Britannia Main Beam stuck on

[herrbartlet]

New (recent) electrical problem:
Side lights & headlights (low beam) working correctly from dash switch.
With headlamp switch on or off, using stalk to 'flash', the main beam will stay on after stalk released.
When on low beam, pushing stalk forward will engage main beam but pulling it backwards - they stay on 'main'.
Switching off headlights or switching off engine does not cut power to main beams.
Checked stalk with circuit tester. Flash and main beam terminals seem to work.
My inexperienced electrical knowledge leads me to think: faulty relay.
603 wiring diagram shows one 22/26RA type relay in the circuit. However there are mostly 1" cube (28RA) 30/85/86/87 (5 pin) relays around the battery and removing one of these does cut the power to the main beam. Reinstalling said relay allows me to use the sidelights & headlights (on low beam only). If I accidentally engage main beam, the only way to cut the power is to pull out/reinstall this relay.
One would think swapping this relay for a new one would cure the problem, but it does not.
It leads me to believe there is a second faulty relay but unsure if I should be looking for another 28RA or the larger 26RA or where it might be located. Any suggestions welcome...

[PEU186F]

I am not familiar with the Brittania or its wiring, but here are some thoughts which are based upon Clive Fosters wiring diagrams which are to be found on this site, specifically page 14. These diagrams are for the 603, but the Brittania may be similar, except I think that the dip switch will be on the column, not on the floor.

The first thing that I note from this diagram is that the dip beams are driven directly from the dip switch, which one would expect to be man enough to handle circa 9 Amps for the dipped beams but perhaps not the circa 20 Amps taken by the main beams. So a decision has been taken to drive the main beams via a relay, which is actuated by the dipswitch, via the coil windings W1 and W2

The second thing I note is that the power to the main beams is not taken via the Light Switch, which is also unlikely to be up to the task, but from a separate supply taken from 21 on the fuse box. This appears to be an unswitched and unfused supply derived directly from the battery applied via the relay contacts C1 and C2

The third thing I note is that the supply to the headlight flash switch is taken from the battery via the fuse 17 - 18 and thence to the main beams down stream of the relay. Clearly the headlamp flash switch is reckoned to be up to the task, perhaps because it's only used intermittently. Can I just emphasise the point that this means that the headlamp flash switch does not drive the relay.

So, what could possibly go wrong? The symptoms you describe indicate that the relay is latching on when the main beams are selected, via either the headlamp flash switch or the dipswitch. You can get a relay to latch on by connecting the output contact to the field winding i.e. W1 to C1. If this is done a momentary supply to W1&C1 will mean that the relay will come on and stay on after the momentary supply is disconnected, as power is now fed back through the relay. This can be useful if your trying to catch a momentary flicker from a brake fluid warning device, but not in this application. To unlatch the relay it is necessary to disconnect either C2 or W2, which you have achieved by unplugging the relay.

So I surmise that your car is suffering from a misconection somewhere in the wiring that is connected to W1 and C1, as although this fault could perhaps have arisen in the relay you have already checked that out. Whereabouts this might have occurred is more difficult to pinpoint but I'd suspect that it's occurred somewhere in or around the column dip-main-flash switch. What you don't want, presuming that the wiring colours are the same as for the 603, is any connection between the wires coloured Brown-Blue and the wires coloured Blue-White

Good Luck

[herrbartlet]

Belated thank you for the detailed response. Only now had a chance to investigate, due to work commitments. There were a couple of good tips in there which got me looking differently at the problem. I was able to rule out a fault with the indicator/dip switch and tried disconnecting some additional wiring installed by a previous owner, one of which cured the problem. This raises the question as to why this modification was installed in the first place and doesn't give me 100% confidence this is the end of the matter.