dzywicki

My '99 Cherokee was throwing a code for O2 sensor heater. I discovered that the fuse kept blowing. I replaced both O2 sensors and didn't see anything obviously wrong with the wiring to either O2 sensor. The upstream still has the plastic tubing around it and I don't see any breaks or areas where it would be shorting. I replaced both O2 sensors with Mopar. The previous NTK ones had ~50k miles. I thought that fixed it, but the fuse is still blowing. It seems ok for 1-2 drive cycles and then blows. I need to watch it closer and do some controlled tests. Is there anything besides a short in the wire that would cause the fuse to blow? It seems suspect that it doesn't blow right away.
I checked online and didn't see any other relays in the wiring between the fuse and the O2 sensor. Any thoughts or help would be greatly appreciated.

bluejeep2001

I checked the factory wiring prints today...if correct and not modified the fuse only feeds both sensor heaters. So either one of the sensor heaters is bad or one of your wires to the sensor is shorted to a ground. Anything after the sensor is irrelevant electrically. I will try and upload the print or a picture tomorrow. Since it happens fairly quickly, disconnect one without disturbing harness too much and do several drive cycles...see if fuse blows..then replace and disconnect the other one. If it still blows then shorted harness somewhere. If not then one sensor is bad..got a spare one somewhere will try and get a heater resistance reading tomorrow

dzywicki

Thanks for verifying that. I was thinking of doing that experiment. Will report back on the outcome. IT has to be something int he wiring harness, just have to find it.

bluejeep2001

The heater circuit is the 2 white wires, pins 1 and 2 as labeled on the back of the connector. each sensor heater reads approx 4 ohm. If you read from the output side of fuse (A8) the resistance to ground should be approx 2 ohm...2 heaters in parallel.

bluejeep2001

dzywicki

Thank you for the wiring diagram. I had some time to do some more diagnostics. The mystery deepens. The resistance for each O2 sensor individually was good (~1.4 ohms). With both connected, it was ~2.4. I was able to check it with the engine running and it went up to the 110-120 range and varied, which seems normal. I disconnected the front and the fuse blew after a couple fo start/stop cycles. Repeated with the downstream disconnected and had the same results. I did a superficial inspection of the wiring harnesses and the insulation and protection are still intact on the upstream and downstream. Not ready to tear all that apart quite yet.
Then I started poking around the PDC. I disconnected the the clips and pulled it up. I moved the wiring harness around a bit and then put it back. After I did that, the fuse was blowing as soon as I turned the key on. We are in the 3rd winter in Northern Va so still pretty cold out. Next step is to look for some pictures of the PDC and get a better look at the underside and wiring going into it. Should be loads of fun. I haven't done anything to the wiring and noone else has touched it so I'm guessing there is some corrosion on the underside. Dealt with this on my Audi a few months ago. A relay got corroded and kept the electric fan on and would kill the battery. All the other relays around were fine and showed no signs of corrosion. Hopefully it's something simple.

Both O2 sensors attached

Ohms when running

Resistance on front O2 sensor only.

PDC top

dzywicki

Son of *****. Took the PDC out and didn't really see anything out of the ordinary. I did clean and tighten the fuse connections. I examined the wiring harness and still not findng anthing that looks like a short. It seems to be ok for 1 start and then blows. I went through all my fuses so need to get some more to continue testing. I'm thinking I'll try adding some wiring in line and then check the current draw. If I can't find anything, I may just run new wiring and see if that cures it.

dzywicki

Did some more troubleshooting today. Checked the downstream wiring harness and it is in great shape. Took off the wiring harness to see if there was anything going on to the front sensor. Everything looked great there too. Checked multiple different ways for shorts between the heater circuit and couldn't find one.
I finally went back to the circuit diagram posted by Bluekeep2001 and the only other piece of the circuit is the ASD relay. I had a problem a few years ago where a rebuilt alternator from Napa was way overvoltage and t kept blowing the ASD fuse so I am pretty familiar with that circuit. I don't recall ever changing that relay out but may have when I was troubleshooting the alternator problem. I pulled the relay and pulled the same relay on my 98. The one from the 98 looked like a Jeep OEM relay and had several identifying part numbers on it. The one from the 99 (the one I am troubleshooting) did not look at all the same. The only numbers on it are 0310 and SHO49. I have been to the pick a part many times and always pick up a few relays. I also have a 95 YJ that uses a lot of the same relays. I had a few spares laying around. I put that in and did a full trip and several restarts with no blown fuses. Previously, the fuse would blow after the 3rd or so restart and then blow almost immediately one the key was turned.
Hopefully this is it. It would makes sense since nothing else changed. I did suspect corrosion somewhere causing a short so maybe I was right, just not exactly as I thought. Will give it a some trst trips and report back.

Inside the wiring cover along the valve cover

Front O2 sensor wiring inside the plastic covering

ASD relay from the 98 XJ

RElay that was in the 99 seemingly causing problems

dzywicki

I took apart he mystery relay and didn't see anything out of the ordinary. I also did. few more starts with no blown fuses.

bluejeep2001

Electrically the ASD relay should not be able to blow fuse directly...BUT if the elec connection from the output side of the relay down leg 24 is poor and the voltage drops, you could pull more amps through the fuse and blow that circuit.. Its also possible that the factory diagram is not totally accurate and something else is in the circuit possibly shorting. I have found inaccuracies.in the diagrams before. Try it and see what happens

dzywicki

Have had a few short trips and many starts. No blown fuse yet. I was doing some testing and the mystery relay and then tossed it in the trash. Hopefully won't get a return of the blowing fuse.

dzywicki

Happy to report back that everything is good after the relay swap. Would have never guessed it.

dzywicki

Dang it. It's back. Been great for several hundred miles and over a month. Blew the fuse again today. May have to figure a way to get an amp meter hooked in there. And to amke it even better, my 98 decided to not crank. Seems to be the NSS so have two jeeps with problems at the same time.

bluejeep2001

I do not think your meter has the capability to read the 15 amps or more required to blow the fuse. I would c heck the resistance again from the output side of the fuse to ground, with the fuse removed obviously. If you do not know which side is the input, have the fuse removed with the truck running..The input side will show 12v to ground...the output side will be no volts

dzywicki

This multimeter is a crappy one I got from HF for basic diagnostics. I have a nicer one I use for more in depth troubleshooting. Good suggestion on checking resistance. Something is obviously driving the current up. The only other thing I can thing of is something with the alternator again. 5-6 years ago I had to replace the alternator. The replacement was fine at idle and up to a certain RPM. Once it went too high, it would spike and blow the ASD fuse. Took awhile to figure that one out. I actually have a spare, brand new alternator I could swap in to test that theory too. It's my wife's cherokee and she has a bit of a lead foot.