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Electrical Issue? Possible short?? Where to Start?

blaqueowl

NAXJA Forum User
Location
Oregon
Hey y'all. Okay bear with me kinda long just trying to be thorough. Been searching Google and every Jeep forum for a while now and can't find an answer to the problem with my 1998 Cherokee 4.0L. About a month ago I was driving on the highway about 50mph when the Jeep gave a couple quick bucks. The next day it died at a stoplight. Bucked and then died while in reverse a few times.
Cleaned the TB and IAC after that. Drove around the next couple weeks (mostly under 50 and some trips up to 65mph) and everything was peachy.

Drove 6 hours to Spokane bucked maybe a couple dozen times on the highway real quick like half a second then it was fine. Finally threw a P1391 code during the last hour of my drive. Searched on the forums for solutions, thought it might be a bad Crankshaft Position Sensor (bought havent put in). Next day I cleaned all the grounds EXCEPT the braided one on firewall. Why? I noticed it was connected to the firewall and the other end was resting on the exhaust header! I discovered it really connects to the last head bolt. "No way. Could this really be why I'm having these issues???" So i reconnected the ground the head bolt. What an easy fix!

It wasn't.

Oh boy. The Jeep was not a fan of that ground being reconnected. It was hard to start had to give it gas and once it started it up began misfiring, choking and eventually backfired through the intake!Disconnected the ground again everything was fine!

Noticed there was a bolt missing on the bellhousing right where the CPS is, so I replaced that. Cleared the CEL and drove another 3 hours to Missoula and everything went smooth no bucks or CEL.

I'm guessing there is probably an electrical short somewhere. I don't know where to begin. The O2 sensor wiring sheathing had melted a couple years ago but thats been fixed.
 
What year? I just fought an issue that made me go get a new to me wiring harness from a jy. Turned out to be the crank position sensor wiring where it zip-tied to the bracket. I would backfire and pop and ping so bad I finally gave up and had it towed home. It's kinda a PITA to get to but they way to check ( atleast what I found) is to turn the key on and wiggle it all around. If you hear the fuel pump try to prime when you move it a certain way then cut the zip tie and look for broken wires. I have a 2000 with a stroker motor and she's a blast when she's not a b****.


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My '99 diagnostics manual says that the distributor rotor or rotor drive may be loose. Remove the distributor cap and check for side-to-side play in the rotor. Some rotational play is normal, excessive side play is unacceptable. It is very odd that the braided ground wire causes the symptoms.

Check the wiring harness from the distributor, along the firewall, to the PCM. Check for it being penetrated and exposing wires.

The CPS harness enters the fuel injector wiring conduit then exits out the end where it curves up to the main harness along the firewall. Check the area of the harness where it exits the conduit and curves to the main harness. There have been cases where the harness chafes on the end of the fuel rail, and chafes on the end of the rocker cover.


Possibilities (from 4.0L FSM):

Symptom:
P1391-INTERMITTENT LOSS OF CMP OR CKP

DISTRIBUTOR INCORRECLTY INDEXED
CMP SENSOR OUT OF SYNC
DAMAGED TONE WHEEL/FLEX PLATE (CRANKSHAFT)
DAMAGED TONE WHEEL/PULSE RING (CAMSHAFT)
IRREGULAR LAB SCOPE PATTERN OF CMP SIGNAL
INTERMITTENT CKP SIGNAL LOSS WHEN WIRING IS WIGGLED
IRREGULAR LAB SCOPE PATTERN OF CKP SIGNAL
WIRING HARNESS INTERMITTENT
INTERMITTENT CMP SIGNAL LOSS WHEN WIRING IS WIGGLED
SENSOR CONNECTOR/WIRING
SENSOR CONNECTOR/WIRING
CAMSHAFT POSITION SENSOR
CKP SENSOR CONNECTOR/WIRING
CRANKSHAFT POSITION SENSOR
 
It's probably due for a tune up its been about 40,000 miles..I was gonna do that regardless. If it is just in need of a tune up wouldnt I have these issues all the time and not just at certain speeds? Around town it runs and idles great. Tim-I'll take a good look at those wiring harnesses maybe a wire is exposed causing that ground to short out. I'll report back when I have more time to check everything out.
 
No, when I was young and dumb, mine had a bad plug wire and would only very intermittently cause problems. Bucking, jerking and sometimes it would shut off. Took me a while to find that...
 
I also have a 1998 and I had a similar issue.

I am not sure if it is ok to post a link to another forum, but I will try it as I have more info over there about how I was able to track it down.

Basically, on mine, the end of my fuel rail closest to the firewall had rubbed through the flex loom on a bundle of wires and the wrong one was being shorted out. I have a couple pics in this thread with some wonderful MSPaint skills used to show where I was talking about. I was able to patch up those wires and use some wire ties to make sure it no longer rubbed and it immediately was fixed. I was even able to reproduce it by forcing the contact before I fixed it. Now, I am not sure that this is your problem, but I hope this helps you out.

http://www.cherokeeforum.com/f2/strange-behavior-highway-speed-engine-cutting-out-171122/

Paul
 
UPDATE! This finally got resolved!

It turned out it was several things: 1) A wire with cracked sheathing back by the firewall was causing a short to ground. 2) The ECU got shorted out and failed 3) The crankshaft position sensor failed

I had this looked at the automotive department at a local community college and this was their assessment. Something (possibly the wire by the firewall) caused the computer to short out and fail. After the computer was replaced they were able to determine the source of the voltage spikes and found it was the CPS. Replaced the CPS and everything was back to normal.

I was skeptical when they told me the computer was shot because from my research these things are usually pretty hardy, but I suppose with an electrical short ANYTHING is possible.

It took about 6 months for them to resolve, but I don't have to pay the labor costs only the price of the parts :thumbup:
 
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