Hi all,

My tail lights aren't working - head ligths, plate lighting and brakes are ok.
Bulbs are fine too - tested with external power source and the second filament lights up.

The BPM is properly powered.

It sounds like the wire is cut off somewhere between the BPM and the trunk?
i have identified the concerned wire, which is the green one on the side of the rear lights connectors, and gets no power.
I don't suppose the BPM itself is buggy, and I haven't tried to disconnect the battery yet.

The cut wire seems the likely cause to you guys? And how in earth do I find the location?

 

Worth checking this connection here

 

Just did, and as I thought there is no issue here, all wires properly soldered.
Only the tail lights are down, all others seems to be fine, hence my supposition that their power cable is cut off somewhere...

 

Further tests: after reading more subjects about tail lights issues, I figured there might be a broken ground cable at the security control module. I've remove the fuses and relays box from the trunk and checked the module thoroughly, nothing to be seen here..

But even if it were a severed ground cable, I suppose I'd see a voltage at the power wire ? This wire remains at 0V always.

I'm at a loss here really

 

Have you checked the following fuses?
# 3 / 15 amp RH heelboard fuse box

# 5 / 15 amp LH heelboard fuse box

# 7 / 15 amp LH heelboard fuse box

 

Yes, definitely check the fuses if you haven't. I'm attaching the tail light wiring diagram from my 98 XJR. Not sure how much it will help, but you might be able to locate the wires at the BPM and see if you have power to them at the BPM. If so, then you probably have a broken wire someplace. I hesitate to believe that since neither light works but it's possible I suppose. Most likely a blown fuse.

 

ok I've rechecked these fuses and they are ok. Removing one of them (number 7 LH if I'm correct) shuts down the license plate lights.

from the diagram, I understand that the tail lights have nothing to do with the SLCM and are only controlled by the BPM.
I should probably check whether there is power on the concerned BPM pins (FC15-28 and FC15-54) when the lights are supposed to be on ?

The issue appeared right after I changed the bulbs from the center brake lights actually. One was fried, and 3 others went out when removing the assembly, so I ended up changing all 5. I don't see how this could have had any impact on the tail lights, but there may be a relation here.

 

Amusing, sort of: prior to dismounting the BPM, I checked for power at the 5A fuse in the trunk -- fuse #21, which is the one for the tail lights. There IS power when the lights are on, 11.20V or so. It falls to 0V when the lights are off. Not sure how I skipped this verification when I first started investigating.
This power does not reach the concerned bulbs, so something must be severed along the way.
I've started following the outgoing wire (a red and white one I think, although I'll need to double check), which merges into the trunk harness.

I'm not sure I read the wiring diagram correctly. This RW wire just seems to change to red and green and go back to the front and the BPM ?

 

You will need to find the wiring manual for your year/model. I’m sure I found mine here on the forum someplace so you might do a search. The wires could be different.

Something I thought of: you didn’t put LED bulbs in the tail lights did you? Are you getting a bulb failure message on the dash?

 

I think the manual I have is accurate, just having a hard time understanding it

The bulbs are the filament type, and yes, I do have the amber light and message "check rear lights".

I'll keep tracing the wires tomorrow and give feedback (might help some desperate soul in the future, who knows)

 

Checking the input and output voltages at the BPM level is proving difficult.
As far as I can tell, there is no outgoing power on the tail lights pins, but I can't make sure the 'on' signal is received.

Another thing I just realized is that the front side lamps aren't working either - the small bulbs at the bottom of the inner front lights.
Bulbs are OK. There is no fuse for those it seems.
I can't see a relation between both issues, but they may be related anyway... Must've happened recently, had the MOT this summer and they would have noticed, I guess...

 

It’s not unheard of for the BPM to go bad. Mine did. You could get the part number from yours, check the VCATS label in the trunk, and try eBay or google to see if you can find a used replacement. They’re not terribly expensive. Pop it in there and see if it will fix your issues. In my case, the little security light by the shifter would occasionally glow dimly at random, and blink dimly with my blinkers. The car would occasionally display a ‘rear bulb failure’ message when there was no failure. I tried a number of things until I replaced the BPM, which fixed it. That was years ago and it’s still working fine.

Edited to add: I just thought of something else. There may be a relay that controls the tail lamps and possibly other marker lamps. Check your wiring diagram for a relay first. I don’t remember if there’s a relay for those lights or not, but if there is, it would be cheap and easy to replace it.

