240 General: Bulb Failure Warning Light - puzzle

[Clifford Pope]

I didn't think the bulb failure sensor recognised the headlamps, but it seems it does in some peculiar way.

A day ago the warning light came on when I switched from main beam to dip. It went off again on switching to main, then on with dip again.
I checked the headlights and they are both working properly. All other bulbs are working.
It only does it when switching from main to dip. When I turn the lights on straight to dip, not going through main, the warning light stays off. I can make the light go out by momentarily turning the switch to the middle position then back to headlights.

What's going on?

[Triple-S]

Interesting - I have similar on my 240. My symptoms slightly different: turn from OFF to sidelights and that failure bulb lights up. Then turn to either dipped or main headlights and it goes out, then turn back to sidelights and no warning light comes on.... Again all bulbs are fine.
My thoughts (but as I don't drive at night not important) is that the failure light relay thing (behind my glovebox I believe) is on the way out, but I stand to be corrected.
Sorry no help really, just comparing.....

P

[Patrick Garland]

I had a similar problem with one of mine many years ago. I think all I did was take the bulb out and clean the connections on the unit and the connections on the bulb itself. Although the bulb worked fine it appeared that slight corrosion or whatever that green stuff is that builds up on connections was enough to trigger the bulb failure warning light. Worth a try.

Regards

Patrick

[Clifford Pope]

Originally Posted by Patrick Garland

Although the bulb worked fine it appeared that slight corrosion or whatever that green stuff is that builds up on connections was enough to trigger the bulb failure warning light. .

By "the bulb" you mean the orange warning bulb on the dashboard?

[Patrick Garland]

Hi, sorry, no, I meant the individual headlight bulb. Although the bulb seemed to work fine because it had some corrosion on the connectors it seemed to affect the bulb failure warning sensor so sometimes it would come on as a failure and sometimes it wouldn't. Sorry should have explained it better. May not be your problem but it may be relevant.

All the best

P

[Clifford Pope]

I'd cleaned all such contacts quite recently when I got the car, and reassembled everything using proper electrical grease.

But I can't see on the diagram that the headlights are connected to the bulb failure sensor anyway. Or are they? I've never noticed the orange light come on in the past when a headlight has failed - presumably the driver is meant to notice anyway?

But if not connected, how can the headlights trigger the sensor, and why only when flicking from full beam rather than turning on at the switch?

[rtbcomp]

One point about this unit is that the warning light will only come on when 1 bulb fails out of the pair as it measures the current difference between the two sides.

if you have an intermittent or weak connection on one side you could get a current difference enough to trigger the relay, but the light could still come on.

Check ALL the bulbs that are monitored by this relay, especially those at the rear of the car, it's sometimes possible to get stop/tail bulbs fitted the wrong way in their holders.

A problem in one circuit, not in itself serious enough to trigger the warning, could be compounded by another problem, thus causing the relay to call.

If you've got a trailer socket make sure irs clean, current leaking in the brake light circuit could be a problem.

Normally the trailer brake lights are connected straight to the pedal switch, before the relay, if that is the case the trailer socket shouldn't cause a problem.

Make sure the bulbs on each side are identical.

EDIT

"and why only when flicking from full beam rather than turning on at the switch?" - I suspect that main & dip will be on at the same time momentarily as you switch beams. If one side has a poor connection or a mis-matched or blown bulb (even on the unprotected circuit) the relay might trigger

[Clifford Pope]

Well, it's resolved itself, as a headlight bulb failed yesterday just as I was turning into the garage. Presumably the slight bump on the ramp triggered it.
It's interesting that the sensor seemed to know that a bulb was going to fail, about a week before it did.
Thanks everyone for your input.

[rtbcomp]

The bulb was probably intermittent before it failed and triggered the warning relay.

[140]

The bulb failure unit is a number of reed relays one for each circuit which will trip if there is a difference in the magnetic field around that particular reed relay so even a very small difference caused by different makes of bulb or a slightly dirty connection will cause the dash light to illuminate.