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'01 XJ electrical issue - all the symptoms of a CPS but not the CPS

B

BlacknBlue_XJ

NAXJA Forum User
Location
Longmont, CO
There are a ton of factors in play, so I'll do my best to explain everything clearly and concisely.

The Jeep is a 2001 XJ.

Symptoms:
  • The symptom started as what you would expect from a heat-soaked injector; a rough idle for a couple seconds after a start and then it would run/drive fine.
  • The starts got harder (cranked for longer before firing, idled poorly for longer).
  • Tried to pull the CEL and two parts shops couldn't get their scanners to connect.
  • No Bus on the cluster.
  • Relays in the PDC click and buzz constantly with key on; removing ASD relay or fuse #11 in the kick panel stops clicking
  • Car died on the side of the road in typical crank sensor fashion.

Replaced crank sensor with no change. Engine grounds are good. I started to poke around and discovered one of the wires to the fuel pump got pinched and grounded, but not for that long. Thought that would fix it, but no change after repairing the wire. I didn't check the relay, but I did jump the pins in the PDC and it struggled to start as before. I'll check/replace the relay tomorrow.

I guess I'm looking for someone who has chased electrical gremlins through this system before and could give me some ideas. If the new relay doesn't do it, I'm thinking the fuel pump may have taken damage from the short, or (hopefully not) the ECU got fried. Cam sensor? Pulling my hair out on this one - thanks in advance!:cheers:
 
To me, the symptoms describe a short circuit and the additional symptoms suggest it is related to the CPS or one of the other 5 volt engine sensors. Test the related sensors, including the O2's. Visually/physically inspect all the sensor wires and use your volts/ohms multi-meter to test continuity and to test for short circuits. Wire harnesses melting insulation or rubbing on sharp metal edges are common. Also common are failed wire splices.
 
Thanks for the reply. I've tested the crank sensor and TPS, but I can't readily locate reliable test info/voltage for the cam sensor or others. If anyone has links handy I'd appreciate the help.

Both upstream o2's are new Bosch units. I was under the impression that the downstream wouldn't interfere with any functions.

I'll be going through the circuit from the PCM to the PDC and fuel pump to test for continuity. Let me know if there's anything else I should specifically look for.
 
A down stream O2 short is still a short circuit. The cam sensors is a hall effect sensor, inspect for damage, and test. Youtube has some videos.

Just an FYI and probably not directly related to the CP like symptoms, Jeep 4.0 and Bosch O2 sensors do not play well together. The Bosch O2 sensors can cause driveability issues. You should be using NTK O2 sensors.
 
Last edited:
Sounds like you have a couple separate issues here

1. The no bus is a common issue and basically you need to pull the cluster and clean the contacts

2. The heat soak issue is common on 00/01's. There is a tsb for it, but the part is basically a cardboard shield for the injector. I fixed mine with hood vents.

3.where did you get the new cps from? Bad ones out of the box are not unheard of.
 
I'll look around for a test procedure for the downstream o2. I'll try anything at this point.

Cam sensor tested fine.

I got into the cluster this evening and the contacts look brand new.

It's not heat soak. I blew it off as heat soak since it started getting hot here as my hard starts began to occur.

New cps is a Mopar unit that tested ok.

At this point I'm thinking it has to be the ecu. Every ground has been checked and I have appropriate voltage and continuity everywhere. I'm going to pull it apart tomorrow and check the circuit. Not looking forward to replacing that.
 
IslanderOffRoad said:
Sounds like you have a couple separate issues here

1. The no bus is a common issue and basically you need to pull the cluster and clean the contacts
Click to expand...

No bus is not only caused by the cluster not seating. If an ECU were dead you would also get it for example.

OP:

I think you have something else going on. Grab the pinout from the factory service manual, then start testing your power and grounds.

I have a spare ECU in fort collins if you want to test it.
 
I have tested just about every wire that I knew of/learned about in the 5v sensor circuit and I have good continuity and voltage. I've followed the advice posted in this thread over on JeepForum. All was in spec.

I'm checking the o2 sensors today and I'm to the point of rewiring my trans temp gauge to see if I somehow messed something up there. A test ECU would be awesome! Are they interchangeable? From what I gather the only major difference would be a SKIM system, which I don't have. I'll shoot you a PM.
 
I have fixed similar symptoms to this with a simple TPS swap several times on a few different XJ's (most recently a 98 with failing fuel pump symptoms, and a 94 with classic failing CPS symptoms).
When the TPS starts to go I have seen it cause a NO BUS error, random stalling, failures to start, rough idle, and general rough running.


Sent from my SGH-I337M using Tapatalk
 
zluster said:
No bus is not only caused by the cluster not seating. If an ECU were dead you would also get it for example.

OP:

I think you have something else going on. Grab the pinout from the factory service manual, then start testing your power and grounds.

I have a spare ECU in fort collins if you want to test it.
Click to expand...

Good point.

I figured starting with the free/easy solution was the way to go
 
If the remote keyless entry module or its wire harness is shorted to ground it can cause problems. Same for the driver door and tailgate wire harnesses which are both commonly a source of short circuits. Simple enough to unplug any or all from the body main wire harness as a test.

Installed the NTK O2's yet ?
 
Ah, never though of the keyless entry. Is there a plug that does the whole door/hatch at one time or just unplug the individual harnesses in the door?

I'm not convinced about the o2's. The car ran fine for several months with new Bosch sensors before this problem started. It also seems like many of the issues were over a decade ago, but I didn't put that much time into researching that. Back o2 passed the proapne torch test. I'll test the upstreams today if I get time.

I'm going to Napa to buy all of the sensors. The local Napa is good about returns as long as I don't get them all greasy. Worth a shot, I guess.
 
The Keyless can be unplugged from at or near the fuse box in the passenger foot well. The doors unplug from just inside the rubber boot. The tailgate unplugs from behind the cargo area plastic trim, I think on the passenger side.
 
I printed and organized every diagram I could find and started over, beginning with a trace of the fuel pump circuit. An engineer friend came over and we followed it from the pump forward, and found the fuse in the PDC had blown. Popped in a new fuse and the key-in-run clicking was gone on the first try. Starts and runs fine.

It wasn't a huge deal as I have another vehicle to get me around, but I still couldn't believe it was as simple as a fuse (THE fuse that should have gone if the pump wire grounded as it did.) Myself and a far more experienced mechanic missed it the first time around. We're thinking we simply checked the wrong fuse. Thanks for putting labels on the diagram under the PDC lid, Chrysler.
 
BlacknBlue_XJ said:
Thanks for putting labels on the diagram under the PDC lid, Chrysler.
Click to expand...

LMAO! You thought Chrysler put thought into their labeling. Shame on you! ;)
 
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