1996 XJ12 unable to pull OBDII codes consistently
 

I have a beautiful 1996 XJ12 that runs great other than an occasional check engine light that comes on. I am unable to reliably pull diagnostic codes from the ECU and have tried to do so using several generic bluetooth/wifi OBD adapters, an Autel diagnostic tablet, and even official NC inspection equipment at a friend's shop. 99% of the time attempts to connect over OBD result in a link error.

Once in a blue moon it will connect to the ECU while driving having a OBD bluetooth adapter plugged in and I can reset the code which is map sensor related. I have a new map sensor on the way but this obd issue will be a problem in the future.

I purchased a used ECU thinking there was a problem with the original one keeping OBD from working but while the car runs great on the second ECU it still won't pull codes. I read that all the modules use the instrument cluster as a hub for OBD communication so I tried replacing the cluster and that didn't help anything. I have also tried jumping wires straight from the obd port to pins 22 and 23, the pink and orange serial communication wires, on the ECU without success.

I have checked and tightened all the grounds shown in the electrical diagram. None of the ground points had any corrosion and there is no corrosion on the ECU connector.

Is it normal in XJ12s for the OBD to be this flakey? I have a 1996 XJR as well and OBD functions perfectly in that but I know the ECU is completely different in the XJ12.

If anyone has any ideas on this I would greatly appreciate it. Thanks!

There where from what I understand engineering problems in getting all the bugs out to integrate a OBD 2 ECU to this engine . US government apparently stopped sales in the US for this reason .

Might be something in the TSB's :

http://www.jagrepair.com/XJ6XJ12TSB.htm

Otherwise this my be helpful :

http://www.jagrepair.com/images/Trai...20-%202000.pdf

The V12 starts on page 193

Great! That training guide looks like it has a lot of good info, I'll read it over and see if I can figure anything out. Thanks!

I have no troubles reading codes from my early build 1995 XJ12, using an older generic code reader. However, I do occasionally get a check engine light with a code saying that the code library is not found! When I hook up, it finds the code library as usual and I can reset it. Other than that it has always worked fine.

The OBD2 system is somewhat buggy in the 300-series as you would discover on the forums here.

Seems like you have done some worthwhile checks. If I recall, there is a fuse for the OBD system, meaning more electrical connections to check in that area. I'd have to look it up for you later.

Edit:

I actually found two fuse locations for power to the data link connector: F10 (5A) in the right-hand heelboard (an ignition switched supply), and F6 (also 5A) in the trunk fuse box (always on). Check those out.


The XJ12 didn't have the proper misfire monitoring under the OBD2 rules. It was already decided that production was changing over to the V8 engine. So Jaguar decided not to sell the XJ12 in the USA market for 1997, rather than invest in a solution.

Welcome to the forum. Not many XJ12 owners around anymore, we are always happy to have another come along. Say hello in the Intro section and post up some pictures if you can!

This didn't make any sense.
 

Now it makes sense:

The code I occasionally get is not included in my scanner's library. It simply can't display what the code means. It displays "DTC definitions not found; refer to service manual." For a while, I would just reset it. I took it to mean that the ECU lost its code library! Seems odd. But that's not the case. If it said "definition" singular rather than "definitions" plural, I might have got it sooner. Nevertheless, it did display a code: P1106. When I finally looked in the diagnostic manual, I discovered the code is for... guess what... MAP sensor, bank B!

The scanner doesn't have V12 specific codes, and P1106 and some others are not used for the AJ16. Now I know. :icon_doh:

On a positive note, now that I figured this out, my OBD2 seems to work quite well! I don't notice any difference in performance when that code shows up either. Tests on the MAP are pending.

Jag North Carolina, are you with us? Looking for any updates please.

The MAP sensor should read on the middle wire 1.7 volts DC at idle according to the 96 wiring guide breakout sheet . But the reference voltage from the ECU also ties into the throttle position sensor so you can remove and clean that connector before or after testing as it my be sucking the signal down .


https://cimg0.ibsrv.net/gimg/www.jag...7b3a89ba5b.png