 

Not to muddy the waters, but a year or so ago I suddenly had no brake lights or turn signals....and the "rear bulb failure' message. I chased my tail with cleaning contacts, new bulbs, etc., etc., but the cure was a new battery. Even though the car didn't give any other signs of a failing battery it seemed the cars electrical system detected the battery was at the end of its life and protested in this way. A new battery and everything was back to normal.

 

About that
while making some tests I accidentally shorted a circuit, blowing the rear fog fuse. After replacing it, I decided to call it a day and went to start the car and park it properly.
Aaaaand : no crank, with a P1260 (theft detection, engine disabled). Checking the battery before disconnecting it (after erasing the fault code in the ECU), it had dropped to 8.5V (was 12V earlier).

I charged it for a while and thankfully I could start the car again, but that's odd and maybe the battery will soon be dead. Annoying as it is barely a couple years old and wasn't cheap.
Nevertheless, even when the car is running and the battery at 14+V, the issue is present.

Simplest solution might be to replace the BPM indeed, as they're not expensive, but I would have preferred to get a definite diagnosis (and there is no relay involved, unless I read the manual and diagram wrong)

 

Guys,

As per the VCATS label, my BPM is LNC2500CH/014.
I understand that the /014 refers to the geographical market and potentially implies some parameters to match the country's laws and regulations.
(e.g., in Sweden or other country in the area, all cars must have their headlights on at all times, if I'm correct, so that would be hardcoded in the BPM. Again, that's just my understanding, possibly wrong).

I found a used BPM with same software version, from a UK seller. I'm guessing therefore that the donor vehicle is a british version, RHD -- the market code isn't specified.

Could that cause any issue if I mount it on my car? I'm not aware of any major differences in driving laws between UK and France, but if the lights become inverted or something, it can be annoying.

 

I can’t imagine why Jag would have numbered the software versions the same if there were actually differences across markets. I think it had more to do with which options were available on the car. I don’t see what difference RHD would make, but I have no experience with that.

Someone else may weigh in on whether the software versions were different in different countries, even if the numbers are the same in this case. Seems to me like it would be a tracking nightmare to have the same number mean something different in different countries.

I don’t know what else you can check...fuses, relays, battery...I think you’ve eliminated each of those as the cause. The odds of each of your symptoms being caused by wires that suddenly broke at the same time are pretty remote. I’d try the BPM if it was me.

Edited to add: Something else you could try first: wiggle on the wire bundle that goes into the BPM and have someone watch the tail lights as you wiggle it. You might also check for corrosion at the BPM connector if you haven’t done that yet.

 

Actually I infered that from this document -- page 3, first paragraph. But it indeed mentions "features" as well, so the options of the car.

Mine originally came without any -- didn't even have cruise control --, so I suppose it's relatively safe to install any BPM with same software : worst case scenario, it will be programmed with more options, that couldn't be activated anyway.

Probably best to wait for someone else to chime in though

(And yes, I agree with you : last option would be a severed cable, which sounds very unlikely to happen at random. I'm going to replace the BPM.)

 

I just edited my response to add a couple other tricks you can try. You might not have seen it, but might be worth trying.

 

Tried that already, yes. Spraying electrical cleaner, too, but no change

 

Fun times.

Until I could take a decision regarding the replacement of the BPM and the version to use, I decided to wire the output of trunk fuse #21 directly to the tail lights -- I'd have the dashboard error, but at least, lights.
Removing the tail lights assembly, I banged the left one a bit too hard, and noticed the second filament of the brake/tail bulb had broken down. Dammit. So I went to buy a spare.

Mounted it : EVERY. THING. IS. WORKING. PROPERLY. Tail lights are back on, as well as the front side markers.

I don't understand. I'm guessing the original bulb was misbehaving even though both filaments looked fine, but I can't see how it would wreck such havoc. It's not like the tail lights and side markers are mounted in series? I think.

The battery, unfortunately, seems to be on its last legs... currently in charge, it didn't have enough power to start the engine.
Not to add insult to injury, especially since I'm the insulted one, but I strongly suspect that the battery was fine until I did a short circuit a couple days ago and put the car in lockdown/antitheft protection.

All this to say: sorry for having wasted your time with a nonexistent problem