Rear lights go off when headlights are turned on.

[vulgalour]

Bit stumped on this one.  Car is a 1951 Lanchester LD10 that I've almost finished fitting a new Autosparks wiring loom in.

Front sidelights and indicators aren't yet wired up. We don't think this will have a bearing on the issue, but I mention it just in case it does.

Turn on sidelights - running lights and number plate light come on at the back as they should

Turn on headlights (dipped beam) - headlights come on normally. Running lights and number plate light go off.

What I can't figure out is why it's doing this. What would cause the rear lights to go off as soon as you turn the headlights on?

[xbl]

Is the switch in good order? Power to the rear from the switch in dip position?

[Dez]

Id guess switch isn’t wired correctly. There will be a separate terminal for the rear lights that’s live in both the side and headlight positions, the rear lights shouldn’t be teed off the sidelights as some switches turn the sidelights off when the headlamps are on to save the bulbs being used unnecessarily.

[vulgalour]

xbl: Actually, that's a good point. I tested for power to get the rear lights working and then forgot to test it for when the switch is in the dip position so I should probably do that. I don't know whether the switch is actually good, it's one of the few bits I haven't pulled apart and rebuilt so it could have issues internally.

Thank you, it's something for me to check that I'd been completely blind to. Couldn't see an obvious thing to check for trying to find the problem elsewhere.

[vulgalour]

May 5, 2022 18:31:52 GMT Dez said:

Id guess switch isn’t wired correctly. There will be a separate terminal for the rear lights that’s live in both the side and headlight positions, the rear lights shouldn’t be teed off the sidelights as some switches turn the sidelights off when the headlamps are on to save the bulbs being used unnecessarily.

Possible.  I followed the wiring diagram instructions for it but I suppose if there was a change to the switch I might have got it wrong?  I'll double check when I next get chance and report back.

[colnerov]

Hi, It could be any number of things, as Dez says just a mis-connect of the switch terminals. Also dirty contacts in the switch through lack of use, try working the switch through all its positions to hopefully clean the contacts somewhat, even better with a squirt of switch cleaner. Check all the earth points for the loom, putting the headlights on could be putting enough load that the rear lights do or appear to go out, have they actually or are they just very dim, where they are finding an earth through other bulbs. Are there any more switches? I can remember one car, I can't remember what now, the light switch only had on and off and a second switch powered from it switched the headlights on or off.

Colin

[vulgalour]

Rear lights are definitely fully extinguishing. Fortunately there aren't many spots to check and all the lighting earth points are connected and appear good. Dirty contacts are very likely, that's been a problem throughout the car probably with it being sat idle for last 36 years or so.

My gut feeling is this is a wiring issue on my part and I've probably got two connections the wrong way around due to inexperience and poor instructions. This is my first time rewiring a whole car so it's been a learning experience.

[gtviva]

Certainly sounds like the wires are on the wrong terminals on the light switch

[misterm]

Is it a typical 50s light switch? Pull for side/tail, then slight rotate and pull to second stage for heads?

[vulgalour]

May 6, 2022 10:18:26 GMT misterm said:

Is it a typical 50s light switch? Pull for side/tail, then slight rotate and pull to second stage for heads?

No pull function.  There's the ignition barrel in the middle and then a switch that rotates around it, looks like this:

[jimi]

May 5, 2022 18:31:52 GMT Dez said:

Id guess switch isn’t wired correctly. There will be a separate terminal for the rear lights that’s live in both the side and headlight positions, the rear lights shouldn’t be teed off the sidelights as some switches turn the sidelights off when the headlamps are on to save the bulbs being used unnecessarily.

This ^^^
The dip switch only switches its output between main & dip, rear lights should be on the input side not output, so dirty contacts on it shouldn't come into it, the only switching on the rear lights should be at the main switch, if there's enough power passing that to light the headlights then the contacts are good.

[vulgalour]

I've checked my wiring against both wiring diagrams I have, the wiring the car came to us with, and this diagram which is the one that pops up when searching for info on this switch which is a Lucas PLC6.

What I've learned is that it doesn't matter what order the wires are in, it either doesn't work at all, or the problem with the rear lights going out when you turn the headlights on comes back. That suggests it must be the other option of dirty internals and, on closer inspection I can just see some green through the gaps in the casing. I'm going to carefully open it up and see what's going on inside, it probably needs new grease and cleaning like everything else.

EDIT:  Scratch the dismantling idea, all I succeeded in doing there was jabbing a screwdriver into my hand so I'm going to just squirt cleaner in it and hope for the best.

[vulgalour]

This morning I got straight out to the garage to take off the switch and test its functions. This time I spent the time to make a little diagram too, because at least then I have something I can look at easily, like so.

The wires that should go to this are:

red - rear sidelights and number plate light

red - front sidelights

yellow (x2) - A at switch to A1 at voltage regulator

white (x2) - A3 a switch to A3 at voltage regulator

white/black - Clock/dashboard

blue - headlamp dip switch

I had the switch wired as below, which I have to assume is incorrect since I have nothing going to L.  I'm not sure which socket the white/black should go to.  Also bear in mind the way I'd wired it is in line with both of the wiring diagrams I had that are supposed to be for this car, neither one suggests putting anything in L.

I don't know what L is doing or what's supposed to connect to it.  That it comes on when the switch is set to sidelight and goes off when headlights are selected suggests something is going on.  Whether or not that something is correct I don't know.

[xbl]

It's not for a tell-tale for the side lights on the dash or anything simple like that?

[vulgalour]

The only telltales are for Oil and Ignition, nothing else has a telltale, the car predates the requirement.

[Rich]

Having done some specific reading, I suspect if the switch is dropping the side/tail output at a the switch when switched to headlights then there is an internal issue with the switch. I’ve serviced many pieces of switchgear from this period and it’s fairly straightforward stuff. I imagine one end of the sliding contact isn’t connecting properly. Worst case the switch is available as a ‘new’ part although I appreciate with the patina of the car you would want to do your best to keep the original.