XK8 / XKR ( X100 ) 1996 - 2006

The saga of the P1797 and U2500, TRAC, ABS and Transmission Failure

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Old Jul 15, 2020 | 10:27 PM
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Default The saga of the P1797 and U2500, TRAC, ABS and Transmission Failure

In the hopes this lengthy story helps someone...

So, it's like this. Starting over a year ago just after I purchased it, car shows TRAC, ABS and Transmission failure, mostly the engine stalls, but occasionally goes into limp mode. This happened fairly rarely, but often at speed, and often enough that I developed a 'cure'. Wait about 10 seconds as I coasted and put on the flashers, put the car into neutral, turned the car off and on and drive away like nothing happened. This terrified the wife and daughter (once at night on interstate 95, daughter wasn't fond of me turning off the car while moving about 80 mph without lights, but I didn't let her interfere with my 'checklist'), so I figured that there might be a better plan, fix it.

Codes that appeared always were P1797 in the transmission (CAN TCM loss of communication with ECM) and U2500 in the ABS (CAN general loss of communication). More rarely were P1260 (security input, key module), P1797 in the ECM (CAN ECM loss of communication with ECM), and assorted other random and nonrecurring errors. Most everything pointed to CAN bus problems. I looked at everything I could find on this and other forums and pretty much did everything anyone suggested for this problem that several others appear to have had. So I

1. Removed the ABS module and resoldered the power connector and several of the data connectors including both CAN bus pins.
2. Removed the gear illumination unit resoldered everything. It looked like crap anyway, and I needed to replace the light bulbs.
3. Removed the instrument panel and resoldered the CAN bus pins and the power pins (and assorted other pins that looked like they had been soldered by a blind person).
4. Checked all of the grounds. No problems, 'fixed' them anyway by reseating them.
5. Checked the CAN bus connectors everywhere and resistance across segments and the whole, sprayed everything with gallons of electronics spray. No problems.
6. Checked the battery. No problems since it was brand new anyway.
7. Checked the alternator. No problems.
8. Fixed the fan in the ECM/TCM box which wasn't working. So, maybe it was just heat in the box? (foreshadowing... not heat in the box.)

Bottom line, still a rare issue. I had figured out that it most often happened after I had been driving for a while, turned the car off for some length of time (15 minutes to an hour or so), restarted and it happened within about 5 minutes. It was associated with external temperature, most often happened ~30C (~85F) or higher. So, since it was still happening through the summer, I got IDS, but learned nothing that I didn't already know. So, I

8. Decoded the CAN bus command set for the XK8 2001-2002 (this took quite a bit of time) since I figured that might help. Eventually it did, but not immediately.
9. Measured the temperature on the ECM, since the ECM appeared to be part of the problem, maybe it was overheating. Turns out in the worst of the conditions I tried (high heat outside (~39C - 102F), high heat in the interior (full heat on HI), high speed for about 15 minutes (I'd swear it was never over 80 MPH, officer), almost killed me, but didn't get above about 70C on the ECM. The spec is 80C in operation, so probably not the problem.
10. Purchased a CAN logger so I could tell what the bus was doing at all times.

But, everything still pointed to the ECM. The thought was that perhaps when the ECM/TCM fan didn't work, it damaged the ECM? Meanwhile, I happened to catch two of the events on the CAN logger. What the heck? The ECM WAS ACTUALLY TURNING OFF DURING OPERATION for about 3 seconds causing stall and the codes!

So, now completely convinced it was the ECM, I tried to schedule someone locally to do the swap. The Jag dealer wouldn't touch it unless I bought an ECM from them (cheaper to buy another car), and the local independent I trust never got me on the schedule. I waited about 6 weeks (now early fall) . In retrospect I had no idea how simple it was, so I suppose they had little interest in billing half hour when they were booked solid for weeks/months. On the other hand, if they had told me the actual swap would take minutes... In any case, based on Bob's advice in the forums, I went and

11. Purchased a 2002 ECM and swapped ECMs using IDS. This seemed to fix things (but keep in mind it's fall) and nothing happened for weeks so I
12. Bought two more XK8s because I love driving them and wanted to spread the love among the family.

