While attempting to upgrade the trunk bulb to LED, the light stopped working altogether. I checked the original bulb with a multimeter and it checked good.


Question: which location and fuse controls the trunk/boot light?

 

gagman66, looking at the diagrams I have, there is no fuse for the trunk LED per se. The only fuse between that light and the battery is a 40A fuse (BCM-3?) which I would have to assume powers a lot of other stuff which I would think would be evident with use of the car. Where I would expect the fuse to be on the drawing is a large dotted box (as if the drawings are not complete). But, this should be powered from the fuse box in the passenger compartment.

 

Thermo,


Thanks. According to my owner's handbook, there is no 40A fuse located in the passenger's compartment fuse box. I need help identifying the fuse number that controls the trunk light, and what the voltage should read when touching the leads with a multimeter. Thanks.

 

gagman66, looking at the diagrams, if you have the trunk open, you should have 12VDC at the bulb.

Reading the service manual, the central junction box is the "brains" of the trunk light. It senses when the trunk lid is open and then uses an electronic switch to then supply power to the trunk area to turn on the lights. So, what you may discover is if you have the trunk open for a long period of time, the car will automatically turn off the interior lights to the vehicle. Because it is using an electronic switch, during this state, you may see a low voltage if you are putting the multimeter in place of the bulb (this is due to the very high resistance that a multimeter has when in the voltage/resistance mode). But, if you were to say drive the car then immediately get out and pop the trunk, remove the bulb and measure the voltage then, you should get 12 VDC. If you are not, then you either have a bad wire leading to that bulb socket or the electric switch is having issues.

Please note, the controls are from the central junction box (ie, in the dash), not the fuse box in the trunk.

 

[QUOTE

Please note, the controls are from the central junction box (ie, in the dash), not the fuse box in the trunk.[/QUOTE]



Thanks. How to find/identify the central junction box in the dash?

 

I'm enclosing the response from XDave's private message that resolved this issues.


As always, XDave's knowledge and willingness to help is unparalleled.


Hi,

Checking the schematic the trunk lamp is an output from the central junction box without a final fuse. That means it will be controlled by components on the PCB itself which suggests there is probably a mosfet with current sensing involved. It may be that it requires a certain resistance for the circuit to supply the full voltage, and the sensing is holding the circuit low assuming the bulb has blown. They use a similar technique to detect blown headlamp bulbs (and LEDs).

I have been out to test this on my car and it does appear to behave in the same way. With the bulb present and on there is 12.8V across the bulb. With the bulb removed there is 1.33V between the terminals. When the bulb is then replaced the lamp will not come on until the car is reset (either wait 20 minutes with the doors all locked, or start the engine, stop it then lock and unlock the car). Once reset the bulb is them illuminated again.

It may be that your LED replacement is fine, but the car needs to reset first as it believes the bulb has blown so does not try to illuminate it again that 'session'. Can you try it and see if that helps?

Regards,
Dave

 

LOL, I had the same issue with my XK, and starting and locking and unlocking got my trunk lights working, how ever, I had to put the original bulbs back in to get it to work, I"m wondering if the LED's don't show the same resistant as the original?

 

Hi I'm having the same issue. 1,3 volts with meter but i did replace the bulb with a like and still nothing. reset and still nothing. next look, maybe the micro switch in the trunk release? Driving me nuts. but that's a short drive.

 

Hi has anyone else had this issue? no trunk lights.
I have checked every fuse, none missing, none blown. I removed and checked the micro switch on the trunk release.
the lights do work when applying power with a jumper. but not any other time.

soon i will have to pay for a dealer to troubleshoot the issue. or just put in a tilt switch.