MWildmann

OK. I've got a 2000 XJ with the 4.0. It's an automatic with the California emissions package (so 4 O2 sensors, blah). It keeps blowing the fuse that is on the O2 sensor heater circuit, so it's throwing just about every O2 sensor code possible. It used to blow the fuse only intermittently, but now i'll replace it, and go to fire it up and it blows it immediately. I've replaced all 4 O2 sensors, and it's blown the fuse before and after doing that, so that kinda eliminates a problem with the O2 sensors themselves. I've tried the search tool to no avail. If anyone else has experienced this and can give me an idea of where to start on this issue, I would really love to fix it. Tired of looking at my dashboard lit up like a freakin christmas tree! If I can't fix it, it's going to have to go to my mechanic and probably cost me a fortune. Thanks all!

SeriousOffroad

A fuse that blows immediately when current flows through it is a clear indicator that there is a short circuit. Stop throwing fuses at it and find the short circuit.

Start at the O2 sensors and work your way back along the harnesses until the problem is found.

MWildmann

Thanks for the reply. I know there is a short circuit. Maybe i should've been a little more clear. What i would like to know is if there is a common place that this happens on XJs. I've already checked the O2 sensor harnesses and the more accessible wiring under the hood. Nothing i could find wrong. What other loads are on this circuit besides the O2 sensor heaters? I don't have a service manual, so if someone else does and can look it up, that would be most appreciated.

SeriousOffroad

I'll poke around in my FSM tonight and see what I can find for you.

Marcucco

Find/borrow/buy a multi-meter (doesn't have to be a fancy one).
One of those cheap probes that looks like an awl with a light bulb in the handle might work, but the multi-meter would definitely be much better for this.

Be sure the ignition is off.
Unplug the harness from all the O2 sensors.
Unhook the positive battery terminal.
The negative battery terminal needs to stay hooked up to the battery.
Pull the fuse that keeps blowing.
Hook the red lead from the multi-meter or the clip on the probe onto the positive battery terminal.
Set the multi meter to Ohms (i.e. resistance, the symbol looks like an upside down horse shoe)

Touch the black lead from the multi-meter or the pointy end of the awl/probe to each of the contacts in all the O2 sensor harness leads.

I've never probed out a 2000 and I don't have the book for it so I can't tell you specificly what readings you should get on the multi-meter. What you're looking for is one set of O2 sensor leads that's way out of whack from the others.

With the multi-meter, you're looking for a particular lead with a very low resistance (like an indication between point something amps to a 0.0 or an "OL" indication) on one or both terminals in the lead.
With the probe you're looking for the light to come on very brightly on one or both terminals.
Check all the leads, you will probably find that you're getting some resistance on one or both terminals (with the meter) or a weak glow with the probe.
Again, you're looking for an O2 sensor lead that doesn't match the others.
Once you've found the bad lead, you'll have to trace it back.

There is another possibility here.
If all the leads have similar results, try again with the ignition on (leave the positive battery lead unhooked) and re-run the test.
If one of the results now comes out differently then it did with the ignition off, there's a strong chance that one of the modules is bad or is sending bad info to a relay.

That's about all I can do for you without being there. If it helps any, this is exactly what your mechanic is going try first.

Good luck

Mark

Marcucco

Update:
With the bulb in the probe eating up most of the voltage and the diodes in the circuitry, the chances of you damaging the "brain" are pretty slim, but you might want to play safe here if you're using a probe.

If you're using the probe, unhook the connection to the computer/control/brain module.
Leave the negative battery terminal attached and the positive battery terminal removed.
The alligator clip for the probe should be attached to the positive battery terminal.
Probe out all the leads from the O2 sensors.
If you get a light while probing any of the leads, that lead is absolutely shorting out to ground and you need to trace it back. You'll find the fault.

SeriousOffroad

TESTING FOR A SHORT TO GROUND ON
FUSES POWERING SEVERAL LOADS
(1) Refer to the wiring diagrams and disconnect or isolate all items on the fused circuit.
(2) Replace the blown fuse.
(3) Supply power to the fuse by turning ON the ignition switch or re-connecting the battery.
(4) Start connecting the items in the fuse circuit one at a time. When the fuse blows the circuit with the short to ground has been isolated.

TESTING FOR A SHORT TO GROUND
(1) Remove the fuse and disconnect all items involved with the fuse.
(2) Connect a test light or a voltmeter across the terminals of the fuse.
(3) Starting at the fuse block, wiggle the wiring harness about six to eight inches apart and watch the voltmeter/test lamp.
(4) If the voltmeter registers voltage or the test lamp glows, there is a short to ground in that general area of the wiring harness.

Marcucco

SeriousO offers a good place to start, but since you you already know theres a dead short from one of 4 possible sensors, and you're sure the sensors are good, (and it sounds like you don't have a wiring diagram), it would probably be quicker to figure out which sensor lead is bad and work your way back from there rather then the other way around.

MWildmann

Wow, all of that is getting printed. Now i just need to find a couple hours to follow those suggestions. Thanks very much!

Marcucco

Good luck!
The process isn't that complicated if you don't let it intimidate you and it wont take more then an hour, just don't lose patience and you'll find it.

SeriousOffroad

What fuse number is blowing?

MWildmann

I don't know what number it is and i'm out of town right now, so theres no way for me to look at it. If memory serves, it's in the fuse box on the passengers side fender and it's the second fuse from the front, closest to the engine.

MWildmann

Oh, and it's a 15 or 20 amp fuse.

freegdr

common issue check wiring above drive shaft/transfercase area falls down on to driveshaft and shorts to ground....

MWildmann

I'll definitely check that out too. Thanks very much!