PCI Bus stuck High 8-10 volts PIN 1 of Diagnostic Junction Port

Brian Owner Parma, Ohio Posted   Latest  
Question
Driveability
2002 Jeep Grand Cherokee Laredo 4.0L (S) 4-spd (42RE)
Start-Stalling

Called into a job that a few others have been working on (as usual) Crank start and stall. They wanted me to work with tech to verify that the PCM was faulty. Quickly found PCI bus ( DLC Pin 2) was stuck high @ 8 volts. Disconnected the diagnostic junction port and tested the individual circuits, all had activity except for Pin 1 ( PCM, SKIM, PDM, CD changer{not equipped}, amplifier {not equipped}. Pin 1 stays High ( 8 -10 volts, fluctuating vary slowly over a few minutes). I disconnected the Skim, PCM, and PDM and still have the 8 volts. I see no other modules on this circuit, I am wondering if the schematics are missing something. The vehicle does have a aftermarket remote start that I completely disconnected.

There is something keeping 8 volts on this line I am just baffled as to what , I did try disconnecting the ABS, Radio, Over head message trip computer, DDM, and cluster in case of a misprint in the SI.

Just to make sure that I was not missing anything major, I got a little more intrusive than I would like, I cut D25 at the PCM , SKIM, and the diagnostic junction port and ran a jumper to pcm and skim, I had proper square wave/ comm activity , added them to the rest of the PCI bus , bypassing the circuit on terminal 1 of diagnostic junction port, and have proper vehicle running no lights or DTC's. diag​.​net/file/f1m7dsuna…

I am all ears for any Ideas , hopefully I looked over something simple. My plan is to head back tomorrow and start separating D25 at c106 and c200 to try to isolate the problem.

diag​.​net/file/f2yt9kg1h…diag​.​net/file/f3i7un9x8…

I will keep every one posted !

+4
Chris Diagnostician
Lansdale, Pennsylvania
Chris
   

Brian, I haven't had a chance to pull up a schematic yet, but it definitely sounds as if there is another module on line 1 as it would be rather convenient for a rub through/short to arbitrarily show up as 8V. I noticed you have the TCM crossed out on the schematic but didn't mention isolating it that I saw. Can I assume that was isolated from the circuit or maybe overlooked, as I've done

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John Instructor
Beaver, Pennsylvania
John
 

You are on the right track, let the car help you. You will have to continue separating the circuit until you isolate the one that is feeding power onto the data bus. Then start reconnecting the circuit using the modules that are working and clear all codes. Now you can tell what module has the problem by seeing what codes wont clear, the missing module will have the working ones coding against

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony
 

Hi Brian: I'm a bit confused by your post. It may be me though as I'm having some fun with a sinus infection. The confused part is you wrote that you disconnected the PCM from the DJP with no change. Then you mentioned cutting the wire with change. Here's what I think that I know. For you to have voltage higher than normal but lower than battery voltage, you lost the integrity of your low

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Brian Owner
Parma, Ohio
Brian
 

Hi Guido , I do do have trouble getting my thoughts out properly so that is part of the problem. I was trying to say that with the diag junction port plugged in , and cutting the ckt for pin one that on the pcm side of this things did not change, ( stuck high) and the rest of network was up and running, More to come below..

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Mario Diagnostician
Weston, Florida
Mario
 

If all modules have been disconnected then there's nothing left but wiring integrity. I'd check those connectors c106 and c200.

+1 Ð Bounty Awarded
Jim Owner/Technician
Charlotte, North Carolina
Jim
 

I have seen a lot of broken/ shorted wires to the driver door module, between the door and the body, check there behind the rubber boot for possible bad wiring,

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Brian Owner
Parma, Ohio
Brian
   

Ok I got it. A serious tail kicking. LOL about 5 hours to get here. Short story, circuit shorting with in connectors c200 and c106 due to water intrusion. Thanks for all the input every one. Long version.. Day 2 I first wanted to restore all circuits back to the way they should be to retest base line , I twisted or jumped all ckts together. put the scope on Pin 2 at the dlc and flat line 5

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony
 

Hi Brian: I'm having a hard time wrapping my head around what I think that you're saying. Here's why. The bus voltage is nominally 7.5 vdc. You had readings of 8-10 vdc. There is no transformer (and I doubt that it is FM). Where did the rise in voltage come from? TIA, Guido

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Geoff Diagnostician
Lahaina, Hawaii
Geoff
 

G, I didn't look up those connectors in SI but I figure they are likely 12-way (or so) and some wire in there is 12v.

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Brian Owner
Parma, Ohio
Brian
   

The rise in voltage is some sort of insulation break down, corrosion, or short to other ckts internal to C106 for the D25 ckt. I could not see anything obvious with out taking apart the entire connector. No other problems with vehicle noted, and all modules communicating properly after bypassing that connector. I think I got hung up on this vehicle thinking it was a module problem due to:

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Mario Diagnostician
Weston, Florida
Mario
 

Thanks for the update Brian! Nice find

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