Permanent live or ignition live......

[jagnut66]

Hi,
Having put Sally back together I have discovered that I seem to have wired the lights to an ignition live rather than a permenant live circuit.
Everything works as it should and now when the ignition is off, so are the lights.
This doesn't bother me, for one thing it would mean that there is no chance of me accidently leaving the lights on and draining the battery.

But does this matter for a reason I'm not aware of?
Which is why I'm throwing the question 'out there' to see what the opinions of others on here are.
If there is a valid reason or an electrical safety aspect, that dictates it is better policy (and safer) for them to be on a permanent live circuit, then it is easy enough for me to change the set up.

I look forward to your answers with interest.
Thanks and best wishes,
Mike.

[svenedin]

I think the main reason why you should not do this is that with the headlights on there is a lot of current going through the ignition switch. It is just not rated to handle this current.

From what I can find, the Lucas 31973K (the original part number from my 1969) is rated at 5 amps on the ignition circuit which is pretty close to what the coil and the fuel pump draw. In other words, the ignition switch is already almost fully loaded. The sealed beam headlights (as an example) are 65w/55w. So with the headlights on dipped that is 110w at 12 volts =9.17 amps. Far too much current for the poor ignition switch. It will overheat and melt!

Obviously your car is earlier but still the ignition switch is not meant to handle that current.


Stephen

[jagnut66]

It will overheat and melt!

Noted Stephen, a very valid reason indeed.
I will alter the wiring accordingly.
In the process I shall fit an inline fuse, what amp rating would you recommend for headlights / sidelights?
Thanks and best wishes,
Mike.

[svenedin]

I am a bit puzzled as to which circuit diagram in the workshop manual is applicable to your 1954.

I am looking at N29 as an example. There is a wire coming from terminal A1 of the control box (36 brown/blue) that goes to the lighting switch terminal A and it is permanently live. That is the live to the lighting switch. Then another wire also brown/blue goes to the ignition switch from lighting switch terminal A. So, the lighting should not be ignition switch dependent. In fact the lighting switch is permanently live and is a junction for the live to the ignition switch.

The later cars have a 10A inline fuse for all of the lights but not the headlights which are not fused.

Stephen

Screenshot 2026-08-19 at 22.21.28.png (385.02 KiB) Viewed 2163 times

[oliver90owner]

Headlamps generally operate separately to the ignition (not why it is called that) switch. They generally have a separate switch. Therefore……

[jagnut66]

I am looking at N29 as an example. There is a wire coming from terminal A1 of the control box (36 brown/blue) that goes to the lighting switch terminal A and it is permanently live. That is the live to the lighting switch. Then another wire also brown/blue goes to the ignition switch from lighting switch terminal A. So, the lighting should not be ignition switch dependent. In fact the lighting switch is permanently live and is a junction for the live to the ignition switch.

Yes, that is similar to the diagram I used when rewiring my car, I shall have to double check my workshop manual to see if it is the exact one but I believe they all show that arrangement.
I shall have another look at the wiring at the same time.

Headlamps generally operate separately to the ignition (not why it is called that) switch. They generally have a separate switch.

Headlamps and sides operate via the light switch, or are you thinking of main beam / dipped via the separate, floor mounted, dip-switch?
Both switches operate as they should, they just appear to be ignition fed at present....
Which I intend to rectify.

Best wishes,
Mike.

[Bill_qaz]

Apart from the technical issues discussed, you should be able to have side lights on with ign off and when car is parked on an unlit road. Imagine breaking down and having no lights, thats why hazard lights work directly without ign on. You may need to get out of the car in the dark and it safer to have the lights on.

[svenedin]

Apart from the technical issues discussed, you should be able to have side lights on with ign off and when car is parked on an unlit road. Imagine breaking down and having no lights, thats why hazard lights work directly without ign on. You may need to get out of the car in the dark and it safer to have the lights on.

Exactly. Remember you can't just turn the ignition on and leave it on without the engine running because you need the lights; you'll burn out the ignition coil. Wasn't it a requirement in the Highway Code at one time that sidelights must be lit when the car is parked at night in certain situations?

