Thanks for looking, but please, before you reply, keep in mind that I already know the 12 code indicates a power loss to the computer.

The 12 code never goes away, 50 starts or 500, she always shows a 12 code.
It wouldn't bother me except for the fact that it would seem to indicate that the computer is continuously running the engine at the default settings.
The engine runs well, and the OBD does seem to work otherwise, (it coded out for O2 sensors this spring), but with a quarter million miles on the clock, I kind of think the stock settings might want for some tweaking.

I did search the archives here and saw someone mention that perhaps the computer was not be properly grounded.
I have already gone around and made sure all the grounds were clean and tight and even used OxGuard between the terminals and sheet metal. (Including behind the dash.)
On close inspection of the PCB under the hood (drivers side) I noted that it is deliberately isolated from the chassis by plastic grommets, so I don't think that's the issue.

I was wondering if anybody knew weather there was a capacitor or battery dedicated to storing memory on the OBD2 board itself that may have gone west which could be replaced (I have some skills with electronics)

I considered grabbing another computer in the boneyard, but I doubt I'd find one that did't have the same issue.

Thanks for looking in.

 

OBD I DTC "12" is the battery disconnect code.


The '96 is OBD II which will have a "P" code. The '98 FSM shows a cross reference to the two digit Hex codes to the "P" codes. In this case, the Hex code 12 = P0443, which is a fault in the Duty Cycle Evap Purge Solenoid circuit.


Is your Evap Purge Solenoid clicking when the coolant temp gets up to around 150*F?

 

Ah- ha, Thanks, I'll check that in the AM.
As it didn't trigger a CEL, I never bothered to look any deeper then the "12"

 

It should have triggered a CEL.


Does your CEL bulb work?

 

I have the Chrysler FSM for the 96. Following the "Using the Malfunction Indicator Lamp (MIL)" directions I found the following information:

The Diagnostic Trouble Codes list shows the MIL 12 code as "Battery disconnect" with no related P code or hex code.

Your correct that the P0443 with a hex code of 12 is the Evap solenoid indication, however this indication would trigger The CEL lamp and the MIL code code for that is 31.

If the CEL light wasn't working I wouldn't be able to read the codes would I

Thanks for your time tho.

 

The CEL bulb not working has nothing to do with reading codes from the DLC.

 

The CEL light blinking is how I'm reading the codes, it would therefore appear that the CEL is working and in communication with the PCM and that the OBD is functioning.
I don't have a scanner and am not using the DLC.

 

See if you can get a auto parts store to use their code reader to pull up a code.

 

I noticed you said you have a '96 FSM. If you obtained this manual from Pacific Coast Manuals, it's more than likely a '97 manual. I bought a '96 FSM from them and it turned out to be a '97 manual. I emailed them about it and they never responded.

 

Do you have an Android phone? If so, you can buy a bluetooth OBDII adapter for around $20 on Amazon and use the $6 Torque app to turn your phone into a full-featured scan tool. It will not only read and clear codes, but it will also run emissions readiness tests and display real-time engine monitors. There's also a free version of Torque (Torque Lite) that just reads codes. When the code comes up on the phone, you can tap it to search for information about that code on the internet.

If you have an Iphone, you can do the same thing with a wifi OBDII adapter and the DashCommand app. It's a bit more expensive, but it's still cheaper than most scan tools.

Also, as Ken said, almost all local parts stores will read codes for free. They walk out with their scanner and read it right there in the parking lot. They won't clear it, though.

 

My 97 always reads out the 12 code when I do the key trick. However my scan tool showed a pending code for evap. Then last week it lit the cel and and threw a P0441 and 1486. Not surprising as a vacuum line to the purge is unhooked. No other codes for battery disconnect with the scan tool yet the 12 will show up still. Just throwing this out there.

 

Hiya.

Nope, thinking I am totally incorrect. Feel free to ignore my original submission below.

A suggestion, double check the code descriptions you are using. I am thinking that code 12 was "loss of signal from distributor" or similar to that not loss of power to computer. When pulling codes, engine is off, so no signal from dizzy, automatic code 12. This would be normal and not a problem.

Tim

 

I really appreciate everyone who has responded to this thread, I get that your trying to help.
Replies are posted in chronological order.

