Wiring diagram or location of PCM Ground? - JeepForum.com
 
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post #1 of 14 Old 11-28-2016, 12:45 PM Thread Starter
kb9mfd
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Wiring diagram or location of PCM Ground?

I have a gremlin that I cannot find. I have had several posts on various parts of this, but now I am looking at the wiring harness. Basically the symptoms are a jerky voltage gage (I have checked the voltage with a meter off the battery and its fine, also the auto place tested the charging system and its fine.) Oil pressure is also inconsistent and moves all over the place (even when the jeep is not running), but the temp, tac and speed seem to be fine. Cruise will not hold, it works for a while then just stops, I can hit resume to continue. (Already checked the vacuum system and its fine, no issues), and my mileage sucks. I have installed a new NGK O2, new exhaust, new plugs (old plugs showed slightly rich), wires, PCM, changed the transmission from a 42RE to a AX15, re-did the frame ground, re-did the battery connections. I am at a loss. Next I am going to test the battery and ground connections to the PCM, anyone know where those connections are? I will test them at the pins, but how about the location where its grounded? Also where does the volt gage get its reading from? Would there be one connection that would effect both gages, cruise and mileage? I had these issues with the 42RE, and when I put the AX15 in things ran great for about a week or so and then all the symptoms came back.

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post #2 of 14 Old 11-28-2016, 05:32 PM
ZeeJay1997
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Quote:
Originally Posted by kb9mfd View Post
Next I am going to test the battery and ground connections to the PCM, anyone know where those connections are? I will test them at the pins, but how about the location where its grounded?
Those are on the passenger side of the block somewhere. Should be black and tan wires

Quote:
Originally Posted by kb9mfd View Post
Also where does the volt gage get its reading from?
The voltage regulator is in the PCM. Not sure how it interfaces with the instrument panel on your model year.


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Originally Posted by kb9mfd View Post
I had these issues with the 42RE, and when I put the AX15 in things ran great for about a week or so and then all the symptoms came back.
That's a tell tale sign of a bad PCM


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post #3 of 14 Old 11-29-2016, 04:45 AM
Uniblurb
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I've tended to think Zee that the 94-95 might be like the 93 where the PCM ground is tied into the negative battery ground.

In looking at the 95 FSM 4.0 PCM connector pin-out cavities/pins 5, 11, 12, are all grounds for the PCM. These are the black/tan colored ground wires you mentioned so there could be multiple grounding points.

It should be noted the FSM mentions the data link connector is also on the same ground as the PCM referred to as circuit Z12. It states, "If the system loses ground for the Z12 circuits the vehicle will not operate. Check the connection at the ganged-ground circuit connection."

I wonder if the 'ganged-ground connection' is the gang of wires/eyelets on the inner right fender next to the battery since it sounds like it.

Regardless here's a couple diagrams where they are showing grounds on both the coil bracket stud and next to the battery on the inner fender.

Unfortunately they are always vague on which grounds are which and those 'G' numbers are just the connection number.

Here's the wiring and connector diagrams.
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96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #4 of 14 Old 11-29-2016, 06:41 AM Thread Starter
kb9mfd
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Quote:
Originally Posted by ZeeJay1997 View Post
Those are on the passenger side of the block somewhere. Should be black and tan wires
I'll look for those. The Haynes manual is rather worthless on the wiring aspect but it did mention a frame ground in the left kick panel and I do remember it now. That ground seems to end up at the instrument panel. I re-did the frame ground by the battery, and I know where the engine ground is but I have not taken that one apart yet. Since the charge system checks out fine, the ground between the frame and engine must be good.

Quote:
Originally Posted by ZeeJay1997 View Post
The voltage regulator is in the PCM. Not sure how it interfaces with the instrument panel on your model year

Unless I can locate a diagram of the panel, Haynes has the connector but not the panel, I think I may take apart the extra one I have to see how that gage wires to see what pin/wire its on.


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Originally Posted by ZeeJay1997 View Post
That's a tell tale sign of a bad PCM
Then I have two bad PCM's with the same issues. I have the one from the '95 auto and the one from the '93 manual. I am currently using the '93, but I had all the same issues with the '95. I used the '93 because I noticed it already had the caps replaced and it was the manual transmission computer, I don't think it matters but I use it anyway. The '95 computer probably should have the caps replaced.

I don't have any evidence yet to back this up, but it does seem that the power and mileage is better after a do a reset of the PCM and gets worse over time. That would lead me to believe the PCM is getting incorrect data within proper limits about the engine. I guess that could be a sensor or a bad ground / power messing with the reference.
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post #5 of 14 Old 11-29-2016, 06:47 AM Thread Starter
kb9mfd
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Quote:
Originally Posted by Uniblurb View Post
I've tended to think Zee that the 94-95 might be like the 93 where the PCM ground is tied into the negative battery ground.

In looking at the 95 FSM 4.0 PCM connector pin-out cavities/pins 5, 11, 12, are all grounds for the PCM. These are the black/tan colored ground wires you mentioned so there could be multiple grounding points.

It should be noted the FSM mentions the data link connector is also on the same ground as the PCM referred to as circuit Z12. It states, "If the system loses ground for the Z12 circuits the vehicle will not operate. Check the connection at the ganged-ground circuit connection."

I wonder if the 'ganged-ground connection' is the gang of wires/eyelets on the inner right fender next to the battery since it sounds like it.