Seemed like all was good with the world, until one warm day in the fall, it happened again. What the hell? So probably not the ECM. Probably not the CAN wiring either, but I

13. Checked all the wires and grounds again. No problems.
14. Checked the alternator and battery. No problems.
15. Checked the connectors on the instrument panel. My solder joints were still better than the original, so I resoldered more joints, some of which looked great anyway. Not hopeful on this 'repair' since it actually looked fine before I started resoldering because I had a soldering iron and i was already in there.

It only happened a couple of times in the winter, every time after I had driven for a while, stopped the car for a while, then right after starting again. Did I mention I rarely lock the car door, either at home or when I'm out. That may be important later....

So, the last major thing suggested in the forums was replacing the instrument panel, but I had been in there and the traces and joints looked great. In any case, I

16. Purchased an instrument panel on ebay. I did avoid the one 2 from my VIN in case the problem was generic to a model run and got a 2002 with about the same mileage.

I dithered on replacing it throughout the spring because COVID took up every waking hour with increased workload. The ECM symptom/stall/reduced performance mode still happened, but rarely, perhaps once a month. I might mention now that until today, I only had one trickle charger and 2 XK8s in the garage. That might be important later...

At some point, with some minimal prompting/help from another forum member, we

17. Decoded the CAN mode 22 PIDs for the 2001-2002 XK8 (this took some time). I figured that might be helpful. Note: as far as I can see this is one of the most complete public datasets for CAN bus commands for any car (or the most complete open dataset. SCP bus is next...).

So, I could now look at many many things with the logger while the car operated. About three weeks ago, it got warm (over 35C) and the failure happened twice on one out and back run (turned of the car in the middle). Unfortunately, the SD card wasn't in the logger and I had no data. The next day, I had the logger on when it happened again after driving to a restaurant at night, eating, then happened on the restart. Lots of data, but nothing clear and I didn't have time to look through all the data. Irritated, I

18. Replaced the instrument panel the next day.

It was still hot, in general, and it happened twice the next day and on each of the two days following. With the new INST, fascinatingly enough, the ECM doesn't turn off, but the same thing happens, TRAC, ABS, Transmission, limp mode (less terrifying for the wife since the car is still under power). Now, really irritated, I

19. Looked at all of the CAN data I had on the three or four most recent failures.
20. Recorded comparison data on the XK8 driven most often next to mine by day and by night.

By now, I had a sneaking suspicion what may be happening. One of the mode 22 CAN PIDs is IGN+ or ignition positive on the TCM. I happened to be recording this during a couple of the failures. Other helpful things are the logged timings of the amber and red lights, and INST messages, engine RPM, wheel speed, etc.

Reference data from the 'good' XK8 shows a low of about 12 V when starting and about 13.75 V under power, constant no matter how long one drives. The 'bad' XK8 recently started on a typical failure day at about 10.5 V (or even lower, surprised it started) during the ignition and bounced up to about 13 V while driving, eventually declining to about 12.5 V by the end of the 15 minute drive. Crucially, on restart, it started at about 11.5 V, but never made it back to 13 V before stalling, bouncing around 12 V. On restart after the event, the voltage climbed back up slowly, presumably as the alternator saw more airflow.

So, what's happening? I now think heat sensitivity in the voltage regulator (perhaps diode related) is causing weak generation in the alternator in exactly the condition that I see the failure (you can actually see this in the data at night with the AC on where the voltage hovers around 12.25). This is exacerbated by

a. Heating up the engine, but then sitting without airflow into the alternator making it nice and hot on the restart.
b. Use of the AC increasing car electrical load
c. Use of the lights increasing car electrical load
d. The summer heat decreasing convective heat transfer from the alternator when it's sitting without moving.
e. Dale generally letting the car battery drain by leaving the doors unlocked in the garage and using the charger on the most rarely used XK8 (Wife's, she prefers to be terrified in my car.).
f. Dale generally letting the car battery drain by never locking the doors anywhere. Power draw on the unlocked car is much greater than the quiescent draw with the car locked.

