Full beam kills dash lights

[Mini]

Both number plate lights working and full beam seems to have self healed. Only problem now is bulb failure on for no reason.

[ptjs1]

Mini,


A few things to mention:

The bulb failure system doesn't have any connection to the headlight bulbs. It only monitors the front & rear "sidelights" (which aren't on the "sides"!) plus the number plate lights at the rear.

However, the XJS has a Dim/Dip system which means that a filament in the headlight bulb is illuminated when the "sidelights" are turned on. This is NOT linked to the bulb failure system. The only bulbs at the front of the car which are linked to the bulb failure system are the 2 small independent bulbs that are also located inside the headlight shell. It is quite difficult to see if these are actually working as the Dim/Dip headlight bulb filament is much brighter and obscures their visibility.

So, first verify that the actual front sidelight bulbs are working by turning OFF the ignition and then turning on the sidelights. Can you now see a bulb illuminated inside each headlight? Those are the bulbs that are monitored by the bulb failure system.

Also just to clarify, when you turn on the ignition, and then turn on the sidelights, the bulb failure light IS supposed to illuminate for 25-30 seconds. Are you saying that it illuminates but then stays on permanently after more than 30 seconds?


Now, IF you have verified that the small front sidelight bulbs, plus the 2 taillight bulbs plus the 2 number plate bulbs are working, AND the bulb failure light is staying illuminated after more than 60 seconds, we have to verify where the problem lies.

The way to do this is as follows:

First locate the 3 sidelight Bulb Failure Modules. There are 2 at the rear of the car. These are mounted with a single screw beside each rear taillight bulb-holder. See pic.

Then find the front module. This is mounted behind the under-dash knee bolster panel on the left-hand side. See pic.

There are 3 wires on each module. Disconnect the wire on each module that is marked WL This is at the opposite end of the module to the setup adjustment screw. See Pic.

With all 3 WL wires disconnected, turn on the ignition and the sidelights. The bulb failure warning light should now stay off.

Turn off the ignition and the sidelights. Reconnect the WL wire on the front module. Again turn on the ignition and the sidelights. The bulb failure light should illuminate but then go off after 25-30 seconds. If it doesn't, the problem lies in the front circuit. If it does illuminate but then goes off after 25-30 seconds, the front circuit is intact.

Turn off the ignition and the sidelights. Disconnect the WL wire on the front module.

Now connect the WL wire on one of the rear modules. Again turn on the ignition and repeat the test. Once complete, turn off lights and ignition and disconnect the WL wire.

Repeat the whole test for the other rear module.


In this way, you'll pin down which of the circuits is causing the problem. Post back and let us know how you get on.

Good luck

Paul

[PaulGover]

AFAIR the bulb failure warning is triggered by a difference between the LH and RH sidelight current flow, usually down to one or t'other having a bulb blown, but it can be down to using different rating bulbs across sides. A possible cause is to use an LED bulb on one side, or two LED bulbs but only one with the inbuilt resistor to draw the necessary current to fool the detector circuit.

[ptjs1]

Paul,

That's not actually how they work. There's quite a lot of misinformation floating around as to their operation. I recently had to establish how they work as part of my LED fitment project which is due to be published in the mag over the next few months.

The sidelight modules don't work on comparative resistance. They work independently for each circuit using a bi-metallic strip inside the module which heats up, deflects and breaks the warning light output signal. (That's why the warning bulb comes on initially, until the wires have heated up sufficiently to turn off the warning bulb) That's why I am suggesting disconnecting them all and then trying out each circuit one at a time to verify the problem circuit.

Cheers

Paul

[Mini]

Thanks gents

That a fantastic set of guidance. Will get to this on Monday and report back.

Regards

[Mini]

Followed the procedure. Front one is the culprit.

Do I just leave disconnected and live without detection on the front or are the modules fixable / available ?

Thanks Simon

[ptjs1]

Mini,

Although I'm sure it's possible, I have never seen a failure in a warning lamp module before. They are a very simple concept using a bi-metallic strip to break a contact which extinguishes the light output signal.

You've confirmed that you followed my suggestions, but did you perform the initial step of turning off the ignition completely (remove the key) and then turn on the sidelights and confirm that you can see a small bulb illuminated in each front headlight?

Paul

[Mini]

Forgot to say. Someone before me changed the headlights to the 4 light US version.

So just to confirm I can see the two working side light bulbs. In fact this all started with having to change one of them. Which I did with an LED bulb. It works but could this be the reason for bulb failure light being triggered ?

Thanks

[ptjs1]

Mini,

That's absolutely the reason for the whole of your problem. It's always helpful to describe what changes you've made before you noticed the problem, as the issue almost always lies there. You'd have saved yourself a lot of time and angst if you'd mentioned that you'd fitted an LED bulb!

The LED bulb draws significantly less current, so the bi-metallic strip in the failure module doesn't heat up enough to break the contact with the warning lamp output.

The good news is that there is possibly a way around it. You will see my explanation in the next edition of the magazine or possibly the month after, where I describe the fitment of LED bulbs to a late XJS.

In the meantime, I would leave the wire disconnected on the front warning lamp module so that at least the rear warning circuits are working correctly.

Then before, you seek to implement my suggested solution, I would consider changing the other front sidelight bulb for an LED. Otherwise, you will need to readjust my solution again, when you next change the bulb.

Cheers

Paul

[Mini]

Ok apologies school boy errors.

Good to know and I look forward to the article.

Help greatly appreciated.

Regards