Regardless here's a couple diagrams where they are showing grounds on both the coil bracket stud and next to the battery on the inner fender.

Unfortunately they are always vague on which grounds are which and those 'G' numbers are just the connection number.

Here's the wiring and connector diagrams.
Thanks for the information and diagrams. I have the PCM pinout now and see the connections. I will need to pull the cap off the connector so I can test it while its running to make sure the ground and power are what they should be. What book did you get those out of? Is there a complete connector and wire diagram showing all connectors and harnesses?
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post #6 of 14 Old 11-29-2016, 07:04 AM
GOLDWING
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You can buy a FSM on a PDF download for $8 from Pacific Coast Manuals. That will have everything you need.

GW

2 '98 4.0 ZJs
Hers is stock, mine, OME shocks, BB, Aussie, 2x6 sliders, 31s, etc.
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post #7 of 14 Old 11-29-2016, 07:13 AM Thread Starter
kb9mfd
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Quote:
Originally Posted by GOLDWING View Post
You can buy a FSM on a PDF download for $8 from Pacific Coast Manuals. That will have everything you need.

GW
Awesome! Thanks! I just bought a copy and downloaded it.
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post #8 of 14 Old 11-29-2016, 07:15 AM
GOLDWING
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Happy to help another (I hope) Badger FAN!

GW

2 '98 4.0 ZJs
Hers is stock, mine, OME shocks, BB, Aussie, 2x6 sliders, 31s, etc.
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post #9 of 14 Old 11-29-2016, 11:52 AM
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Quote:
Originally Posted by kb9mfd View Post
Thanks for the information and diagrams. I have the PCM pinout now and see the connections. I will need to pull the cap off the connector so I can test it while its running to make sure the ground and power are what they should be. What book did you get those out of? Is there a complete connector and wire diagram showing all connectors and harnesses?

You're welcome and glad GW turned you onto downloading the 95 FSM where those diagrams came from.

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #10 of 14 Old 12-01-2016, 07:45 AM Thread Starter
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A bit of update along with some questions....

Over the weekend I found the positive battery terminal was cracking, so I got a new one and replaced it on Monday. (That would have reset the PCM)
On Tuesday I found out the oil pressure sending thingy was so loose I could unscrew it easily without effort, so I replaced it with a spare I had and tightened it down, that fixed the oil pressure gage issue.
I downloaded the FSM and discovered that fuse 22 supplies the power for the gage. I checked it with a volt meter and found the voltage kept moving around. Oddly enough that is also the second of three fuses for the airbag controller. Strange they did that. Swapping fuses 18 and 22 (Both 15 amp) I discovered the fuse is rather loose in the socket. I bent the pins a little for a better connection and now my gage is more steady. First question is there anything I can do about a making that fuse connection better? Is there a way to remove the contact to tighten it?? Strange the air bag controller was not complaining. Also the volt gage still moves around round a little bit, I wonder if the air bag controller is having issues?

Now for the most confusing part, I did a lot of driving yesterday and now my volt gage is steadier, the oil gage is steadier, and now my cruise doesn't quit and my mileage is better again.

All that because of a bad connection for the air bag controller?? It has nothing to do with the PCM. A better but not less complicated explanation would be resetting the PCM on Monday fixed the cruise and mileage issues, because when I did the transmission swap and put the PCM in it would of been reset and then all the issues came back a couple of weeks later. Now its working correctly again and if in a couple of weeks the issues come back and I reset the PCM.... , any idea what would cause it to be degrading the PCM engine memory and causing the cruise to quit often, but a PCM reset fixes it?? Is there any way to read the sensor inputs in real time out of the PCM?? (ODBI) I am assuming there must be a sensor that is not completely failing, but giving wrong readings so not to trip the CEL (I have never had a check engine light or a error code from the PCM yet.) and over time causing the PCM to learn the wrong emissions causing the degrade in mileage and tripping off the cruise? Any thoughts?? I have had this identical issue with TWO PCM's and TWO different O2 sensors (I replaced my 6 month old Bostitch with a NGK), so neither of those could be the culprit. Should I just replace all the sensors? I need to clean my IAC as I noticed I am idling too low again and two years ago doing that fixed it. Am I suppose to clean that every two years?
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post #11 of 14 Old 12-01-2016, 06:48 PM
ZeeJay1997
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Originally Posted by kb9mfd View Post
First question is there anything I can do about a making that fuse connection better? Is there a way to remove the contact to tighten it??

Yes you can release the terminal from the housing using something like a jewelers screwdriver or heavy needle. If you look in the cavity with a bright light you can see a very small catch tab in there that has to be defeated. Once it is released, it will pull out of the bottom of the housing. You may need to do some disassembly to get to where you need to be.

If you're so inclined, here are the part numbers for Jeep terminals.





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post #12 of 14 Old 09-11-2017, 04:23 PM
vikkilynn
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thats great i so needed this
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post #13 of 14 Old 09-11-2017, 06:53 PM
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thats great i so needed this
Hi, if you have a 1999, you might want to check out the WJ forum, because, although we're pretty friendly here, and will try to help, there were a lot of changes made between the ZJ (1993- 1998), and WJ.
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post #14 of 14 Old 09-13-2017, 09:54 AM
jamesmsmall1
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Should be black and tan wires


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