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Old 06-17-2014, 09:32 PM
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98 jeep tj crank no start

Hello everybody i have as stated in the title a 98 jeep tj with a 4.0L
and M/T non C.A. emissions. I currently have a crank but no start issue. After checking spark and fuel I found I have neither.I also hooked the jeep up to my snap on scanner and have no communication. So i checked the under hood fuse boxs for blown fuses had none. So I did some googling and found these videos (https://www.youtube.com/watch?v=a-d6-jrGidA) Watched them all then went out and checked for 5 volt reference signal at the tps sensor had no 5 volt signal Did as in the video and unplugged the tps map cam ps and crank shaft ps and still no 5 volt. so everything so far is pointing at the computer. I got out my test light and started checking the with it and found out fuse 21 has no power at all and not blown. Then i check fuse 21 for a short to ground and found that i have 3.3 ohms on the orange/ Dk Green wire which feeds the injectors and coil and o2 sensors. The i checked the other side of the fuse for short to ground a had 2.28 milly ohms on the dkgreen/ pink wire which is the ads sense wire. heres a link to wiring diagrams (http://www.blackgto.com/jeep/98wiring.pdf)



Guys were should I go from here? Replace the computer or check the wiring harness with a fine tooth comb? Any help would be greatly Appreciated! Ask any questions i can get pics or video just let me know I wanna make this thing live again!

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Old 06-17-2014, 10:18 PM   #2
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Check fuse 11 and fuse 6. 6 provides power to ASD relay and to fuse 21 when the AND relay is energized. Fuse 11 provides signal to IGN Power is PCM. PCM won't turn on if there's no signal. If those fuses are good then check PCM grounds. If all checks out then PCM is at fault

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Old 06-17-2014, 10:34 PM
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Alright so i went out and checked the fuses there good then the a31 and 31 In fig 10 From the diagrams i linked in first post. I went from batt pos to a31 and a32 with a test light and got light so there good can i safely say its the pcm now. And thank you Yyang for the quick response!
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Old 06-17-2014, 10:46 PM   #4
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Just because the fuses are good doesn't mean the PCM is bad though. There may be breaks between the fuse and pcm. Use test light between ground and a2 and a22 to see is power is getting to the pcm
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Old 06-17-2014, 10:48 PM   #5
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Check a2 with key on engine off
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Old 06-17-2014, 10:55 PM
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power a22 with out key and with key on a2 only with key onbut i pulled to conncetor and checked from front aka conector not pluged in to computer should i try and back probe it?
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Old 06-17-2014, 11:00 PM   #7
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Should be fine checking it with the connector unplugged. Looks like all your checks are good. Ground circuits and power circuits are good. Leaves only one thing left
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Old 06-17-2014, 11:04 PM
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The pcm?
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Old 06-17-2014, 11:09 PM   #9
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Wait u said fuse #21 has 0v on both sides of the fuse? Recheck and trouble shoot there. Should be 12v. ReCheck your ASD relay
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Old 06-17-2014, 11:11 PM   #10
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When you checked fuse 21 did you have key on? It should have 12v with key on
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Old 06-17-2014, 11:15 PM
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correct no power on either side fuse is fine switched asd relay and horn relay after testing horn still nothing. I also check for ohm readings on fuse 21 orange/Dkgreen had 3.3 ohms Dkgreen/pink had very little like 2.285 mohms. What should i check now? Prehaps unplug the Dkgreen/pink wire from the pcm and recheck to see if pcm has a short. Or should i start unplugging things from the orange/ Dkgreen side until short is gone?

Ps Yyang do you have a steam account so we can chat faster or origin?

edit just rechecked with key on got nothing with test light
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Old 06-17-2014, 11:23 PM   #12
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No i don't have a steam account. You won't get 12v at fuse 21 if the pcm is dead. PCM controls the ASD relay. I'm pretty sure your pcm is at fault. The pcm provides power to all the sensors you've checked. Pcm is getting power and ground. It turns on when it receives an ignition signal which it is. The pcm is not working
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Old 06-17-2014, 11:26 PM
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Alright Yyang thank you very much for helping me double check this out. I will get a computer on its way and let ya know how it goes!

