Rkferg13

i have a 97 2wd xj with the 4.0. about 2 weeks ago i drove the jeep to pick up my cousin after it sat for a week because i was out of town for work. it drove fine but when i went to leave it was surging and anytime i hit the brakes it died. this was at night. i turned the headlights off and it drove ok back to my moms house. i took the battery out to have it tested (i noticed the dome light was very dim when the door was open.) the battery was fine. I replaced one of the terminal clamps bc it wouldnt tighten enough to keep it from moving. then it ran fine for 3 days or so. i noticed that i have no reverse lights now and it threw two codes P0700 and P0705. anytime i start the jeep it will run fine idling but *** soon as you touch the brake pedal it immediately dies. i have done some reading some people say to rebuild the NSS. i also noticed the serpentine belt is loose and shiny on the top. if i could get it running enough i would take it to oreillys and test the alternator. i live in an apartment complex and they freak out when you work on your vehicle here. does anyone have an idea? i would really appreciate the help. I've been pulling out what little hair i have.

extrashaky

I would look for a problem with the brake booster.

Rkferg13

I actually just found the problem. After more searching through posts. I found that a ground wire in the driver side trunk was the problem. It's the ground for the brake lights and turn signals. It's amazing that one freaking ground wire could cause all those problems. Thanks fr the advice though

RocketMouse

lol... welcome to Jeeps..hehe.... glad you found it.... I was going to mention electrical fault when you stated about the issues with the tail-lights.

Glad you got her fixed.

Suprmn

That ground wire might also be the one that goes to the fuel pump. Its in the same compartment.

Rkferg13

I'm glad I got the dirty little ***** running again. This is my second Cherokee. I wouldn't trade the Jeep for anything. I paid 1400 bucks. It's only got 151,000 miles so, it's got a lot of life left in it.

Rkferg13

I actually found the post on here about the ground wire. Someone was having a similar issue. They found out it was a loose ground wire and took pictures of the location. One wire caused my jeep to die f I touched the brake, threw codes, and made it run rich lol. I know it's off topic but if anyone lives near spring tx and has a welder I'll buy beer if they let me use it lol

jordan96xj

I think I got my head around this one.

The XJ (at least on the 96, can't speak for all), does an usual thing with the circuits that run to the tail lights for brake lights. When the brake is depressed, the brake switch applies positive voltage to the circuit that ultimately feeds the brake lights. -HOWEVER-, when the pedal is released, instead of simply -opening- the circuit, like a switch normally would, it switches that positive voltage to ground. So the circuit is always either positive, or ground, but never just open. I learned this when installing my brake controller, because this has implications for the manual control on most low-cost brake controllers.

What this probably means is that if that main ground back there gets compromised, certain components, most likely the fuel pump, are finding an alternate ground in the brake light circuit, and continue to function. -UNTIL-, the brake is depressed, and that wire goes from ground to positive, effectively removing the ground that the components are depending on because the primarily ground is loose, disconnected, or corroded.

Next time you have a tail-light assembly taken out for some reason, take your multi-meter and watch as the bulb circuit goes positive when you press the brake, but then goes to ground when you let off the brake pedal.

I asked a few knowledgeable folks about this behavior, which I guess used to be more common back in the day. But really, I didn't get any straight answers. The best they could come up with was that it was designed this way to help reduce RF interference in the circuits that could lead to problems with ignition, etc. I can detect some RF interference on the circuitry in my Jeep, so this idea has some merit. But I don't know for sure.

But the next time you work on a tail light problem, especially involving brake/switch, keep this interesting design feature in mind, as it has implications for what you are observing when troubleshooting with the meter. (really, who assumes that a single wire is going to be positive one moment, and then negative the next?).