cj2822

New owner of 2000 XJ 4.0 A/T, first Jeep ever. 190k miles. Keeps blowing 40A fuse #2 (red wire, feeds 12V to many systems and accessories via the ignition switch). Fuse blows without warning and for no apparent reason. Happens while driving (!!), immediately shuts down the vehicle while in motion (probably because of power outage to auto shutdown relay, which is one of the systems downstream of fuse #2). Does not blow this fuse every time I drive it, or during any specific conditions that I can figure out. I have replaced ground G100 (from battery to frame) and replaced ignition cylinder and ignition switch (thinking maybe that was the location of an intermittent/random short). In between fuse-blowing incidents, I've driven it several times, sometimes up to 30 miles without a problem. Then out of nowhere it happens again, while driving, coast to a stop, push it home or replace the 40A fuse on the side of the road (I carry spares now). This has happened 4 times in the last few weeks.

Additional background info that may or may not be relevant:

1) first time it blew a 40A fuse it was fuse #3. Again with no warning, in a grocery store parking lot. Had it towed, mechanic thought it was due to a loose ground G100, which he said he tightened up. Next time was fuse #2, after which I replaced entire battery negative connection to ground (it was bad, now is 0 ohms). Fuse #3 has never blown since, but clearly a bad G100 ground was not the issue. I have checked one other ground, but not all grounds. Most other grounds are far downstream of fuse #2 and should blow smaller fuses if there were other grounding issues... or so you'd think but who knows.

2) Vehicle has aftermarket remote start system, which I have never used. The system gets power from red wire / fuse #2, but has it's own 30A input fuses that do not blow. Pulled these fuses out entirely to disable the system, and still blew fuse #2 today again while driving. So it would seem that the short is not in the remote start unit, but who knows.

3) electric door locks stopped operating during one of the many fuse-blowing episodes. But I can hear a relay clicking in the passenger side when I push the driver's side lock button, so there's power in there somewhere. Also the passenger window never worked, and the driver's window stopped working, but the back two are fine. All of these systems are downstream of fuse #2, but are protected by smaller fuses that do not blow. So it does not seem like the source of the problem, but again who knows.

4) the radio used to contiually stay powered on, even when the key was removed (had to remove faceplate of aftermarket stereo to shut off the power). This issue was fixed when I replaced the ignition cylinder (satisfaction in at least fixing something in this POS since I bought it 2 months ago). I used to think maybe this was related, but the ignition cylinder/switch have been replaced and are fine now.

I own the service manual from Pacific coast, and I've studied the wiring diagrams a lot. What's frustrating is that the problem is so random. Except that it always blows the same fuse, there doesn't seem to be any pattern as to when or why it blows, or any warning.

Any ideas are welcome.

Turbo X_J

Replace the ignition switch connector?

jordan96xj

Start by inspecting the ignition switch connector. Particularly the condition of the insulation on the wires right by the connector and near the steering column. If the shielding has melted or developed burn holes their load can jump directly to ground (the steering column steel) and pop a 40 amp fuse. Most of the items fed by the ignition switch have additional fuses down stream, and if you don't find any of them blown, then the ignition switch connector is the primary suspect at the moment. Also, a loose ground won't typically result in 40 amp load (blown fuse), but instead will result in components not working (motors, lights, etc).

cj2822

Thanks guys, that makes sense... so I checked and I don't see any burn marks where the red (fuse #2) wire enters the connector, and it doesn't feel loose. The braided wire sleeve feels solid, especially by the metal in the steering column. Is it worth taking apart and re-building the ignition switch connector anyway?

Or would you suggest tracing the connection between the remote start unit and the factory red wire, if maybe that's where it's shorting? That tap/splice has to be nearby somewhere - probably tapped the red wire ahead of the ignition switch connection just below the steering column, but it's all wrapped in sleeves or tape so I can't see it.

There's nothing else aftermarket on this jeep that I know of, so I would think the red wire would be continuous between the PDC/fuse block and the ignition switch except for the remote start connector. Are there any other common trouble spots between the PDC and the ignition switch where the insulation could have rubbed off?

The weird thing is that I haven't found anything specific that sets it off - road bumps, braking, turning the wheel etc - it seems to be totally random. I've run it hard over a huge bump at the end of my driveway and that doesn't do it, but then earlier today it blew during a gentle acceleration on a smooth street. Previously it blew during soft braking near a stop sign.

Here's the view of the ignition switch connector and braided wire sleeve. Sorry if the pictures end up a little big - not sure if there's some way to downsize them...

