rompnXJ

Oh jeep overlords, i request thy help

I APOLOGIZE IN ADVANCE FOR THE HUGE WALL OF TEXT. ive got one hell of a gremlin. ive been searching and cant find any relevant threads.

'97 auto 4x4 XJ, 192k miles. daily driven, no recently changed parts or modifications.

went on a camping trip, all was fine till we found some real deep mud that we really had to hammer through, im talking 3/4+ throttle in 4lo for a good 5-10 minutes. during that, both e-fans were running, and heater was on full blast. tstat is also removed. engine did not overheat, stayed below 215 during all that. she usually runs 190ish on a hot day so i pulled off the trail, popped the hood and let her cool. we take off again and all is well till we find some more mud. i laid into the throttle and the engine starts backfiring randomly. i keep my foot in it cuz i dont wanna get stuck and the truck shuts off. all gauge needles freeze where they were, and then a few seconds later fall. turned the key off, then went to start it and no gauge readings; no fuel level, no temp, no battery info. more importantly, the check engine light would no longer light up when you turn the key on. odometer still working properly, airbag, low washer, & check gauges light still work properly. cranks, no fuel or spark. found it blew the 20a fuse in the passenger kickpanel; left row, 4th from the bottom labeled IGNITION/RUN. replaced it, everything was fine, so off we went. then the fuse started blowing repeatedly whenever i would hit the gas real hard. it started blowing more and more frequently, with barely any throttle. then the exact same symptoms occured, but this time the fuse did not blow. parked it, set up camp and let it sit overnight. woke up the next morning, still no gauges or CEL. spent 2-3 hours pulling out the dash panels and physically inspecting the wiring harness leading from the interior junction block to the PCM. no wiring faults found, dissassembled and cleaned the 4 main harness connectors that are on that harness, along with the PCM connectors. no luck. ive not been running a front o2 sensor for about a year now, i noticed the o2 sensor connector somehow ended up with one of the pins bent and touching another pin; one of which was a ground. fixed the pins and taped off the connector. still no luck. heard that an o2 sensor short can burn up the ASD relay, so inspected it, a couple of the slots on the ASD relay looked like they had gotten pretty hot. replaced relay. still no luck. let it sit for another 4 hours trying it periodically every 20 minutes or so. eventually i turned the key back on, all of a sudden my gauges were back, and the CEL was lighting up again. it fired right up. 20 minutes later blew the same fuse again. replaced the fuse again, fired right back up and then drove home for 30 miles as if nothing was wrong at all. i could go WOT and the engine wouldnt miss a beat. made it home, and ive been drivin it around the neighborhood, it is showing no symptoms. let it idle and shook the entire wiring harness, shook both inside & outside junction blocks, and tapped all the relays with a small hammer. cant get it to act up again.

I'm at a complete loss here, im thinking the computer is shot and randomly crapping out. can anybody help me out here ??

Bustedback

Look and see if the O2 sensor wiring is melted to the exhaust manifold and shorting out. Going through that deep mud may have pushed the wiring up against the exhaust. Put the thermostat back in it, you're not doing the engine any good by not having it. And put the front O2 back in, the engine will not run as well as it could with a working O2.

rompnXJ

i appreciate the fast response.

i found out how to duplicate all my symptoms. you guys are not going to believe this, but this problem occurs every single time i turn on my blower motor, immediately. (like when turning on the heater). i will pull it apart tonight and let you all know what i find out. has anybody seen anything like this before ??

salad

That's messed up! I'd first assume that something got wet, but possibly the blower motor relay is done.

This is for 1999 but you should find some similarities to your 1997

rompnXJ

also, if i leave the blower switch in any of the ON positions and turn the key on, the CEL will not light up and i get no gauges. turn the blower off, and the CEL & gauges come back and it will fire right up. also, if i unplug the blower motor while its idling, the engine will die immediately.

rompnXJ

thank you very much for the schematic; that helps a lot

rompnXJ

the fuse indicated at the very top left of this schematic, is that the fuse that i kept blowing? it was inside the passenger kickpanel, left row, 4th from the bottom, labeled IGNITION/RUN & it had a 20a fuse in it, schematic indicates its a 15a but perhaps the PO replaced with a larger fuse

salad

Fuse 11?

On a '99 that fuse drives:

PCM
ASD Relay
Fuel Pump Relay

salad

Might be worth a peek behind the junction block. Remove the two screws holding the trim by the seat and you'll be able to remove that whole panel around the JB

rompnXJ

i did pull that JB out, and removed all the connectors and cleaned them, but i didnt dissassemble the block. ive seen some pictures of that block all corroded on the inside due to moisture, and mine has been dunked quite a few times. i'll pull it apart tomorow and let yall know what i find. if anyone has anymore suggestions let me know, thanks!

rompnXJ

gonna bump this, still hunting for this problem. I've been driving it around with the blower Off for the last month or so.
Symptoms are the same, engine will immediately stall and become a crank/no-start condition if blower is switched on or if any part of the blower circuit is unplugged. Also will occasionally blow fuse #11 in passenger compartment junction block, especially when applying heavy throttle from a stop on dry pavement.

I have dissassembled and cleaned the passenger compartment junction block, I have replaced the blower motor relay, and I have verified good solid ground at pins A-31 & A-32 on the PCM, which is the connector closest to the headlamp.

It seems to me that the PCM has a weak ground somewhere and is somehow finding its ground through the blower circuit, and engaging the blower interrupts that ground circuit.

Does anybody know where the blower fan ground is located ?? It is labeled 'G-108' in the schematic 7 posts above this one.

Thanks for any & all help !! lol

schematics or links to schematics are greatly appreciated

belvedere

This makes sense to me. Wish I could offer more help. Problems like this can be a real b!tch to track down.

scoobyxj

Fuse #6 under the hood is the blower motor fuse. It provides power to the blower motor through the relay. The motor is then grounded through the blower speed switch on the HVAC control unit. G108 is in behind the left side of the dash. Roughly where the headlamp switch is.

I would try pulling the blower fuse to isolate it from the blower, or HVAC control circuit. Also try unplugging the HVAC control module. IIRC the LG wire goes to the ECU to tell the computer to run the A/C compressor. The 97 HVAC system is different from the later ones because it uses a manual control to change the temp where as the later ones use an electric motor. I think the blower circuits are pretty much the same though.

Radi

Have you verified a solid ground between battery (-), the engine block and the chassis?
I think you're right, the blower is forcing the chassis ground to rise above ground level.

cruiser54

Good advice here^^ from the night shift. Re-do ALL your grounds before proceeding on to any other rabbit trails.