Would you recommend i get mine refurbished by cardone or should i just buy one from a junkyard? With my commercial account at autozone its gonna cost 333.49 for a refurb or 100 for a used good one.
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Old 06-17-2014, 11:35 PM   #14
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Do check that fuse 21 though. You should get any ohms. Try unplugging the pcm and all injectors and ASD relay and Recheck for continuity between ground. If you get OL reconnect each connector at a time until you get an ohm reading. It may be what caused the pcm to fail.
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Old 06-17-2014, 11:38 PM
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ok i will do that tomorrow morning and report back here.
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Old 06-17-2014, 11:58 PM
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Yyang fig 10 in the diagrams i linked on the bottom left the fuel injectors control the two wires that come off of fuse 21 are somewhat attached or no i have no idea what part of the diagram mean.
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Old 06-18-2014, 01:38 PM
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Ok Yyang I did some testing with the multimeter. used battery ground for ground end of multimeter and then used positive probe to measure the ohms on both sides of fuse 21. fuse 21 was removed from jeep during testing.

orange/Dkgreen Dkgreen/pink
2.6 ohms----- OL- pcm unplugged
2.6----- OL- asd relay unplugged
2.6----- OL- Injectors unplugged
2.6----- OL- Coil unplugged
5.5----- OL- Upstream O2 unplugged
OL----- OL- downstream O2 unplugged

so i left the downstream unplugged tested out some more things no ref 5 volt still from pcm no fuel pressure still and still have no communication with the pcm also my dash volt meter and dash gas gauge do not move.


Can we safley say the pcm is dead and i have a bad O2 sensor?
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Old 06-18-2014, 02:15 PM   #18
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Yes your o2 sensor is shorting to ground. Have you tried plugging in everything else except the faulty o2s. And see if the pcm will power up. That's definitely the root of your problem.
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Old 06-18-2014, 02:24 PM
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yes I have still nothing just cranks over and with the o2 sensor unplugged i still have no power at fuse 21
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Old 06-18-2014, 02:35 PM   #20
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Replace o2s and pcm
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Old 06-21-2014, 08:09 PM
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Alright I ordered a pcm for it Gonna get O2's on the way will update thread soon.
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Old 06-21-2014, 08:38 PM   #22
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Hope it works but not sure a new PCM is the issue thre is a guy with a 97 just put in a new PCM with no improvement

I gather you believe the PCM is failing to close the shut down relay as that is its only function on supply of plus 12 to coil, o2 sensor heater and injectors
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Old 06-21-2014, 09:16 PM
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And the fact that i have no 5 volt refernc3e signal to my cam sensor my crank sensor my map sensor or my tps sensor and i have no communication with my scanner. I am more then happy to preform any test you could advise I am all ears man.
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Old 06-21-2014, 09:32 PM   #24
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Originally Posted by xvMr.Greenvx View Post
And the fact that i have no 5 volt refernc3e signal to my cam sensor my crank sensor my map sensor or my tps sensor and i have no communication with my scanner.
I recently thought that I may have an ECM problem and talked to a couple of shops that repair same. They both mentioned the failure of the 5 volt output as being one of the most common ECM failures.
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Old 06-21-2014, 09:46 PM
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So your saying That I'm probably on the right track then?
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Old 06-21-2014, 09:48 PM   #26
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So your saying That I'm probably on the right track then?
It looks a lot like that to me.
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Old 06-21-2014, 09:51 PM
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Good, I found a pcm at a jeep junkyard for 100 bucks. They said its guaranteed To work and if it doesn't they will either swap it out or refund me they said. Much cheaper then 338 bucks to send it in to Cardone for a rebuild.
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Old 06-21-2014, 09:53 PM   #28
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If not tried already a good clean solidly connected triangle of ground wires with the three corners as bat negative a mounting bolt on alternator and a good solid clean firewall bolt
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Old 06-21-2014, 09:55 PM
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Have not tried that but have tested the ground to the pcm feed and there fine will try that tomorrow.
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Old 06-21-2014, 09:58 PM   #30
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This may be a dumb idea but it just happened to me, the ASD COIL fuse behind the dash, (#10 I believe), blew when my sensor shorted, I was about to replace the whole pcm as well.
One more indication you may want to look for is the fuel and battery gauges wouldn't work and the check engine light didn't come on. These all indicate the PCM is failing to power up as well.
i did a google search for the no check engine light and the fuel gauge issue which led me to several forum threads, one of which, (I don't remember which), led me to the problem.

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