PS Mike, I have never really had cause to look at the earlier wiring diagrams because my car is so much later. In some ways the early diagrams look more user friendly with the pictures of the devices (dynamo, fuel pump etc) but actually tracing where a wire goes is horrible compared to the late diagrams. I feel for you! Perhaps print out the diagram and then trace the wires on the diagram you need to address with highlighter pens or coloured pencil of different colours. It might help to annotate your print out with the wire colours too.

Stephen

[Bill_qaz]

Looked through several wiring diagram variant in the manual and all have live feed from regulator battery live to light switch feed and to ignition switch feed as described by Stephen.
Likely the light switch feed is connected to wrong terminal of ignition switch, an output, rather than sharing the input.

[geoberni]

. Wasn't it a requirement in the Highway Code at one time that sidelights must be lit when the car is parked at night in certain situations?


Stephen

It still is the law and the criteria hasn't changed much, it's just that street lighting is far more common as are speed limits.
Though with many local authorities switching street lighting off at midnight I do wonder if there is a bit of a hole in the Regs these days....

The Headlamp switch has always been fed directly from the battery so that lighting is aways available.

In the case of the Minor that means being fed directly from A1 on the Voltage Regulator which internally connects to A, which is connected to the battery.
This is the earliest diagram in the Manual as Stephen says a pain to follow the lines...

Screenshot_20260820-155938_Drive.jpg (94.95 KiB) Viewed 1841 times

I do doubt Stephen's Lucas 31973K is only rated at 5amps, because aside from supplying the Coil and Fuel Pump (unfused) it also supplies much of everything else on the car, via the green wiring.
I suspect it is more like 25amp which still had to be born in mind when adding electrical load to the car.
I'm not home at the moment but when I'm back I'll look up the tech data I have and see if I can get a rating...

[svenedin]

. Wasn't it a requirement in the Highway Code at one time that sidelights must be lit when the car is parked at night in certain situations?


Stephen

It still is the law and the criteria hasn't changed much, it's just that street lighting is far more common as are speed limits.
Though with many local authorities switching street lighting off at midnight I do wonder if there is a bit of a hole in the Regs these days....

The Headlamp switch has always been fed directly from the battery so that lighting is aways available.

In the case of the Minor that means being fed directly from A1 on the Voltage Regulator which internally connects to A, which is connected to the battery.
This is the earliest diagram in the Manual as Stephen says a pain to follow the lines...
Screenshot_20260820-155938_Drive.jpg

I do doubt Stephen's Lucas 31973K is only rated at 5amps, because aside from supplying the Coil and Fuel Pump (unfused) it also supplies much of everything else on the car, via the green wiring.
I suspect it is more like 25amp which still had to be born in mind when adding electrical load to the car.
I'm not home at the moment but when I'm back I'll look up the tech data I have and see if I can get a rating...

Berni, yes the later ignition switch (key start) 31973K also know as 47SA is widely quoted at a rating of 5 amps for the ignition and 16 amps for the start position

See:https://www.autoelectricsupplies.co.uk/ ... SAYCpyp_9K

and

https://www.holden.co.uk/p/lucas_type_i ... _body_only

Then this company says 16A

https://www.vintagecarparts.co.uk/produ ... ition#pic2

However, Autosparks quote 25A continuous which is quite different to what everybody else states: https://www.autosparks.co.uk/lucas-ignition-switch

I cannot find the actual Lucas technical information which would be rather useful! The Lucas Automotive website is hopeless.

Regardless of what the actual amp rating of the ignition switch is, the headlamps are not supposed to be wired via it!


Stephen

[geoberni]

I cannot find the actual Lucas technical information which would be rather useful! The Lucas Automotive website is hopeless.

Regardless of what the actual amp rating of the ignition switch is, the headlamps are not supposed to be wired via it!

Stephen

Which is getting us back on topic
I've just been doing some online browsing and can only find 'vintage' Lucas Parts catalogues, not technical specs.

