Ever since I've got the car, the only OBD error code I've scanned, every time I hooked the tool up, has been P1797, marked as "pending" (aside from the usual P1000/P1111). Erasing the codes does nothing, the error still comes back on the next startup. However, the CEL turns off when driving. I'd assume if the P1797 was actually pending, it would illuminate the CEL.

Does this code get stored in the TCM in a way that required higher level software to clear it? Otherwise, does anyone know what could cause it? The merc OBD error code table lists it as a communication failure between TCM and ECM. I keep forgetting to test the CAN wiring for faults, but I'd assume if there was a break between the two then there would be many more issues/car wouldn't start or shift/etc. I pulled the connectors and cleaned them for both the TCM and ECM, no change. The default action can be one of two things: limp home mode if the ECM token message is missing, or substitute engine torque and speed values for high ones to prevent transmission damage. It definitely isn't in limp home mode, and it seems to shift fairly smoothly, which I'm guessing it wouldn't for the latter action.

I also saw in the OBD transmission guide that there are codes that are "non OBD II DTC" which I'm guessing means they also need higher level equipment to read them? I figure there could be some that may point to a more specific fault.

The fault code is near the bottom of this PDF http://www.jagrepair.com/images/Auto...tem%201998.pdf

I did some slummin' and came across this post on JaguarForum. It was the last one in the thread and by the OP, so I'm guessing it was his answer and hopefully give you some direction...
"Well hopefully ive finally sorted the gearbox fault,just returned with the car from my local garage as i was still getting error messages ,hes cleared all fault codes again and found that the connecting wires that go into the rear of the gearbox were contaminated with oil,as the seal is starting to perish so hes recommended i have a gearbox sump and fluid change in the near future and change the seals that go around the wires at the same time."

Hi,

Since you got the Mercedes transmission built-in, you probably got the same fault I experienced 2 years ago. I didn't had any codes but noticed an ATF leak after a couple of weeks of downtime. The O-rings of the connector going into the transmission (front right side of the oil sump of the transmission, coming from the TCM) are known to fail. As they do, the oil will pass therethrough. There are two O-rings. As the inner O-ring fails, you wouldn't notice it necessarily since the oil enters the connector. As the outer ring fails too, you'll have an ATF leak and you'll notice it one day on the garage floor. The tiny tiny amounts of oil entering the connector (and therefore the wires as well) are known to be drawn into the leads of the wiring loom (and in the worst case up to the TCM) because of capillary forces. This can cause the DTCs go nuts.
If you browse through the Mercedes forums, you'll find quite a number of drivers having issues with that. Some of them even had an oil barrier implemented (I did consider this too, I even have the parts on stock but didn't install them yet).
Just replace the connector with the upgraded version or O-rings only (Connector Mercedes Benz Part. A 140 270 02 5, O-rings A 026 997 40 48 and A 026 997 41 48, or e.g. FEBI Bilstein 36332) and you're good. Easy job when changing the ATF. Also possible without draining the ATF when front of the car jacked high up.
By the way: it's not the connector being the faulty part, it's the O-rings. Their diameter is too small so that they don't seal properly anymore when aging.
If you need some more info, I'd be happy to help.

That's some good info xjr2014_de....is that what appears the gentleman is speaking of in my post? That came from a thread from 2010 and they never got more specific regarding the repair.

Thanks, my pleasure.
Yes, the gentlemen's issue could be quite similar. Even though the S-Type didn't ever had the Mercedes W5A580 transmission fitted (since the thread is from the S-Type forum). The 4.0 V8 NA S-Type had a Ford 5R55N transmission. And when the S-Type R 4.2 emerged, it shared the stronger ZF6HP26 transmission with its XJ350 brother. I didn't know that their wiring connector seals are prone to fail too, but a quick google search has shown that these boxes share the same issue with the W5A580 :-) And yes, the connector of the ZF boxes is located at the rear of the transmission - as the gentlemen describes.
Cheers, Alexander

I'll take a look when I get a chance. It seemed like the connector didn't handle CAN signals when I read through the description, but I guess it could still be related, especially if it's drawing ATF all the way to the computer modules.

When you said with the front of the car jacked up high, do you just mean for the sake of reaching the connector? Or will the fluid level be above the connector when the transmission is level?

Exactly like this. If the car is level and you unplug and unscrew the connector, you'll mess things up since the ATF is leaking badly. You can prevent this by jacking up the car at the front, so that the connector is above the ATF level.
In my case, I didn't jack up the car enough, so I still had some ATF running out. Was quite a mess with the ATF all over my hands, arms and my face while lying on the floor.
So I recommend the jack-up method only in emergency cases.
In the first place, just pull the transmission oil pan drain plug, let the ATF drain, replace the drain plug and the copper seal ring with new, do the connector and refill the transmission with ATF.
Optionally you can pull the oil pan after draining and replace the oil filter and the pan gasket as well, as it costs just a couple of minutes more when you drained the transmission anyway.
As parts you can use regular Mercedes or aftermarket Mercedes parts. They're much cheaper than the Jaguar parts.
I do recommend replacing the drain plug as well, because it has just a internal hex socket, which wears quite fast. And since it costs only a few bucks, it's not worth bothering with a stripped screw head.
Here's the shopping list when I was doing the oil change:

• Fuchs Titan ATF 4134
• Mann Filter H182 Kit (pan gasket + oil filter)
• Mercedes A0009976532 (oil pan drain plug)
• Mercedes A1402710060 (drain plug copper seal ring)
• as oil dipstick you can use any dipstick offered on e*** for the W5A580/722.6/NAG1 transmission

In general you can use the regular aftermarket service parts for the MB W5A580 transmission as Jaguar didn't customize the parts used for the XJR.

By the way, here we go with the corresponding TSB :-)

Sounds good, how many liters did it take for the drain/refill? A trans fluid change was on my list for the future, I guess I'll push it forward a bit and add those parts to my shopping list. I had actually just ordered a dipstick the day before so I could check ATF levels and condition, should be in next week.

For draining and refillig via the oil pan you'll need to replace around 4 liters...in total, the transmission with torque converter, oil lines and cooler contain 9.5 liters of ATF.
Since you got the '99 XJR, you might have an additional drain plug at the torque converter too - then you'll get even more out of it. The early /R's had this additional drain plug, but at some point in MY99 it was deleted. My car hasn't it (MY2000) - so I can't share any experience with that.
As the oil pan doesn't have a built-in magnet and the drain plug isn't magnetic either, I bought a small, strong neodym magnet (size of a penny) and put it into the oil pan...just for my peace of mind ;-)
Cheers, Alexander

EDIT: please find attached the torque settings for the oil pan bolts and the drain plug