A few times while indicating to turn right I've had people blow their horns at me and I thought it was impatient drivers. It turns out not so lol. Last night my son was behind me and he said every time I indicated to turn right, my left brake light would flash with the right indicator. I checked it this morning and lo and behold.... it does.

indicating right, flashes the left brake light simultaneously until the stalk is off.

Any ideas of where to start looking?

2006 XJ8. Thanks in advance as always

 

Since these odd issues pop up a few times on here now. My rear lights had a different but similar strange fault and it was due to a faulty rear fuse box (the one in the boot or trunk) I picked up a used one cheaply enough and very straightforward to swap over.

 

That's a classic symptom of a bad earth (ground) for the R/H unit. Check your ground connections but be warned, the ground connection studs on the car's aluminium frame get very fragile with time and can easily break; quite often people have ended up having to make new ones.

 

I 2nd the earth suggestion, this used to be a common fault back in the eighty's with Ford ( and other ) car rear lights, back then you could just run a second independent earth wire from the cluster to a ground bolt on the body. As mentioned above though beware the earth bolts are aluminium and risk seizing and snapping off.


You could take a length of ordinary sheathed wire and temporarily touch it to the point on the cluster and to a suitable earth point and then try the indicator flasher again and see if it cures the problem, a quick and cheap test !

 

Christian,

The torque specification on the ground nuts is just 6.5 ft. lbs., or just a little more than hand tight. The ground studs are attached to the body by a method similar to spot welding and are easily pulled off of the body if the nut is overtightened.

Cheers,

Don

 

Thanks everyone for the input and I have one more question although its not for not spending a lot of time looking at both the car... and google lol. Where IS the ground wire for the rear lights? I've looked at both the left and right sides and I'm only seeing the cluster plug going into the units, and none of them seem to have a ground wire. Ive even looked at the units listed on ebay to get a better view of them off the car.

 

You have to remove the boot carpet on the right side to get to the earth points, checked mine before I got the fuse box, the rear electric control module is also there.

 

I take it all back !

The rear light units don't have an earth/ground line as such ...

Each bulb has a 12v supply which is always on, or always on with the ignition, on one wire and the other wire goes to the Rear Electronic Module where they are switched to ground to turn the lamp on. This is to enable the module to detect a failed bulb; the return line should sit at 12v when the lamp is off; if it goes to 0v when the lamp is off the the bulb is open circuit.

 

Thanks for the info Patrick & Aaron. Quick update. Pulled the fuse box and everything seems to be squeaky clean and all connections are solid. All earth straps appear to be intact and in good order with no corrosion at all. as this seems to be the only issue I'm actually stumped as Patrick says, the rear lights aren't actually grounded so I may have to take it into someone that knows what they are doing, or do a lot more research myself. If I work it out. i'll post what I find here.

 

*****SO UPDATE*****

Ive managed to get the indicators to work correctly (lawd knows how) UNTIL I brake. then all hell breaks loose again. Ive read several posts where people say switch out the rear fuse box, Aaron above mentions it but how do you know the one you buy used isn't as bad as the one in the car? thats too much of a shot in the dark when Ive had this one out and it "appears" to be fine. this is becoming a challenge lol.

I have her booked into a shop next week, hopefully at $70 an hour, they find it fast.

 

Wow $70/hr is cheap. It's $100-110/hr for independent shops and $125-150/hr for dealers around here.

 

I bought mine from a jaguar breakers through eBay with a 30 day warranty. My old fuse of looks mint. The problem seems to be with the PCB on the bottom which I believe has a series of diodes in which for whatever reason fail and cause all kind of strange goings on with the rear lights. I was lucky only cost me £35 to fix.

 

So you guys who said the fuse box were right. Kudo's to you all. Got one from an 04 that fixed the rear light issue which is a bonus! A tiny minor issue is this box doesn't have a fuse for the TPMS and now my dash has the low pressure warning light on.... lol I would rather have this than iffy rear lights! However does anyone know which relay the wire for the TPMS are located so I can at least bypass the fuse? I CAN live with it but would rather not.
This is the old (06) one with the fuse location F37



This is the 04 with the fuse missing. It's the only difference.

 

I am having the same problem with my 2004 Jaguar XJ8 with right turn signal causing left brake light to flash also. It seems when it warms up after about two to 3 minutes the brake light quits flashing. Also causes left and right backups to glow flash very dimly, but once the Brake light quits then they all resume normal process with right blinker still flashing and brake lights steady. I have repeated the process several times and a couple times the brake light would continue to flash blink even after 10 minutes, but most of the time after just 2 to 3 minutes.

