This tale is pretty long because I feel I should include all of the symptoms my car showed in case another person's car shows the same ones. Sorry if this topic has been covered before but I searched the forums and couldn’t find anything quite like this. In January I bought a 2004 XJ8 to replace a beloved 2002 Super V8. The new car came from a dealer in Florida with 77K miles, recent service and front air shock conversion. Immediately fell in love with the new car! About a month after its arrival I was driving on the interstate, flat surface maintaining 70ish MPH when suddenly the transmission kicked down 2 gears, hung there for about 2 seconds then upshifted back to normal. This occurred several times over the next few weeks so I changed the transmission fluid (and converted it to a metal pan). The fluid coming out was perfectly clean with no debris of any kind in the pan or filter. I used Global ATF from O'Reilly to refill on the recommendation of a Jag tech that worked for me for many years and is extremely knowledgeable with the X3xx cars. After this the car still exhibited the same odd kick down characteristic at various times, some days more than once per trip and then perfect for several days. Nothing I did while driving seemed to affect it one way or another. Then one day the engine started missing badly on one cylinder and the check engine light came on. I replaced a faulty ignition coil and not only did the engine smooth out but the kick down issue disappeared (albeit temporarily). Note that there was no discernable miss until this point, the engine was silky smooth during the kick down issues. One evening a few months later whilst sitting at an extended stop light the transmission "bumped" like you shifted from neutral to drive except it was already in drive and had not disengaged. A few seconds later it did that again and then twice more at the next long stop. From that point it seemed like the transmission could not figure out what gear it should be in. It would upshift too soon and then kick down 2-3 gears sending the engine RPM's to 4K in a matter of seconds. It happened often enough that I could feel it pull back a bit just as it kicked down and I would let off the throttle. The trans would then upshift to where it should have been but the uncommanded kick downs and "bumping" at extended stops in drive kept getting worse. I spoke to another local tech with a ton of ZF experience and he instructed me to replace the seals between the trans valve body and case (while my Jag tech said he has never done this). I replaced the seals and used the ZF fluid as instructed by tech #2 then we set the trans adaptations using the program he had. The trans ECU showed no stored or pending codes and this repair made no difference. Then, suddenly, the car started running rough again, not a dead miss but lighter erratic misses (but no check engine light or pending codes). My Jag tech said I should replace the other 7 ignition coils which I did (using Denso coils) and the transmission issues completely disappeared. I now have over 700 miles since then and the trans is perfect. Apparently when an ignition coil starts to fail it will send an electrical charge backwards into the electrical system and this charge was affecting the trans ECU even though I could not feel any engine anomalies until the coil failed completely. The hardest part of this was the fact that nothing showed up on any engine or transmission scans. So if your transmission starts acting crazy and nothing shows up on the scans you should investigate the age and condition of your coils.

 

Thanks for posting this.

Rob

 

Thanks for the detailed description. No doubt, it will be helpful to others. It is great that you persisted, through process of elimination, to solve the problem!

Here's to you having many years of trouble-free driving!

Steve S.

 

I had the same problems with my 2005 Super V8 with some notable differences. My car with 110k on it would sometimes jump gears and "flare" but only under hard acceleration. It would bang the 1-2 shift even after multiple adaptation resets, it would clunk going down to 1st at a stop and it often refused to downshift for gentle passing at highway speeds. The error codes were always related to TCM CAN bus failures. I won't go through the more than 14 months of trying to figure this out but there were 2 categories of problems that had to be solved. 1. The trans needed a total rebuild which you can WATCH at ( ) 2. Even after the rebuild fixed all the other problems, the flaring under hard acceleration persisted. This was even AFTER a full new set of Denso coils. The last thing I did was actually 2 things at once so I don't know what finally fixed it. I replaced the new Denso coils with new OEM Jaguar coils (made in Japan) which I found for $70 each AND I replaced the 2 noise suppression capacitors. One capacitor hangs off a metal bracket under the transmission, and the other one is up on top of the right side of the engine. These are intended to suppress voltage spikes from the coils and keep that noise from bringing down the CAN bus!

My theory is that (particularly with supercharged cars) the combustion chamber pressures under hard acceleration can get high enough to prevent the spark from jumping the plug gap if everything on the coil side isn't working perfectly. The spark energy has to go somewhere so it either bounces around in the secondary coil dissipating as heat, OR it finds it's way to a ground source. If this happens near enough to CAN wires it can cause multiple modules on the BUS to reset. The existence of a suppressor capacitor adjacent to the TCM connector tells me that Jaguar knew about this problem! Most modules will reset and you might never notice, but If the TCM gets zapped in the middle of a hard acceleration event, you wind up at full throttle in neutral during the reset. Since the TCM wakes up at 60 mph with the engine on the rev limiter it freaks out and goes into limp mode. This can be maddeningly intermittent since a hundred different things can affect events inside the combustion chamber AND where a voltage spike might wind up.

$600 worth of OEM coils and 2 cheap capacitors and the problem has NEVER reoccurred. This was the most difficult battle with a car malfunction I have ever had and I was close to selling the thing multiple times trying to figure it out.

Glad you figured it out too!

Jeff

 

Same thing here. Fought it for 1 1/2 years. Everything replaced or rebuilt in trans except hard parts. Finally got a checksum error from tcm. Looked at obd2 misfire counter and found anomilies but not enough to set ecm codes. Had already purchased a nice used trans with convertor and new oem coils and plugs. Coils and plugs came in on sat. Trans was not due until mon. Decided to do coils and plugs while i waited. Boom. Everything fixed. Along with the ignition parts and rebuilt convertor replaced a few weeks before car never and i mean never ran so sweet. Went from 22 mpg max to 28 mpg on first long trip. Went on 4 longer freeway trips since and always 27.8 to28.1 mpg. Amazing. If i would have just swapped trannies i would have solved nothing. Apparntly the tcm is way more sensitive to input glitches than the ecm is. I plan on keeping car for as long as it will have me so having a nice zf6hp26 R model trans and convertor at the ready is very nice although i feel will never be needed

 

I first felt too lazy to read such a long story, but it's really really a very valueable information.
Hopefully I never get into this troubles, but it made me feel more confident

 

So, my friends Super V8 would literally jump out of DRIVE and redline AT 2500 rpm and the re-engage as revs came back down.NO CODES or any indications at all.
So after changing transmissions and a whole bunch of other things (like wheel speed sensors and every other thing we could find on the internet , (I even have SDD - nothing!!!) , it eventually turned out to be a small hole in the plastic intake manifold which we eventually found by smoke machine check. Cheap party machine works! no need for huge expensive ones. Changed that for my known good one and PURFECT. straight away.
We manufactured an entire intake replacement for under $90, using silicone connectors and stainless pipe and We are now confident that we will never have issues with that $400 PIECE OF CRAP AGAIN.

 

I had a similar problem on my 2005 Super V8 about 2 years ago. The transmission would feel like it was slipping when I would accelerate at anything more than a slow pace. I can't remember the exact codes, but if I recall, they indicated a misfire on #3 cylinder. I had read somewhere on the forum that it could cause the TCU (transmission control unit) to give bad data to the ECU. Garbage in/Garbage out. So, I replaced all 8 coils and plugs and reset the codes. Car has been running smooth and no transmission problems ever since. These cars have many control units and computers and they all talk to each other. If something isn't right, it passes that info on to other systems and can cause seemingly unrelated problems.

 

exactly. The tcm is the most sensitive i have found and shuts down the can network. All driveability defaults to each module base program and limp home. Canbus is ecm tcm abs instrument cluster on my car. Once one of the goes no comm the network shuts down