I would do a load test by measuring the voltage with engine off and max. amount of lights on (high beam and hazard warning). But as I would not know exactly, which voltage to expect then as "good" or as "fail", I would do the same on another car with a good battery - obviously, the voltages would differ from car to car anyway, but you might be able to determine, if the difference is minimal or significant - latter would confirm that you would need a new battery - which I suspect anyway...

Also: Did you confirm already that you do not have a corroded contact to body-earth? Remove the thick battery minus cable where it is bolted onto the body and clean that contact surface and put it back on. There might me more than one connection cable... Such bad contacts would also have the effect, that you "do see" sufficient voltage, but the current (the big Amps) would not be able to flow thru the "bottle-neck" of a bad contact...

I can explain this further: This is a mistake I made many many decades ago: I had plenty of spare telephone-wires. I wanted to connect the speakers of my stereo in the car. I used that that-phone wire. The sound was horrible (high frequencies only). This does not work because of the reduced bandwidth of the thin wire - or to but it differently: The power for the middle and low frequencies was not able to flow thru the "bottle-neck" "telephone-wire".

 

Well this is indeed a puzzlement. I can go five days or even a week without starting the Jaguar and then when I get in it fires right up. I will head out to the driveway after this to start it up again after a week of not starting it. I fear that this "no start" problem will strand me if I attempt to head out and run errands with the car.The battery with a full load test (lights on, A/C blasting) and I get 13.8v. So the battery seems to be good. Something somewhere is causing P1797 and all the other goblin codes and other intermittant issues with starting. When I turn the key nothing happens from time to time. Well not that anything doesn't happen-the dash/warning lights come on with with the key turned to the second position and then the start position so that means some electricity is getting where it needs to go.

I just don't know what to do with Lord Donnington. And I have an annoying clonk coming from the front left somewhere. I replaced the ball joints and bushings a while ago to try to rid the sound but it is still there. My car may be haunted...

-Scott

 

Some have found that the connecting wires that go into the rear of the gearbox were contaminated with oil,
which produced that problem and code. That can happen if the transmission seal gets too worn.

 

Thanks, Randy,
I read that too and yet my multi plug is dry with no sign of leakage.

-Scott

 

The other thing I found while researching that code was that the brake switch can sometimes throw that code.
Some who replaced the brake switch, found it removed the code.

 

My brakes always put the check brakes light up on the dash when I touch the brake pedal. Replaced the brake pedal switch in 2016 and the warning light was still there. I just live with it. But I can try another switch.

Thanks for the response, Randy.

-Scott

 

I found this - maybe it helps:
https://www.engine-codes.com/p1797_jaguar.html
I also think that the root cause is some kind of communication-problem, i.e. wire issue or module issue.
The problem as you describe is has all the hall-marks of an intermittent electrical failure.

 

I'd bet it's the upper shock mount. Is the rubber in the mount black? Did the same things on my car; finally changed the mounts and the problem went away.

 

David,
I replaced those crumbling upper shock mounts a while ago. They were the crumbling reddish color ones and now they are black Still solid. It must be something else.

Thanks for the reply.

Scott

 

David
You were indeed correct. Addictedtoboost confirmed it as the source of the problem as well. Once I put in the new Meyle upper shock mounts it was all quiet on the Jaguar XJ8 Front....End. My old mounts had sort of liquified.

But the soaring great news is that the intermittant no-start was solved by installing a new starter motor. I bought a rebuilt Denso starter and the arduous task of installing the heavy beast went to a fellow list member who drove 2 plus hours to crawl under my Jag and get the old one out-conquering the stubborn upper bolt and getting the new one in in about two hours with short breaks. It was a truly amazing gift to have someone from the list make that kind of effort to make sure my Jag was back on the road and I am so grateful to Addictedtoboost for doing that for me. The host of other error codes (including the worrisome CAN code) all went away and the ABS light has stayed off as well. The car is its old delightful self and Lord Donnington rides again! Now I just need to get the rear transmission mount installed and figure out why my driver door locks some of the time but not all of the time. And the ever bothersome “Check rear lights” dash warning light which persists in spite of everything.

I am so thankful and humbled by the care and attention so many list members put into solving my issue. Lord Donnington had been off the road for 10 months and driving it has made me love the car all over again. Nothing rides as smoothly or with such gentle inputs and handling. I couldn't be happier to have my Jag back on the roads of Savannah. Thank you all.

 

Update:

The driver door lock issue has been fixed! Addictedtoboost had suggested pulling out the latch unit and cleaning and lubing with silicone spray. Which I did. (Disconnect the door lock to latch mechanism but pulling down on the black rubber bit and then the threaded part pulls right out, I did it another way by removing the circlip and that is a PITA…)

The lock now works effortlessly and consistently,

Now I need to do the other three door locks…

Lord Donnington
Savannah
2001 Carnival Red over ivory hide

 

Knocking out your problems one by one!
Keep posting.
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