LED bulbs and bulb failure system

[ptjs1]

I wonder if any XJS owners have some experience of successfully fitting LED bulbs to the rear Stop / Tail lights?

I've been working on the problem with Gil Keane from www.bettercarlighting.co.uk Gil is great, hugely enthusiastic and professional in his desire to get better quality lighting available for our classic cars.

I have tried some of his new Canbus-compliant bulbs, but I've still got a problem of how to persuade the bulb failure system to recognise the lower current draw of these LED bulbs. And I don't want to rip the system out or remove the warning bulb. I would just like it to operate correctly with LED Stop / Tail and Front sidelight bulbs.

So, I just wondered if anyone if anyone had ever examined or taken apart one of the bulb failure sensors, shown in the second pic, to understand how they operate, if they deteriorate etc. If I even just wiggle the terminals slightly on that module whilst the lights are on, it seems to slightly affect the voltage readings on the module terminal and also the frequency of the bulb failure warning light pulsing. Clearly, these modules are quite sensitive. So I wonder if I can do anything to them, or if fitting new ones might make a difference. I'm guessing they just work by expecting a certain current draw and sending a signal to the failure system if the current draw is less than expected? Or does it compare the current draw between the left and right modules and flag up if it seems a difference? If the latter, then LED bulbs should be fine if fitted to both sides, shouldn't they?

Any thoughts gratefully appreciated.

Paul

[davidr]

Interesting project Paul. Many of us with facelift cars worry that our rear lights aren't very bright with conventional bulbs. Let us know how you get on

[davidr]

Our old friend BrakeBuster has been following this thread and has offered me the following explanation which I will pass on as he can no longer post to the Forum

"The lamp fail modules pass a constant low signal through the whole circuit , this goes through the lamp, not enough to illuminate it, the module sees this signal and its happy , the lamp blows, the signal doesn’t get to the module its unhappy and throws a lamp fail signal to the dash indicator , an LED wont allow the low signal to travel through it , and the module will see this as a break ( blown lamp) even can bus lamps work different, no low power signal transfer, you may/will also back back signal for some circuits given diodes etc built into the looms"

[cafcpete]

Ok, as the module is not going to be doing anything anyway once you fit LEDs, why not just loop the wires at the module that are detecting the low current draw?

[ptjs1]

Vince,

If you see this, thanks so much for your input, really appreciated and hope you're keeping well!

Also, interesting to hear your explanation. I'd previously been led to believe it was a bimetallic strip module, in other words, it only did something when a light was turned on.

I thought the Canbus-optimised bulbs would work ok. It seems my other alternative is to run a ballast resistor on each circuit to emulate the normal higher current draw. Of course, if I do that, the module will work but I'm drawing the same current as standard filament bulbs which loses a lot of the benefit of using LED bulbs.

Pete,

Tks for your thoughts. I'm really trying to find a solution that still keeps the bulb failure system working, so I've got to keep working on this one!

Cheers Guys,

Paul

[PaulGover]

I thought the difference between CANBUS and normal versions of LED lights was the former has a ballast resistor to draw enough current to make it look like there was a normal bulb in place. (So there's no energy saving, though apart from headlights it's unlikely you'd notice in normal use.)

I vaguely remember reading that the early bulb failure detectors measured the current flowing in the LH and RH lighting circuits and lit up if there was a mismatch. That wouldn't detect two blown bulbs, one on each side, but it would allow for replacement bulbs (in pairs) with matching but different characteristics. But I might well be wrong.

[ptjs1]

Hi Paul,

That was my naive view of the Canbus bulbs also, so I need to talk to Gil Keane about why I'm still seeing a problem.

Interesting your thought on the failure modules comparing left and right current flow. I had also surmised whether this was the case, but a guy in the US said he believed that it's a simple bimetallic strip in each module that makes contact when it sees enough current draw / heat. And so, they are not connected. With Vince's thought that it sends a constant low current flow, I admit to being a little confused now as to how the system works. My simple reading of the wiring diagram isn't helping me to understand it either!

Cheers

Paul

[PJAB]

Hi All

I am in the process of fitting LED bulbs to my XJ-S - and have come across the same problem.

Is there a solution to this? I am primarily upgrading the bulbs for safety reasons - both for me to be able to see better in the dark and to be more easily seen by other drivers. It would seem to be a real shame to have to “abandon” this project because the LED bulbs cause the Bulb Failure caution light to be permanently on.

Thanks

Peter

[ptjs1]

Peter,

My articles on this were published in the June 2023 & July 2023 issues of the magazine. This included my answer on how to manipulate the bulb failure warning system to work successfully with LED sidelights.

I still have a problem getting the LED brakelights to work with the failure system and my articles showed how to disable this side. I know theoretically how to solve it; it's just more complex and I'm still working on the solution.

Hope that helps

Paul

[PJAB]

Paul

Perfect - thanks - I’ll take a look at those copies of the magazine. I must have missed those articles because, at the time, I didn’t have this project in mind.

What a great result from this forum - I had a problem, I searched the forum, I queried whether there was an update and lo and behold I’m directed to something that should be very useful.

Thanks, as ever, for taking the time to help a “newbie” by replying to my query.

All the best

Peter

[ptjs1]

Peter,

You're welcome!

Here you go:

Page 73: https://jec.org.uk/magazine/vol-39-no-06

Page 75: https://jec.org.uk/magazine/vol-39-no-07

Also, I'm not sure, but I think there might be 2 failure modules for the front sidelights on your earlier car. I'll see if I can find out definitively.

Paul

[PJAB]

Paul

Perfect - thanks for that - I will have a read of this.

All the best

Peter