Here is the moral of the story upfront instead of at the end:

Greed is NOT good.

Like many owners of decade old cars, I have swapped in a fair amount of LED replacements for my incandescent interior lighting and some exterior lights as well. Swapping the reverse lights might have taken it a little too far, in any case I have created an issue that does not correct itself just by undoing my butchery.

Amazon sells so much LED replacement and non-DOT approved (illegal) headlight bulbs; finding standard incandescent bulbs such as P21W's has become a challenge.

I have tinted glass and wanted to use LED reverse lights to brighten rearward vision at night. I tried several brands claiming "canbus error free" etc., that would sometimes work with one installed, but two would cause them to glow when ignition was completely off, from the super low resistance, and I think from the small current required to check for failed bulbs.

All the "expert advice" available (from teenagers hacking the lighting harness and running Chinese resistors across the circuits) pointed at getting more resistance into the circuit. "One side is sufficient, no need to add resistance to moth sides"

All of which MAY be true, sometimes, and MAY work, sometimes, and while we aren't dealing with Lucas components and hand made wiring harnesses, it still begs the question:

WHAT ON EARTH WAS I THINKING MODIFYING ELECTRICS ON A JAGUAR OF ANY VINTAGE?

Not that I did anything too Frankensteinian, but I DID run a 50W 6Ohm load resistor across the reverse light circuit. This:
https://www.amazon.com/CUTEQUEEN-TRADING-Resistors-License-Cancellor/dp/B00DCULNXS


Which somehow caused a persistent but hopefully not permanent lighting issue. I returned the setup to "pre-borked" condition, including buying a new taillight for the side I spliced wires, after further messing them up lengthening wires that became too short by adding small extensions; clean soldered splices, with heat shrunk insulation, not twisted crap with black tape .

Electrical work is not among my many and diverse talents, but replacing a tail light for a couple of small wire patches was a wasteful diagnostic move.

Current conditions:

• All bulbs are stock, oem. There are no substitutions such as 1156 for P21W etc.
• Running Lights/Brake Lights OK. These are the stock LED sets on the outer edge of the cluster
• In reverse the right lamp is super bright, left super dim. Amazed the right isn't blowing it's super bright.
• When rear fog lights are turned on, they glow dimly, AND the reverse lights glow dimly as well.

These are the only lights exhibiting issues. The light I screwed with , and replaced was the left. Note that Tail lights are listed as changed in 2006 at VIN H01625, so I was careful to replace it with a light post VIN H01626. The change is not cosmetic, so I assume it's the PCB revision for the LED cluster.
The only similar situation I have come across is the following, fixed by replacing rear fuse panel. This I could handle, as well as a rear electronic module, though that looks less likely.

https://www.jaguarforums.com/forum/xj-xj6-xj8-xjr-x350-x358-28/rear-lights-150992/

After stupidly replacing the left light, thinking that since it was the only item I modified, replacing it would restore function, I want to cease and desist diagnosing by random part swapping. The 3 incandescent bulbs all share a common red wire, that logic dictates is ground and individual socket colored leads for positive -reverse= brown, directional=orange and reverse=white. (what is with the British NOT using brown or black for GROUND?)

I HAVE carefully checked and cleaned the 3 ground points nearest the lighting harness.

Apologies for the novel length post. I wanted to provide as much detail as possible to save some of the Q&A that goes on. It's like pulling teeth to get a full picture sometimes, it seems. Thank for any ideas how to resolve this. Suggestions on relay swaps, or SIMPLE testing would help me a lot. Thanks all!

 

This thread gave me an idea:

https://www.jaguarforums.com/forum/xj-xj6-xj8-xjr-x350-x358-28/right-rear-tail-light-brake-light-not-working-37827/

Swapped relays. No change, but put of sheer desperation, I did what anyone who DOESN'T consider himself some hotshot diagnostician. I checked the freaking fuses. Each side uses 2 fuses. One 5 amp for the 3 incandescent bulbs and a 10 amp for the LED lights. Seems backwards, but after just completely over thinking this thing I should shut up with my theories. The 5 Amp Left fuse for Fog, reverse and directional was blown. Replaced and everything works. The mystery? How did the reverse and fogs light, even dimly with a blown fuse in the circuit? Answer: We may never know. Cats are strange. Jaguars are weird. That much we know.

 

I see you have already resolved the issue.
The British do indeed use Black for Ground (well, usually!), but what you're seeing there is the switched negative concept used by Jaguar in their lighting setup, where one side of the lamps are connected to +12v and they are activated by switching the other side of the lamp to Ground (known as 'sinking' current). So, you would have been finding the common +12v rail on the red wires.
6 Ohms at 12V is 2 Amps, so maybe that load plus the existing fog and direction lamps pushed you over the 5 Amp limit of the fuse causing it to blow, or maybe an accidental short if you have +12v and Ground mixed up?
I don't know the power rating of the existing reverse lamps, but if you find this value you could use the calculation I=P/V to work out the current this would draw, and then Ohm's law (R=V/I) to calculate the equivalent resistor value.
For example if the rear lamp was 10w (just guessing): 10/12 = 0.83A. 12/0.83 = 14.4 Ohms.

Ben

 

Thanks Ben! You sir, are one of the rare breed that analyze things to LIFE. I, on the other hand, analyze them to death. Appreciate the knowledge transfer either way.

Example: I need to get someone with steadier hands than I to solder an "aux-in" mini-plug onto my audio head unit -the pins you identified on cassette and CD units. I explained in such detail to a local TV repair shop via email that the reply simply said "we don't do that kind of work."

They dont solder? Like I say, I think I may have over explained the situation, lol.

By the way, I bought a cheap head unit so I could have the jack pre-installed, and then swap out when I pull the console vs. having it disassembled while waiting on the solder job. But the unit is from a non-touchscreen car. Looks like the front casing, top cover and other mounting points are somewhat different. It appears that the PCB is the same. I don't know if you have modified any of these, since your upgrade ideas are based around the screen. I don't mind pulling anything apart and reassembling it into my case (anything but fiddly soldering tiny contact points!) Do you think they are interchangeable? I should probably just find someone who can solder it same day, but that is not complicated enough!

 

The boards in both type of units do look the same, however I would imagine that the firmware in the non-touchscreen CD player is different to support the extra buttons not present on the touchscreen version. I've not removed the full PCB from a unit before and it could be quite a bit of work, I would recommend getting hold of the CD player for a touchscreen unit to guarantee that it's the right type.

Ben

 

After you put this new 10 inch screen on, do your Front parking sensors still work? How to view the full image (parking sensors) - in the 8 inch system, press the "P" button on the left hand side

 

This reply ended up in the wrong topic for some reason.