I did run the wiring after downloading wiring manual and doing an in depth study of it and thought it may be the (REM) REAR ELECTRONIC MODULE in the trunk as it acts as a bad diode or resistor after they are warmed up. However, that did not do the job after replacing the REM so I am ordering a fuse box and will let you know if it fixes the problem as of today it is 01/31/2022 and I want my Jaguar running smoothly. Been working on cars and planes for over 50 years but always a stump when there are this many wires and Modules.

This is what the REM looks like on the right side in the trunk in case new readers are curious. The far left wire plug at bottom of the unit is the input wires for the tail lights. Cost was about $49.00 shipped on eBay.
but you still come across problems that requires input from others. We use to have to do that in the shop discussions.

HeyLanny

 

Good idea. I use jumpers with alligator clips for such trouble shooting. 12"-18".

 

Good idea. I use jumpers with alligator clips for such trouble shooting. 12"-18".

 

As promised I would update what I found on my 2004 Jaguar XJ8 and the right turn signal blinking, but also blinking the left BRAKE light. After going over the trunk wiring I concluded that it had to be the Trunk fuse box. Reason is that in about 2 minutes or so the brake light usually quit blinking while the right turn signal kept blinking. I also noticed a real dim light of the filament in the back up lights which also quit when the brake light quit blinking.

However, the driver following didn't care for a right turn signal and a left blinking brake light. I looked over the fuse box and it has a circuit board inside with diodes and resistors. When they warmed up then the brake light and other bulbs quit their activity temporally. Left turn signal did not give me any problem, but short problem in wiring did show up in my Foxwell computer as a short at the taillights.

After ordering another used fuse box from a trusted company Euroholics https://www.ebay.com/itm/115173657319 I removed the old and installed the replacement and everything went back to normal. I did a couple days research and found one post suggesting it could be in the Fuse box. Well for those with the same problem get yourself a used Fuse box for taillights and other items with the wiring running through it. Hope this helps someone else and it only cost $49.00

 

Similar behaviour with my 2004 - also same color as @HeyLanny, so eventually it's a hidden feature included by color code .
In my case,
- when right turn signal on and foot on brake, left sopt light and the stop light under the rear window are flashing
- when in reverse gear, left reverse light is on, right only little bit glowing, when foot on brake, right brake light on but not left one and the one under rear window
Conclusion: I will look out for a fuse box.

 

I'm not sure who I'm replying to, I've lost track. The discussion about changing fuse boxes reminds me of the issue I had with my front (engine compartment) fuse box. All the fuse boxes have relays that plug in and can be replaced. They also have relays that are soldered to the fuse box circuit board, and (theoretically) cannot be replaced. I noted from the electrical guide that the feeds to the tail lights and direction indicators (turn signals) pass through R7 and R8 in the rear fuse box. Both of those relays cannot be (easily) replaced.
I had an issue with my supercharger coolant pump not running. Cutting to the end, I replaced one of the circuit board relays in the front fuse box, which solved the problem. Used parts in New Zealand are rare and expensive, so I did the repair myself.
Seeing as there have been a number of people with the same issue of tail light problems, perhaps changing one or more of the relays in the rear fuse box could solve the issue. The solution is inexpensive, but labor (US spelling) intensive.
Firstly, the fuse boxes are dumb devices. There is no electronics or processors inside, just relays and fuses. Once the fuse box is removed from the car, it can be disassembled. The circuit boards, which are in a two-layer sandwich, just clip into the plastic case.
If you have a Dremel-type tool, you can go ahead and cut all the (about 120) pins that are soldered between the boards, roughly in the middle. Once the boards are apart, blow all the copper filings off them and find the relays to be replaced. For the rear box, I think R7 and R8 are the relays that power the rear lights. Each relay has two sets of contacts. These relays (which have the type number marked on the case, are easily and cheaply (about a dollar) obtained from your friendly mail order supplier. Think of Banggood or AliExpress.
Remove and replace the faulty relays with the new items. The circuit boards have a plastic separator, so they just clip back together. Solder the cut pins together, reassemble into the plastic case, and refit to the car. There are threads on this forum about fuse box repair, that were very helpful to me, and you can find them too. Go for it.

Pete M

Circuit board sandwich

Relay

Relay

 

Hi Pete - no worries about who you were answering at: Your answer is highly appreciated!
What you describe is exactly my plan, however I would like to have a replacement here before starting work. The car still drives perfectly with a defect fuse box but without a fuse box it won't.
So: If anybody wants to offer me a good or defect fuse box for a 2004 XJ8, just drop me a message with some pictures of it (destination country will be Switzerland)