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Unread 10-13-2009, 09:27 AM   #1
preacher250r
Registered User
1998 WJ 
 
Join Date: Oct 2009
Location: austintown,ohio
Posts: 4
98 grand cherokee 4.0 no spark

i am way stuck here for 10 days now. i've been reading various posts and checking parts suggested by other troubleshooters. 98 gc 4wd 4.0 (motor replaced 5mo's ago with 94,000 mile 4.0 motor from front-end-wrecked 98 that ran when i got it, so lots of spare parts) just stopped on the wife. we pulled it home and have these symptoms.
---no spark--cranks over great.
---no codes after 3 key on/off turns.
---no 12v gauge, fuel level gauges for sure (they should show levels with key on). the oil pressure gauge moves slightly with the key on so it may work if i had oil pressure and no movement on temp gauge (but it wouldn't till warmed up, right)
---the fuel pump comes on with the key on, but stays on. it doesn't do the 2-3 second cycle as normal
---found the rear oxygen sensor wires rubbed raw on the drive shaft. taped them up, it checks same as new one so i did not replace.
---crank sensor resistance check ok.
---the asd relay energises and stays on.
---5v as tested on back of cam sensor plug was less than .9v. after unplugging everything it came up to 4.4v. found either forward or rearward oxygen sensor being plugged in caused drop. installed new oxygen sensors but either one will cause voltage drop again. perhaps the shorted rear oxygen sensor melted something in the harness?
---replaced pcm with spare 98 gc one, but unsure if i tried it before or after i found the bare oxygen sensor wires. so i could have fried that one too.

i guess the pcm times the fuel pump through the asd and fuel pump relay so i'm really thinking the pcm's bad, but would like to have them tested first.

went to library and got a few wiring diagrams from their mitchells books to try to trace wires from oxygen sensor to wherever, and also circuitry to 12v gauges? chrysler does not check pcms only (they said bring it in for complete diagnostics $100-500 no guarantees they'd find the problem)

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Unread 10-13-2009, 07:31 PM   #2
ZeeJay1997
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1997 ZJ 
 
Join Date: Dec 2008
Location: Sweet Home, Alabama
Posts: 6,871
Unplug the O2 sensors and see if it will crank. It should run in default fuel mode. If it does, there's a bad sensor.

If not, the harness is probably shorted as you suspect. Be sure the bare wires you taped up are not touching each other.

You can check it with a HIGH IMPEDANCE meter by unplugging both sensors and the black PCM plug, then checking each wire for a short to any of the others, except common grounds between plugs.

Check at O2 harness plugs:
Pin 1 heater voltage fed by ASD DG/OR
Pin 2 heater common ground BK/TN -- common to both plugs
Pin 3 sensor ground from PCM BK/LB -- common to both plugs and several other sensors. Will show some resistance to ground by reading through other sensors.
Pin 4 sensor signal to PCM BK/OR

I'm guessing your sensor ground is shorted to voltage or ground.
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Unread 10-14-2009, 08:23 AM   #3
preacher250r
Registered User
1998 WJ 
 
Join Date: Oct 2009
Location: austintown,ohio
Posts: 4
found a bad ground under dash (pin 1 on instrument panel is ground, but read 1m ohm) by steering column, while wiggling the wires with key on i heard a small pop (thought it was a relay energizeing) under the hood. found that i'd blown a fuse but the new fuse held. went back to under dash and played more with wires and got ground back. installed dash and gauges worked. replugged in the componets i had off and the jeep starts and runs fine. tried wiggling wires again to duplicate problem but couldn't. i did have excessive drain to battery and found wiggling wires to door stopped that, but could not find any bare or loose wires, nor recreate drain again.

so it runs but problem may return. the bare wires to the oxygen sensor were the 12v heater wires, so maybe instead of blowing a fuse it blew a loose connection elsewhere. i'm not exactly happy without a positive fix, but i know where to look hopefully upon reoccurance.
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Unread 06-14-2010, 01:16 PM   #4
preacher250r
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1998 WJ 
 
Join Date: Oct 2009
Location: austintown,ohio
Posts: 4
jeep still runs great, no reoccurance of problem. thanks
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