Weird issue. Crank no start. No spark. - JeepForum.com
 
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post #1 of 14 Old 11-07-2019, 06:52 PM Thread Starter
FlyEagles808
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Weird issue. Crank no start. No spark.

Hey guys. 1999 TJ 4 cyl 2.5. Troubleshooting a no start issue. CKP and CMP sensors both check good. Checking 5v supply and ground to CKP and found the I'm getting 5v also from signal wire from the PCM peg A8 on connector C1. The wire should only be sending 5v signal back to PCM not from PCM. The Gr/Blk wire isnt shorted out anywhere. Had an extra PCM, same thing. Any ideas would be appreciated.

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post #2 of 14 Old 11-07-2019, 06:52 PM Thread Starter
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Here's the wiring diagram for it.
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post #3 of 14 Old 11-07-2019, 08:20 PM
Rubi4MyMrs
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Some clarification.

Are you checking the crank sensor connector with it unplugged? Continuity to ground at the brown/yellow wire & 5v at both of the other 2 wires?

If yes, is the crank sensor the only 3 wire sensor returning 5 v to the PCM?
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post #4 of 14 Old 11-07-2019, 08:44 PM Thread Starter
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Yes CKP sensor is disconnected from connector when I'm checking position 1 (Gr/bulk wire) on the connector, it is still plugged into the PCM though. I have great continuity to ground at positon 2(brown/ylw) and 5v on the other two. Thanks.

Last edited by FlyEagles808; 11-07-2019 at 09:21 PM.
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post #5 of 14 Old 11-07-2019, 08:50 PM Thread Starter
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I just check and youre right, Camshaft Position Sensor (CMP) is also showing 5v at supply and signal. Good contituity to ground at center position.

Last edited by FlyEagles808; 11-09-2019 at 06:45 PM.
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post #6 of 14 Old 11-07-2019, 10:14 PM
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OK, I’m going to guess the signal wires for the MAP & TPS will also have 5V.(?)

How about the speed sensor & oil pressure sensor?
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post #7 of 14 Old 11-07-2019, 10:24 PM Thread Starter
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Yes to all. Every sensor has 5v at signal wire coming from PCM.
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post #8 of 14 Old 11-07-2019, 11:04 PM
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I should have also asked, what happened? Died while driving & no restart? Ran fine then one day just no start?


I hate to say but it sounds like the PCM may be bad.

Every sensor signal wire is independent so even if one had shorted to the 5v wire that would only affect 1 sensor which could keep it from starting. I do not see any way something external (to the PCM) would cause this condition to all 5v sensors so have to consider the PCM. I will look a bit more tomorrow…
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post #9 of 14 Old 11-07-2019, 11:10 PM Thread Starter
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Helping buddy fix his. It died one day. Changed a bunch a stuff and finally a PCM. I tried the old PCM he has and its reading 5v too. While looking today I discovered the IAC had stripped and shorted wires. They were definitely touching each other while plugged in. I separated them and found 1&2 0v and 3&4 read 12v.
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post #10 of 14 Old 11-08-2019, 11:19 AM
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Wow! That is a project! Seeing the wires in that condition makes me wonder about the condition of rest of the harness. Have you looked closely at the wires & connectors at the PCM?

Unfortunately the IAC is not one of the 3 wire 5V sensors & 2 hot (12v), 2 ground is normal for them. It’s a bit of a complicated circuit with the voltage/grounds reversing in various situations to control how far open/closed it moves to regulate the idle speed. So not likely to help with the no start situation. Unless…

Have you verified no spark &/or no fuel? Checked ASD relay function? I ask because with the IAC wires touching/shorting it is possible that the IAC is staying completely closed & will not allow air/fuel to the engine. The way around that to get it to start (assuming you have spark & fuel) is to slightly open the TB as you crank it.

Having tried 2 PCMs with the same results seems to point to something else.

Have you tried checking the signal wires with all 6 of the involved sensors unplugged at the same time?

I’m thinking if yes, maybe try unpinning the 2 5V output wires from their PCM connectors. Then with no 5V to the sensors see if the signal wires still have 5v. That would then for sure have to be coming from the PCM, not somehow back feeding from the 5v output. Or, with all sensors & the PCM connectors unplugged, you could check for continuity between the 5V & signal wires for each sensor. Just to be sure none are shorting together. I don’t think one would affect all but running out of possibilities here.

The 5V supply is a unique circuit (most are 12v, of course) so getting it where it is not expected narrows down where is it coming from so am surprised it does not appear to be PCM or shorted sensor related, especially affecting all sensors.
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post #11 of 14 Old 11-08-2019, 02:22 PM Thread Starter
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Hey RUBI, Thanks a ton bradda! The rest of the harness looks pretty good, and took the backplate off the PCM connector to backstab the wires. It is weird because the signal return to the PCM are all different pins, no shared connections there.

I should have been more clear, crank, but NO SPARK. I am getting fuel pressure and fuel relay works good. ASD should be good bacause its sending 5v out to the senors right??

Im wondering if the shorted IAC was the cause of the original no start issue. I know his troubleshooting wasnt good and it was more throwing parts at it, leading into finally changing the PCM. Could the shorted IAC burn out the old PCM. Then if it was unfixed and the PCM was changed with a new one, immediately burn out the new PCM?

When I found the orignal CKP sensor connector issue, I did check continuity between the 5v supply and the signal wires. Nothing. Im going ot get at it again today and verify no sensors are shorting out. its just very suspect that all the signal wires are all reading 5v.

Thanks again for your advice and input. Mahalo!
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post #12 of 14 Old 11-08-2019, 06:35 PM
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I was recently helping a guy that had IAC issues. It turned out he had one or more (not sure how many but think 1) wires shorting to ground. His case did not affect the PCM. They seem to be pretty robust in that area & are fuse protected too. Still, anything is possible. I have not had a chance to look at some limited info I have but off hand don’t see a shorted PCM sending 5V out like you have. There have been cases where they lose their 5v outputs but a short causing the 5V circuit to back feed into the various signal wires? None I've seen but possible, I guess.


The ASD sends 12v to the PCM (among other places) but not the actual 5v. It does activate the 5v circuit in the PCM as I recall.




This may seem strange but have you tried cranking the engine while checking the cam or crank sensor output? Seeing if it shows a constant 5V? Or maybe some voltage fluctuation?
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post #13 of 14 Old 11-08-2019, 09:14 PM
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Also check the ignition switch. Mine was corroded and would turn over but not start. Wiggle the key and started. GL
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post #14 of 14 Old 11-09-2019, 06:49 PM Thread Starter
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Do you think I'd be safe trying one of my PCMs in another 4cyl wrangler? Someone close by is looking for a PCM and man would that help with troubleshooting. Mine 99 4 cyl 5 speed, his 97 4 cyl auto.
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