I wasn't able to see what was going on because every time I checked voltages on the battery/alternator, temperature was cool and voltages looked great. To check this hypothesis (it was happening every time I went out during this period, often multiple times), I simply charged the battery with a 'regular' battery charger. The transmission voltage data still showed decreases in heat, but from a much greater baseline voltage. So, nothing is failing now. Even though it's hot, I have not had an event since, fixing it for now. So, I

21. Purchased a dedicated battery charger for my car.

And, crucially, I am

22. Replacing the godforsaken alternator (Bosch, but open to suggestions).

Hopefully this fixes this forever. 'Positive' things (pun intended) about this experience are 1. Decoded CAN bus and CAN bus PIDs, I expect this to be helpful in general. 2. Learned an enormous amount about the XK8 electrical system. 3. Have a bunch of likely good modules for future replacement. 4. Got to practice my diplomacy and multitasking skills calming wife while trying to avoid being run over on highway.

Moral of this long, long story, as always, if it's incomprehensible electronics issues, it's probably the battery (or alternator in this case).




 
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Old Jul 16, 2020 | 12:53 AM
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Default Now that’s a saga and a half

Dale,

LMAO! Now that’s a real story! After your forensic CAN bus diagnostic R&D, most of which I don’t understand, it comes down to hot weather, a low battery, and a “dodgey” alternator.

In the extreme climate of the Middle East, I too see some weird warning messages during the Summer especially during a long run out in heat of the day.

A year ago I tried and failed the R&R of the alternator. During this failure, I cleaned the alternator’s connections of all the engine oil spilled during a previous long standing cam cover gasket leak. Thought of trying again to put in the spare alternator this year but a garage tested the battery and alternator and all was good.

Morale of the story - low voltage during a run, causes all sorts of weird problems. Right now for me, I’ve an intermittent ABS and Low Brake Fluid warning. Think I may have to hit the ABS module power pin resolder, but just putting the car battery overnight on the tender seems to minimise this problem.
 
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Old Jul 16, 2020 | 06:44 AM
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Dale,

Not only is that a hell of a story, it is one of the strongest cases of absolute persistence in recent memory....

Instead of replacing your alternator, may I suggest having it refurbished. In July 2018 the alternator in my wife's 2006 XK8 turned on the appropriate red "I'm dying" warning light on the dashboard. I had read the stories here on the forum regarding brand-new aftermarket replacement alternators having various compatibility problems so I elected to find a shop to refurbish this one. Being a Denso unit, I figured it would be easy to find a local shop here in the Raleigh / Durham area to do it. I was wrong. I spoke with at least a half-dozen electrical shops in the Triangle and no one was willing to give me the time of day. A Google search finally pointed me to Alamance Starter & Alternator Service in Burlington (336-228-9654). I called them and spoke with Mark Greeson (the owner), explained my dilemma here in Raleigh, and he immediately said sure, bring it in, we fix Denso units every day of the week, and "Jaguar" doesn't scare us off. So I pulled the unit, brought it to him, had a very pleasant conversation, and left him the unit. He sourced the appropriate Denso kit (regulator, diodes, bearings, and brushes), rebuilt the unit to better-than-new condition, called me within a couple of days to let me know it was ready, and charged me $120 which I gladly paid. I prefer having the original factory alternator back in the car because I know there will be zero electrical compatibility issues with it. And I was certainly grateful to find a shop so willing to work with me....

Before you decide to source a new aftermarket alternator, give Mark a call and see what he can do for you....
 

Last edited by Jon89; Jul 16, 2020 at 06:54 AM.
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Old Jul 16, 2020 | 07:53 AM
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Originally Posted by DavidYau
... it comes down to hot weather, a low battery, and a “dodgey” alternator...
Indeed, there's no escaping the irony...
 
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Old Jul 16, 2020 | 09:34 AM
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Dale,
What a journey!
Hat off to you Sir, for your tenacity and ultimate success.

