty test of the charging system is to put a simple meter on your battery. At a bit of a high idle, you should see approximately 14.4 volts, give or take a little. Any parts store will perform a load test on your battery for free.

The P1391 is "intermittent loss of cam OR crank sensor signal." I know you replaced the crank sensor and the cam sensor (within distributor) within the last 50,000 miles. That does not mean that one of those sensors isn't failing. I've had crank sensors last 200,000 miles and I've also seen them last 200 miles. Yes, even OEM Mopar, although the reliability of the OEM Mopar cam/crank sensors vastly exceeds others. If you continue to throw just a P1391 code, my experience is that is more often related to the crank sensor than the cam sensor.

So verify your charging system and be sure you have a solid battery that passes load testing, then clear the codes and see if the symptoms/codes return. See where you are at after that and come up with a further diagnostic plan if necessary. Use a methodical testing based strategy.

Good luck and keep us updated!

BruceB

Tested voltage at the battery with engine off and then running. No difference in voltage.
Turned engine off, turned ignition on, sent clear codes to ecu. Check engine light went off.
Restarted without shutting ignition off after clearing codes.
Engine runs and idles without any problem or hesitation. Also the voltage at the battery with the engine running went to 14.2v as compared to when the engine is off 12.5v.
Going for a test ride now..

so... the $64,000. question. what to do now. Something caused this. Should I just wait and see if it happens again, or be proactive and get some parts here on stand by?

fyi--
Took about a 12 mile urban test drive with no problems. Acceleration, maintaining speed, idling with no problem. Rechecked for fault codes and there was none.

tjwalker

Can't buy parts as you wouldn't know what to buy. Going to have to wait until if/when the problem repeats.

You still should LOAD TEST that battery. Just measuring voltage at battery really tells you very little about the condition of the battery. Load testing tells you the true condition.

14.2 at idle means the charging system seems to be working, so you're likely okay there.

Mr2Jeep

I don't mean to hijack this thread, however I'm following it. I started having a similar problem (engine cutting off momentarily). After a few weeks it finally threw a P1391 code, no GEN light though. This all started after I replaced the spark plugs, wires, cap and rotor. My first thought was that moving the clump of wires at the rear of the engine to access the spark plugs, I broke one of the wires in the clump. Thus causing a break in the electrical circuit. It's been a while and it's still not resolved.

97grand4.0

sounds like a loose connection on the leads to the alternator, or the fuse block, or the battery, and or the grounds.

Jeepsurgeon

I may have found the answer to this P1391.I had the same symptoms while out driving around
a perfectly running jeep and suddenly missing,rough idle,
jerking and backfiring through the intake destroyed my air filter.Then the jeep sat for four weeks in freezing temperatures as I ventured out to test every plausible test I could do on the cam and crank sensors resistance in
/ constant 5 volts/signal trigger 5 volts
while hand cranking crank bolt
Checked the distributer for play removed it and inspected it
even pulled the balancer tocheck to confirm it didnt jump time.
Totally stumped.
it would start and run very roughly at idie.
I had to recharge the battery twice.Got suspicious about charging the battery twice.because the charging guage looked about right and it did Not throw a gen light.
I then did a voltage test on the alternator which showed 12.2 volts.
Too low.I then did a voltage drop test from the battery to the alternator.The alternator itself was only putting out 12.2
I could nt ind the right size brushs in town so a bought some oversize brushs which I sanded down on some 220 grit sand paper which I soldered in and bingo 14.2 volts
And jeep was running perfectly except for the P0132
and P0171 that suddenly appeared and a little rough running
I clearned the P0171 with a new air filter and cleared the P0132 by pulling the 02 sensor bank 1 upstream sensor
and tested it for resistance less that 10 ohms is good.
blowtorch heat tested for variation between 0.0 0 and 0.90 volts.Cleaned off an excessive amount of carbon with carb cleaner.while the jeep was running tough
During its 4 week sit it was running rough at full rich
which completly covered the 02 sensor.after cleaning and testing it tested good.
the jeep ran great for two days and suddenly had the problem again.i waited until pitch dark and had my wife start the jeep.I spotted the culprit causing the issue
the wire that supply's the voltage from the alternator to the fuze box was arcing on to the block of the motor
the wire comes off the alternator at almost a 90 degree angle before it bends upwards at 90 degrees towards the fuze box.I reached down and pulled d the black/grey wire towards me and away from the block and boom ran as smooth as it had in the previous two days. The heat from the block just broke down the resistance of the insulation.I will replace the Stripped Black/grey wire that supply's the output voltage.and vf that should fix it for good.
Hopefully this scenario can be usefull to somebody
That is struggling with the P1391 code.
I've been driving this jeep for 7 years without any major issues and this one whooped my ***.Hope it helps someone. Good luck

BruceB

Well, my bad...never came back and updated post.
The problem ended up being the cam sensor/distributor.
The aftermarket distributor was a freak when it came to replacing just the cam sensor. Different size I guess because any other cam sensor didn't fit.
Replaced the whole distributor and haven't had a repeat of the problem since.

97grand4.0

And that was almost 3 years ago.

BruceB

yea...my bad...

sorry for the late update...

97grand4.0

No worry. What's actually interesting is your note about the cam sensor. One, yes, they are different ones that all will go under the cap but have different configurations on the connector. Two, you said your new distributor and the sensor that came with it are still going. This was not my experience at all, most box store distributors come with aftermarket cam sensors installed. Mine did, Great distributor but the cam sensor failed in about 2 weeks. With the 96 and 97 years the pcm wasn’t so good at pinning the failure down either. As we know, Mopar only on the cam and crank sensors.

BruceB

Interesting enough I had the original distributor replaced in 2007 due to the bearing wearing out. I have no idea what brand the shop put in then, but it lasted until 2018.
This time it was replaced with a Spectra Premium distributor. Hopefully it will last as finding a new Mopar distributor is like looking for needle in a haystack.

97grand4.0

I think mine is a Spectra also. Distributor is fine, its the CKPS that should be mopar. But what works works.