TD5 alarms sounds when lights turned on

My mate down the street has a 2006 TD5 110.
A few days ago when he turns the lights on the alarm (in the nearside wing) sounds.
The lights don't come on at all, but the alarm goes off.
Otherwise the car is fine, starts, drives, everything is fine, except when he turns the sidelights on.
The main beam flash works, without sounding the alarm.

We have swapped the pink relay for a known good one .. no help.
We have swapped the 'yellow' relays in the fuse box around .. no help.

Any clues gratefully recieved.
The dash is out and all the wires etc exposed.

The standard TD5 just uses the horn and doesn't have a separate sounder. So that sounds to me like someone has installed an aftermarket alarm. In which case, they can malfunction in all kinds of interesting ways depending on how it's been wired up. It happened to me. So don't discount the alarm unit itself, if you can find it.

Td5 has a separate alarm sounder in conjunction with the horn. It is in the same area behind ns headlight.

check the header's for corrosion whilst the dash is off they are a two part male and female connector

It's deffo original and part of the loom ...
The car is a 51 plate.

@mystery - what do you mean the headers ??

So far he has changed all of the light stalk switches.
There is power everywhere but no lights, (other than flash) just the alarm sounding ...

Anaconda said:

Td5 has a separate alarm sounder in conjunction with the horn. It is in the same area behind ns headlight.

Click to expand...

That's interesting, mine's not got that. Doesn't look like it was ever fitted, either.

Brown said:

That's interesting, mine's not got that. Doesn't look like it was ever fitted, either.

Click to expand...

It's the bbus, back up alarm sounder which has its own built in battery. The idea is that even with power disabled from main battery the alarm will still sound. Probably a pointless addition now as the internal battery is bound to be knackered so will only sound when it still has a 12v supply.
Maybe not all models have them?

Would this stop the lights working though ?

I am in no way electrian but sounds like the power is taking the wrong path somewhere and sounding the siren instead of operating the lights.
Trace the wiring I guess and follow Mystery advice.

may help

Cheers all.
Managed to fix it this afternoon.
@mystery wins the prize ...

C0287 had a couple of dark marks on the underside, discovered when the fuse board was disconnected and the cables push up, which revealed the blue and red headers.

The pins had overheated and were not allowing more than a few milliamps to pass.
We cut the 6 (connected) wires and soldered them together, all working a treat now.

Good result, well done.