Turbo X_J

Disconnect the battery and obtain a terminal tool like one of these to have a look inside the connector. By depressing the lock tab on the terminal you can remove each wire without damage. Screwdriver type works 99% time on common spade terminals.

jordan96xj

Basically, 1 of 3 scenarios can blow a 40amp fuse:

1. A live 12v + wire is chaffing and making contact with the chassis somewhere (steering column, seat brackets, dash supports, etc.)
2. A live 12v + wire is near a - wire and the two have either begun melting through their insulation and are making contact, or have been chaffed/crimped and are making contact.
3. A live 12v + wire is feeding a component that has internally shorted and is allowing the + to go directly through the component to ground. This is the rarest, and typically will blow the fuse every time, immediately. This typically only happens when a component or its connector have melted due to resistance heat and the positive and ground wires have begun melting into each other. Normally when a component simply fails, it does not cause a short...but instead just an open (or OFF) circuit.

Both 1 and 2 can happen randomly due to bumps, heat, etc. 3 tends to be more consistent (an internally shorted component doesn't typically become unshorted).

You may want to inspect the underside of your PDC (under hood fuse box) to ensure that there is not corrosion/moisture problems causing issues right at the source. If these get moisture under them, they can cause problems right at the box, and most people assume the problems are way downstream somewhere. But since you are only blowing the 40 fuse in there, and not others, this is worth inspecting.

Normally those high amp circuits (40) don't remain that way once they go inside. They typically go to a bulkhead connector that acts as the main place all of the wires go through the firewall. (on my 96 it is the c108 connector that sits down on the firewall between the brake booster and fender, and forms the back of the internal fuse junction box). A 40 is kind of a "main" line that is typically broken up once it hits the internal fuse box into 2 or more smaller circuits with smaller fuses on them. (like a 30 will often become two 15s). My point is, if nothing downstream from the internal fuse box is blowing smaller internal fuse box fuses, it makes me think that that 40 amp circuit is shorting to ground somewhere between the 40 amp fuse (the PDC) and the back of the internal fuse box.

Take a good luck at any harness that is running right behind the fuel rail (back between the rear of the rail and the firewall). That is a common point for the harness to get chaffed as it rubs on the rear of the rail (just one I've seen on here from time to time).

Turbo X_J

With an intermittent electrical issue it is best to concentrate ones focus on one area at a time. If one molests the entire vehicle they may accidentally "fix" the problem only to have it shortly (pun intended) reappear.

cj2822

Thanks again everyone for the advice. I will keep investigating between the PDC and ignition switch as suggested. The short has got to be in there somewhere.

djgrayxj

Note from DJ
If this was my Jeep I would call my local parts supplier like NAPA, O-Relies or Auto Zone etc. and ask if they can scan your Jeep for codes.

If they will, take the Jeep in and see if it has stored any; if it has any wright them down and post them for us to see.

I hope this idea helps!

cj2822

So I might have fixed it. I checked every cable wrap and harness I could see or feel under the hood, under the dash, everywhere l could reach or see.
​ I couldn't find anything that was a potential short. Then I found something under the passenger dash and seat.

Before he sold the Jeep to me, the previous owner removed an amplifier that was under the passenger seat and left all of the wires and cables behind. One of the wires was a #12 red wire probably for the amp power. The end of the red wire that fed the amp was still under the seat, and had become tangled in the seat track/rail, to the point that some of the insulation had been stripped off and the bare wire was exposed. The other end of the wire ran under the center console, under the dash on the passenger side and into the interior junction-fuse block. The wire had been stripped and jammed into the space where spare fuse #8 should have been. Someone was seriously lazy and stupid.

So the previous idiot owner was using the +12V bus to power the amp with no fuse protection... except the 40 amp fuse in the PDC that kept randomly blowing. So when the damaged exposed portion of the red wire occasionally touched the metal rail or seat mounting, it could have completed the short circuit from ground, through the metal of the seat bolted to the floor, to the interior fuse bus, through the ignition switch, up to the 40A fuse #2 in the PDC. It sounds crazy, but it's definitely possible.

I don't know this was for sure the issue, but so far so good. Since I removed the stripped useless wire we've driven the Cherokee 10 miles over several trips without blowing the fuse. My son pointed out that a few weeks ago he moved the passenger seat back and forth when he was cleaning the interior. After that the fuse always blew when he was driving and I was in the passenger seat, and we never made it more than a block before the fuse blew. But when I had driven it by myself with no one in the passenger seat, no blown fuses. So maybe a stripped wire and my fat *** in the passenger seat were the problems.

Thanks for all of your help and advice, guys!

djgrayxj

Note from DJ
Excellent detective work.

vulcan96

MMMMM.......
Nothing like electrical problems to put a bad taste in your mouth with a new ride. My favorite.

From the sounds of things, I am pretty sure you have figured it out though.