Ken_The title of the thread clearly states it is an OBD2 question.
The MIL/CEL 12 code on the OBD1 and the OBD2 are the same.
The Evap Purge Valve is fine.
I've already touched on the CEL/MIL light condition so...
The FSM I have is a Chrysler manual, copyright 1995 Chrysler Corp. It came with the Jeep which was formally a Fire Chief's vehicle in a little Podunk town in Eastern Ct. (Big money in Ct.)

I don't need a code reader, the jeep is running fine.
Because of the 12 code I suspect that the CEL may not updating performance data and it could be running better.

Shaky_ I do have an Android Phone (and tablet), but again, there's no issues with the Jeep or the OBD outside of the 12 code not going away. But thanks for the info.

97Grand_ I have a manifold vacuum gauge hard mounted to the manifold, it's a very handy tool, everything on DD is within spec.
A "pending issue" means it hasn't failed enough times to code out.
You might want to keep watching this thread, If you're 12 code isn't going away, we may have the same PCM issue. If I'm right, your Grand will run even better.

Tim_ As everything is running fine and the OBD seems to be working perfectly otherwise, , I'm sticking with my "bad capacitor" theory, but thanks for trying.

I'm sure I cheesed at least some of you off but please read this next bit:
The reason the '96 FSM recommends you leave the battery unhooked over night to clear the codes is because the radio also has a battery to retain the time and station presets.
The reason we unhook the battery and turn on the headlights or ground out the positive lead is because it's quicker to discharge the memory in the PCM, but it also clears the radio memory as well.
(Anybody who's owned an early iMac will recognize this process.)
If the Capacitor/battery in the PCM/OBD has expired, it will no longer be able to store data and zero out at every shutdown.

So I'll try and restate my question more specifically:

I strongly suspect the battery/capacitor in my 20 year old PCM has expired. I was wondering if anybody had any info, like a schematic or "point to point" or experience working on a PCM.
I could go to the junkyard and buy another module and look up all the numbers on all the components, but I was hoping for a short cut.

Thanks again!

 

Ok, well, there are more than one caps in the pcm, and more than one mode of failures. There are many other threads on those, and some workarounds.
There are also several guys that have taken theirs off, done nothing but clean it up and put it back on, and the car won't start.

So if you don't like troubleshooting a code 12, you really wont like troubleshooting that. All Ken was saying is, and I was supportive of, is it could be an EVAP type problem; to wit: Mine has an EVAP problem, 2 codes, and it throws the 12 also with no battery disconnect in the last 50 starts or for that matter 500 starts.
Just a suggestion, you might want to purchase an actual scan tool, (thank you for explaining to me what a pending code is)ebay has them for under $40 that show pending codes, unlike the key trick, and will graph the O2 performance as well as read the actual P codes and interpretations. Hope this helps.

**From the 98 FSM:
Hex Code 12 = P0443 EVAP Purge Solenoid Circuit
Description ; An open or *shorted condition detected in the duty cycle purge solenoid circuit.

 

97Grand: I really feel like I've already trouble shot the 12 code. I and I'm convinced that's not the issue as the evap system is working per spec, it opens and closes as it should and it's not coding out on the CEL/MIL.

I get that you and Ken are probably pals, and I appreciated his input, and yours, but you guys are not reading the questions or my responses and you're quoting data from a manual that is of a different generation from my '96 XJ.

This is a 1996, and the data I posted comes from a Chrysler Corp FSM for 1996:
The CEL/MIL code on the #12 error is "Battery Disconnected". There is no hex or P code.
The CEL/MIL code on the '96 for an Evap fail is 31, not 12. Hex codes do not show up on the CEL/MIL in a '96.

I checked the Evap system when I first read Ken's post, I thought I was pretty clear I found that it was definitely not the problem.

I took it for granted there are undoubtedly dozens of caps on the PCM, which is why I asked if anybody had any information/experience regarding the memory storage on the PCM.
Being able to correct this issue would be a great help to the XJ community.

I won't think badly of anyone who doesn't have an answer, but when you're not reading the question, my responses ,or the manual, and answering off the top of your head, I have to make the observation that you are not acting in the spirit of his forum.