I've recently replaced my headlights with LED bulbs. Despite promising built in CAN-bus decoders in the bulb, the bulb out light still illuminated on the dash. During said installation, one of the headlight marker bulbs went out. I've replaced them both with LED bulbs. Despite having purchased CAN-bus decoders (resistors) for the headlight bulbs, the bulb out warning is still on on the dash.
Question: will LED marker bulbs cause the bulb out warning?

 

Well one thing that’s at play here is that the XJ-S doesn’t have a CAN bus. So attempts to defeat it won’t work. Now I would think resistors of some sort would be sufficient, but I don’t know what sizes.

 

Well that’s a little embarrassing…but now I know. It explains why the LED bulb people sent me resistors rather than decoders to fix the problem. Thanks for getting me on the right path.
I assume the marker light bulbs in the headlights would be the same, cause the same issue.

 

XJSs use bulb failure modules front and rear, wired to the front and rear sidelight circuits. (plus an additional different type of module to handle the rear brake lights). The modules use a fairly simple bi-metallic strip that deflects based on the resistance of the current draw on each circuit. the headlight bulb is not really relevant as they deflect based on just the sidelight circuits. (Maybe US call them "marker lights"?) The good news is that I found that it is possible to change the start position of the bi-metallic strip in the modules such that a smaller current draw of an LED bulb will still deflect them sufficiently to break the circuit.

You can of course fit resistors to emulate the original current draw, but that seems to defeat one of the purposes of fitting LED bulbs. So I found my solution was a more appropriate answer to the problem (for me).

Cheers

Paul

 

Hi Paul, How do you change the start position of the bi-metallic strip in the module?
Scott

 

Just to clarify things that might not need clarification… the bulb out warning light does not detect headlights out, it does detect the little marker light inside the headlight as out. If you replace the little bulbs with normal bulbs, and leave the headlights as LED, you will have no dash warning lights and nice bright headlights. I mention this because you said the market light went out while changing the headlights.