[jagnut66]

Hi,
Too many replies now to quote from you all but thanks for all this feedback. And the points have been noted.
Right, upon investigation I find that, although connected to the ignition circuit for some reason, my light switch is not physically connected to the ignition switch. Which I would find odd anyway, because I would expect them to be two separate circuits, despite the link showing on the wiring diagram.
I do remember that when I rewired the car I followed the wiring diagram and then effectively cross referenced that with what I could decypher of the existing wiring, so I don't believe there was ever a physical link between the two switches.
The permanent live feed to the ignition switch is, as expected, a brown with a blue tracer. Tested as permanent live.
The feed to the light switch is also, again as expected, a brown with a blue tracer, which unless the ignition is on is dead.
Both exit the loom and go straight to their respective switches, I have not 'interrupted' them for any reason.
There are several brown wires with blue tracers exiting the loom in the engine bay. as far as I can see they all connect to where they are supposed to, interestingly though, in the engine bay they all connect to a permannet live circuit......
I am now going to study the wiring diagram again.
However, in the absence of another solution, I will also order a length of 25 amp Brown with a Blue Tracer from Autosparks and run a separate perminant live feed for the lights.
So the light circuit will return to being perminantly live and the other cable will be disconnected and isolated.
A spare ignition live if I ever need one.
However I am still puzzled, as from what I've found, logically that cable should be perminantly live at the light switch...........
Best wishes,
Mike.

[svenedin]

Hmm. Odd.

So the Brown/blue at the lighting switch is dead but the brown/blue at the ignition switch is live? Looking at the wiring diagram there should be 3x brown/blue ends behind the dashboard. One will be live and is the wire that connects to the lighting switching. The other two brown/blue should be either end of the bridge wire that goes between the lighting switch and the ignition switch. When I am not sure, I test either end for continuity with my multimeter on the "beep for continuity" setting. If the bridge wire is missing, run the live brown/blue you have already to the lighting switch (the other end should be on terminal A1 of the RB106) and use your newly purchased piece of brown/blue to bridge to the ignition switch. Should be job done.

I can't account for other brown/blue wires in the engine bay as I cannot see that on the wiring diagram. There are lots of brown wires with various different coloured tracers though. Check carefully!

Stephen

[jagnut66]

So the Brown/blue at the lighting switch is dead but the brown/blue at the ignition switch is live? Looking at the wiring diagram there should be 3x brown/blue ends behind the dashboard. One will be live and is the wire that connects to the lighting switching. The other two brown/blue should be either end of the bridge wire that goes between the lighting switch and the ignition switch

Not with my loom, I have two brown / blue tracer wires exiting the loom behind the dash, both should be permanent live according to what I found at the other end in the engine bay.
As you said, odd......

I have ordered a 25 amp brown / blue tracer cable from Autosparks to rewire and solve this.
Best wishes,
Mike.

[svenedin]

So the Brown/blue at the lighting switch is dead but the brown/blue at the ignition switch is live? Looking at the wiring diagram there should be 3x brown/blue ends behind the dashboard. One will be live and is the wire that connects to the lighting switching. The other two brown/blue should be either end of the bridge wire that goes between the lighting switch and the ignition switch

Not with my loom, I have two brown / blue tracer wires exiting the loom behind the dash, both should be permanent live according to what I found at the other end in the engine bay.
As you said, odd......

I have ordered a 25 amp brown / blue tracer cable from Autosparks to rewire and solve this.
Best wishes,
Mike.

Ok. If you can trace the dead brown/blue behind the dash to the other end in the engine bay, just connect to terminal A1 or A (they are connected together) at the RB106 then no need for bridge wire. Quite difficult with the bare wire/terminal screw connections and easy for one wire not to be making proper contact when multiple wires join the same terminal. I am sure this is why they moved on to spade connectors.

