hi all,

a few months ago i went to the USO lowrider supershow with my xjr. it may seem like an odd car to have surrounded by impalas on switches and bel-airs on air. however, everyone was very kind and welcoming and i actually snagged a trophy!
now for the problem. As part of the shows safety tech, i had to disconnect my battery. in my great brilliance, i brought everything i needed except a wrench to disconnect my battery! someone let me borrow one which fit my negative but not my positive, so i disconnected negative only (i think this was my first mistake?) and shut my trunk. my heart then sank because i had never opened my trunk without electrics.

my key wasn't working. in a panic i dug through forums for a way to get into my trunk. to save cutting into my deck lid, i went for powering the bulkhead lead in hopes of pressing the trunk button. i used a lithium ion battery pack to put power to the bulkhead (i assume now this was a bad idea?) no dice. after reading some more i found that lubricating the lock cylinder usually fixes the issue so i worked the deck lid cylinder til it opened! after the show was over i went to start the car. no lights no dash no power ANYTHING. while my girlfriend argued with the tow company i rushed through anything and everything i could think of to get power. no melted wires, but lots of burnt fuses. they pop everytime i replace them.

the car gets towed home DOA. first thing i do is start looking at which fuses are blowing. the ones that became relevant to me after digging through forums were the bcm fuses. this is where my problem becomes different than most. my bcm was bad. im pretty sure its my fault. i blew the varistors on the board. as i learned from you all the bcm (which is located behind the glove box for anyone looking) only came in a few configurations but you need the VCATS number to match to get it to work. i bought one that i thought was right but failed. so i went to my next plan. i desoldered the varistors from my new board and soldered them to my original board. BINGO. it worked. bcm fuses along with fuses related to the bcm stopped popping and the lights came back on. we're still not starting though.

next up on my blown fuse chasing journey (along with my key fob not working) the security module from under the boot fuse block. sure enough, blown varistors. instead of dealing with the headaches that come with swapping one, i replaced the varistors like i did on the bcm and now the fob works. still not starting though!

at this point the ONLY fuse still popping is the abs module. this was a big wall i ran head first into. the board is wayyy more complicated and i cant find anything apparently wrong with it. im not electrically savvy so unless something is crispy, i dont know whats wrong.
im confident the module is bad though as the fuse stops popping as soon as i unplug the module. i replaced the module and pump and awaited driving the car to see if the lights turn off. still not running though.

this is where my journey got sad. i really didnt know where to go. i was scared my ecm (located under the false bulkhead panel) may be fried but once again, advanced board i could not decipher. fortunately for me a friend works at a body shop as a diagnostics tech and lended me an autel with jag specific software. not as good as some others, but it was enough to get some can codes. bingo. my key wasn't communicating. there was one computer in the car i had seen referenced many times but never heard much about in the forums. the key transponder module. i pulled it out from behind the gauge cluster. lo' and behold, blown resistors and a burnt contact trace as well as a missing contact pad. i bought a donor ktm off ebay, then harnessed my inner louis rossman and used a strand of copper wire to recreate the severed contact trace as well as the pad for the resistor then i used a hot air rework station to desolder the resistors from the donor board and soldered them to my original board. i plugged it in and sure enough my car started!

im sure most people will never go through a situation quite like this but i thought it was an interesting case study. jaguar actually did a good job of putting failsafes into their boards. my car didnt catch fire and the memory chips didnt get fried so it was simple as replacing those failsafes to get the car operational. i certainly learned a lot. this was my first time doing component level board repair. i got a lot more practice in with a multimeter and learned a lot of important diagnostics from the generous contributers on this forum. hopefully even if this info isnt useful it was at least and interesting read.

cheers,
Brandon

bad varistor on the body control module

bad varistor on the security module

damage on the key transponder module (as well as my contact trace repair)

 

Looks like the lithium battery pack you used had much higher voltage than 12V or you, perhaps, connected it with opposite polarity.

 

Well, was the lithium battery pack a "12V" unit or more in the range of 48V?
If 12V it would be hard to fathom that this would have caused harm, as one of the many FORD tests during the design (and that would include a 1998 Jaguar) is the so called "24V Jump start Test": The requirement of this test is that no component in the car would suffer any harm, if some deadbeats do that jump-start on the car, who cannot tell the difference between a 12V battery and a 24V battery from a truck. Thus, theoretically, no harm should come to the car when doing a 24V jump-start... A lot of "overdesign" needs to be put into the Jag or Ford to make sure it passes the 24V-jump-start test...

PS:
I found only now the time to read your thread in full. Kudos for knowing what a varistor is (and as I just noticed: Jaguarforums's spell-checker does not know what it is...! )
That was indeed some journey into the world of wonders of electronics.
Disconnecting battery-minus first also really should not cause too much trouble.

"M." might be on to something: Did you connect the external battery with reversed polarity? There is also a FORD reverse polarity test, but hey, why knows?

 

Wow all that from closing your boot with the battery disconnected 😳😳 I think I might get a reminder tattooed on the back of my hand!

I always disconnect negative first - if you touch the spanner to the car body while disconnecting positive, the current can weld the spanner in place and start a big fire. Once negative is off the circuit is open 👍

 

> I think I might get a reminder tattooed on the back of my hand!
sometimes I read too fast, and then I read funny things - like in this case, where I read:
"I think I might get a reminder tattooed on the back of my head!"
Yes, that would help...

 

I think that's where I messed up. it was a 20v. maybe that would do it? still shy of the 24v test. hopefully I'm not sitting on another catastrophic failure due to an unseen issue. I'm confident I got positive on the bulkhead lug and negative on my strut. either way I appreciate the extra info on development. neat stuff.

in all fairness to the boot I'm the dummy who didn't try hard enough with the lock cylinder I wasn't thinking straight with the anxiety of being at my first major show and feeling like a fish out of water with a euro car at a west coast style lowrider show. I didn't really think much about disconnecting the negative 'til the electronics failed as its never been a problem before but its the first thing that came to mind when dealing with stuff I'm not knowledgeable enough to explain.

 

Since the topic here kind of is "Lithium batteries"...:
I just found an interesting article - pretty much brand new:
https://www.theguardian.com/australi...l-sydney-blaze

I am posting this to make people aware that fire-danger and death by inhaling toxic fumes from burning lithium batteries is an integral part of handling/living with lithium batteries.

 

These types of situations show the need to keep your mechanical trunk locks operational! Without use when you need it you won't be able to turn the key as it will be all gummed up.
Be glad that at least on your generation of XJ you have an actual key slot!! The last X351 XJ does not and that can lead to real big problems. One was especially bad when he disconnected the battery and then his buddy closed the trunk lid.
There is a way around this but you must remove the back seat cushions to get at a certain electrical plug.
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@foxygrandpa Well played sir, and a good read!