dalthanar

I have a 99 XJ classic. I was going down a rough patch of pavement, hit a bump, and she shut off. Coasted it in to a parking lot. Has plenty of juice, will crank all day, but wont fire.

I get a "no bu5" code on the odometer after 10 seconds of cranking. I replaced the CPS with a brand new one. Checked all of my fuses and relays. All good. I did find find a broken ground cable going from the battery to the fender. The battery was a little loose, so I think when I hit the bump it snapped the cable, and then the cable touched a couple of times and shorted something out.

At this point I am staring at the ecu and thinking it is the bad part. What else should I check, and how do I test if the ecu is bad?

tjwalker

If you have a NO BUS and the engine won't start, something is "shorted" on that bus.

And it could theoretically be anywhere along the entire computer bus.

Replacing the crank sensor did make some sense, as a shorted crank sensor is probably the #1 cause of the NO BUS you are experiencing.

But something else is the culprit. I'd start by removing the connector for each of the 5 volt engine management sensors one by one, then with that sensor off of the bus, check your dash to see if the NO BUS is there or not. If it's gone, you know what sensor is bad.

Other than that, you could probably try a junkyard computer.

dalthanar

This is exactly the info I was looking for! Thanks! You mentioned 5 sensors. I know the CPS is one, what are the other 4? I will do some searching to figure out what they are, but if you can list them off the top of your head, sweet. I love this vehicle, and other than the electrical issue she is a beast. Engine is clean as a whistle.

dalthanar

I turned on the key, got my favorite "no bus" code, and went to pulling sensors one at a time. Did them all. No change. At all. Ecu gave me the finger. So tomorrow I am hitting up the pickapart yard for a ecu. If I can't find one there I am going to bite the $165 bullet and get a refurbished ecu online.

dalthanar

Turns out the #11 fuse on the inside fuse panel was blown. Now, when I it in drive, and go 2 feet, it blows again. I have checked for loose wires in the engine bay, and have found nothing. I have no idea which wires are attached to this fuse. Anyone have any info on this?

The Rooster

what is fuse 11? I would start there..

dalthanar

Asd relay fuse is pretty much what it is. But I switched the asd relay with a known good relay (starter), and there is no difference.

Tonight I took loose all of the connectors, cleaned them, and put dielectric grease on them. All the connectors on the starter, alternator, and the other sensors, all cleaned and greased. Here is the kicker now....

I can put a new 20amp fuse in the #11 position in the fuse box. She will start, and run just fine. I can put it in reverse, neutral, and drive, no change. Still running fine. As soon as I roll forward, the fuse blows and she shuts down. Now she has a brand new cps in it, so I'm figuring it isn't that. Feels like I am slowly getting closer and closer to a fix. Real slow.

jowint

Does this Jeep have a speed sensor at the transfer case?

Wonder if it got wiring screwed up when hitting the bump and shorts when you roll in motion.

dalthanar

I checked the connectors on the tranny and the transfer case, but nothing was loose. Doesn't mean they aren't messed up of course. I guess I could unplug the sensor and see if it still blows the fuse. Good suggestion.

jowint

I checked the haynes manual.
It says 98 and later models the PCM get the speed signal from the rear abs speed sensor.

dalthanar

I unhooked the speed sensor cable, and soon as I started the Jeep the fuse blew. I have a feeling there is a short in one of those wires. I hooked it back up, and the fuse blew again. I am going to cut that harness apart right there, and just replace some wires.

dalthanar

Jeep is up and running. Ground out wire on the starter. When the jeep went forward or backward, the tranny would spin the starter, which sent juice through the ground out wire. Which blew the fuse.

The wire had old dirty oil deposits on it. The oil filter sits right above the wire, and has been dripping on. Replacing the filter, and changing out the wire. Taped it for now to get to the auto parts store. This has been eye opening experience.

dalthanar

I was still having issues with the fuse this week, and also had issues with my oil pressure gauge and battery gauge. I did a lot of forum checking, and saw where someone mentioned that the wire harness will lay on the fuel rail, and the rail acts like a knife, cutting in to the wires. I checked this on my wiring harness, and sure enough, this was the issue. If you have a xj, please check this. It may not have happened yet, but it will in time.