Stephen

[geoberni]

Hi,
Too many replies now to quote from you all but thanks for all this feedback. And the points have been noted.
Right, upon investigation I find that, although connected to the ignition circuit for some reason, my light switch is not physically connected to the ignition switch. Which I would find odd anyway, because I would expect them to be two separate circuits, despite the link showing on the wiring diagram.
I do remember that when I rewired the car I followed the wiring diagram and then effectively cross referenced that with what I could decypher of the existing wiring, so I don't believe there was ever a physical link between the two switches.
The permanent live feed to the ignition switch is, as expected, a brown with a blue tracer. Tested as permanent live.
The feed to the light switch is also, again as expected, a brown with a blue tracer, which unless the ignition is on is dead.
Both exit the loom and go straight to their respective switches, I have not 'interrupted' them for any reason.
There are several brown wires with blue tracers exiting the loom in the engine bay. as far as I can see they all connect to where they are supposed to, interestingly though, in the engine bay they all connect to a permannet live circuit......
I am now going to study the wiring diagram again.

Mike.

Mike
Depending on age of your car there are only 2 versions of how the supply Brown/Blue (36 on early diagram) is connected.

The original 'Control Box' with inbuilt fuse holders had a single cable on A1 which fed the Ign Sw and a link from the same terminal on the Ign Sw then goes over to the Lighting Sw.
The RB106 with a separate fuse box, whether screw or blade connections is the same arrangement except that it requires an additional Brown/Blue to connect from A1 to the Fuse Box, a function previously built into the combined 'Control Box'.

So from the sounds of it you have connected the connection from the Ign Sw to the output of the Sw instead of adding it to the input. The 2 Brown/Blue belong together.

I don't understand why you have found

several brown wires with blue tracers exiting the loom in the engine bay.

There should only be a max of 3 Brown/Blue ends under the bonnet, comprising 2 on A1 of the Regulator and 1 on the Fuse Box supplying the Purple wires.
Note:
At some point the Brown/Blue cable from A1 to the Fuse Box was changed to be solid Brown departing from A.

Hope that helps .

[jagnut66]

If you can trace the dead brown/blue behind the dash to the other end in the engine bay, just connect to terminal A1 or A (they are connected together) at the RB106 then no need for bridge wire.

I don't understand why you have found

Firstly, sorry Berni, by several I meant 3.
And to answer both of you I have a picture of my loom when freshly installed, two connected together on 'A' and one going to the fuse box to power up the purples:

New Wiring Loom 1 (2).JPG (349.65 KiB) Viewed 1256 times

I assume that one is short and only travels down the loom far enough to reach the fuse box and the other is the one that is wired into my ignition.
Which leaves the other one which appears inside the car, which I connected to the light switch and which only goes live with the ignition on....
I would add at this point that when I bought the loom the chap at 'a certain well known supplier' questioned me and I was very clear about what car I had and the year (1954).
Everything else appears to be wired and work as it should.
Best wishes,
Mike.

[svenedin]

It looks OK but not sure about the light green with the purples. Something added?

Are you sure you don't have 3 brown/blue ends behind the dashboard? Sure there isn't one you haven't seen? This would explain it all. One is live all the time and should go to lighting switch. The other 2 are the ends of the bridge wire from lighting switch to ignition switch. Unless the bridge wire is connected it will be dead. You've foundd one dead brown/blue behind the dashboard. Where is the other end?

Stephen

[jagnut66]

You've foundd one dead brown/blue behind the dashboard. Where is the other end?

Sorry Stephen, I think we have 'crossed wires' here....
I actually said:

The feed to the light switch is also, again as expected, a brown with a blue tracer, which unless the ignition is on is dead.

It's only dead when the ignition is off.
All the other wires are connected up okay. This is why it's puzzling.
According to the wiring diagram, there should effectively be only one Brown / Blue Tracer wire exiting the loom behind the dash, this connects to the light switch and then (using the same colour coding) links accross to the ignition switch.
I am going to disconnect and replace the supply to the light swich, because as we established earlier in this topic it is both unsafe and possibly illegal.
The ignition needs a permanent live for the car to work, so that will remain.
However, (regardless of where this mystery wire originates) wiring the ignition via the light switch (as per the wiring diagram) just feels like a bad idea to start with....
So when I'm done the two will be separate.
Best wishes,
Mike.