I wonder... the OEM alternator would have been a Denso unit:- perhaps the Bosch isn't the correct spec and is a tad short of 'grunt'? Maybe the PO was scared often enough to part with the car...

Heaven knows what this would have cost in diagnostic time.

 
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Old Jul 16, 2020 | 10:04 AM
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Michael,

I interpreted Dale's post to mean that he is considering purchasing a Bosch unit to replace his factory Denso unit. But I could be wrong....
 
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Old Jul 16, 2020 | 10:21 AM
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Originally Posted by Jon89
Michael,

I interpreted Dale's post to mean that he is considering purchasing a Bosch unit to replace his factory Denso unit. But I could be wrong....
Yes, currently a factory Denso unit. Reman Bosch and Denso (from Denso and Delco) are about the same cost (about $120 without core charge).
 

Last edited by crbass; Jul 16, 2020 at 10:33 AM.
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Old Jul 16, 2020 | 10:39 AM
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Originally Posted by michaelh
Dale,
...Maybe the PO was scared often enough to part with the car...
Seems likely at this point. I got a good deal on the car, some minor issues except for this, it had been sitting for a while.
 
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Old Jul 17, 2020 | 03:12 PM
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Great write-up!

I'd stick with an OE equivalent alternator, which I think will be Denso.
 
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Old Aug 13, 2020 | 01:13 PM
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Hello fellow frustrated Jag owners who have or experiencing this problem. I have a 2002 XJR100 with 63K miles 'YES' still a baby. That said, I now believe that this issue is electrically related as it pertains to heat generated on a journey and the condition of the battery. I replaced the brake sensing switch LNB6420BB after reading that that might fix the issue, it did not in my case. I have a 3 yr old good battery (I believe) but will have the alternator checked out shortly. I would certainly like to get this issue solved once and for all, very frustrating for sure. Drive well & be safe!! Trevor - Vancouver, BC
 
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Old Sep 4, 2023 | 10:30 AM
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Dale, was this issue resolved? I have very similar problems with a 2005 S-Type R.

Did you change the alternator? Where does the voltage regulator reside (inside the alternator)?
 
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Old Jan 22, 2024 | 12:36 PM
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Originally Posted by amoeba
Dale, was this issue resolved? I have very similar problems with a 2005 S-Type R.

Did you change the alternator? Where does the voltage regulator reside (inside the alternator)?
I would like to know this, too.

Dale tried to help me last year with my plagued '97 XK8.....all the typical ABS light-up issues. He loaned me his scanner, but I really wasn't capable of using it right I don't think. I ended up having both the ABS module and the ECM "rebuilt" by Module Repair Pro in SoCal. Did not solve the problem so I replaced the whole ABS pump and module unit from a working car (hit in the rear). Still same Christmas tree and same symptoms of occasional limp or dying briefly. I am supposed to do a continuity check on the ABS to ECM CAN wiring, but have not had time.

The alternator rebuild or R&R seems so simple compared to all the other potential fail issues.

SO.....WHAT DO WE REALLY KNOW RIGHT NOW???

I know this is a 3-1/2 year old post by CRBASS, but maybe he will see this and tell us what happened?

Ken
 

Last edited by oldjaglover; Jan 22, 2024 at 12:53 PM.
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Old Jan 23, 2024 | 07:09 AM
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Not sure if Dale (crbass) is currently active here on the forum. He moved to Michigan for a new job last summer and was extremely busy as a result. Hopefully he will check back in with us at some point....
 
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Old Jan 24, 2024 | 12:40 PM
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Originally Posted by Jon89
Not sure if Dale (crbass) is currently active here on the forum. He moved to Michigan for a new job last summer and was extremely busy as a result. Hopefully he will check back in with us at some point....
Thanks. I hope he gets settled in and can still enjoy his Jag(s), but I don't think of Michigan as a place for our seemingly fragile cats. Dave has done so much research and experienced so much with his own cars he is a great asset to the forum. I hope he'll be back soon.

Ken

 
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