Bulb failure warning

[paul4be]

I have a niggle which is the bulb failure warning light always being on. I have gone through all lights several times and all are fine. Cleaned up all connections etc as well.

Previous owner has had a tow bar fitted and wiring is scotch locked into the rear lights. Haven't used this so can't comment further.


Where do I start looking as it's really annoying?

[Alec Dawe]

I had that problem with a 240GL estate.
Finally found that the headlamps were of a different rating, left was 50watt, right was 80 watt.
The bulb failure warning works on the left/right current balance being the same, so this was enough to trigger it!
I'm assuming that this will also apply to the 'other' bulbs covered by the system.
Also could be an earth fault in the trailer plug loom.
Hope this might help

[Clifford Pope]

The device is extremely sensitive. It can be be triggered by bulbs of a different manufacture, even if the same wattage. Also I find that bulbs that are nearing the end of their life can trigger it - in fact it seems to be able to detect this before the bulb actually fails.
I find it is most sensitive to tiny differences in the dual-filament bulbs at the rear.

[Andrew (UK)]

Disconnect (or remove if easier) a pair (left and right) of bulbs at a time until you find the pair that is causing the problem. If the warning light stays on with both full beam and dipped then it is probably not the headlights. Look for noticeable differences in brightness of each pair of bulbs. It could also be grounding for a rear cluster if your car is an estate and have rust issues below where the boot closes it can compromise the ground on either side. The front side lights bulb holders can develop high resistance which can also cause problems.

[paul4be]

Thanks for the replies.
Car is a saloon, so no tailgate issues.

Tried each pair of lights in turn, removing the bulbs.

Made no difference and the bulb warning stayed on.
Checked all of the obvious connections as well.

Any other common issues?

Paul

[Peter D]

remove or disconnect ALL bulbs and if the light does not go out the buld fail relay unit ( a bunch of reed relats ) has failed ot you have a wiring fault. Remove relay is bulb out. Report back. Regards Peter

[penninepullman]

It may well be to do with the towing electrics. When I wired my 244 for towing I connected the towing electrics to the ignition side of the bulb failure unit (this means long wiring as the unit is under the glovebox). Just connecting at rear will mean bulb failure is on when towing, but not necessarily when solo. Are those Scotchloks always making good contact? (they are dry joints after all)

[Glen Morangie]

Scotchloks are the IDC clamp connectors - right? (IDC is 'insulation displacement connector' - where the contacts cut through the insulation as the wire is inserted into the connector)

I would check that whoever did the wiring for the trailer board used the correct size of connector for each of the wires 'spliced' - this can be complex as it's not always the same size of conductors being spliced.

The point being that if a given size of wire is forced into too small an IDC connector, the IDC contacts will cut into the conductors in the wire - rather than just the insulation as they are meant to. This might alter the resistance of the wire sufficiently to trigger the bulb fail sensor

Just a thought if nothing else turns up.

[Peter D]

Who mentioned tow electrics Paul4be didn't. Regards Peter

[Glen Morangie]

Originally Posted by paul4be

Previous owner has had a tow bar fitted and wiring is scotch locked into the rear lights. Haven't used this so can't comment further.

this is what got me thinking the tow electrics had been scotch locked into the rear lights.