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MG MGB Technical - Reversing Spotlight

I am planning on fitting a rear mounted 55w Lucas reversing spotlight on my BGT. The plan is it can only come on when reverse is selected but can be switched off using an illuminated toggle switch. I am not certain what is required MOT-wise but it seems a sensible way of wiring it all up? Hopefully someone has done this and can save me scrabbling about …..

I will use a relay somewhere near the new light, ideally tapping my new switch into the reversing light switch cable to activate the relay. This will allow me to select if the power goes to the relay 'switch side' when I select reverse.


I will need to run a cable from there to my new switch in the centre console and another from there to the relay.

Any ides of the best place to get at the existing reversing light switch cable and thence into the car?

I will also need to get some power to the new light. Is there anywhere easy in the rear light area. At 55w its under 5 amps so it should be doable? Tap into the rear interior light? I will put an in-line fuse on it if I do go that route.

Thanks

Mike

Mike Dixon

Sounds good Mike-
If the rules are anything like here , any rear facing white light 'has to' be connected through the reversing lights.-OR have an ON indicator light---through reverse is easiest.
A relay makes it easy
Rev. light feed from original rev. lights into one side of the relay--wire from the other side up to your switch on the console to earth (won't need to be fused being an earthing wire)
Then power out to the new light and power in from as you say --somewhere, maybe the fuel pump would be a good supply and already fused, but an extra inline fuse would probably be a good idea---actually you can get relays now with a blade fuse fitting built in-but be aware that only protects from the relay onwards---you are better off having the fuse as close to the power source as possible.
Cheers
willy
William Revit

You don't say the year or whether it already has factory reversing lights. If you have a gearbox reversing light switch that's all it needs for the UK MOT:

"4.6.3. Switching

"Reversing lamps must operate automatically when reverse gear is selected and extinguish when reverse gear is deselected."

You can have a switch that disables it when not needed, but on that wording you can't have it purely manually operated even with a tell-tale so that will govern where you can pick up power i.e. it can only be from the output of the gearbox switch.

If you already have that switch find where the gearbox harness joins the main and rear harnesses in the mass of bullet connectors by the RHD master cylinders. Green carries fused ignition 12v from the main harness to the gearbox switch and green/brown brings it back to the rear harness for the lights - assuming you already have them or a rear harness equipped for them.

If you choose to fit a relay and/or manual over-ride then you would fit it there, and perhaps a fuse if you pick up an unfused 12v supply, connecting the lamp to the green/brown in the boot.

Incidentally as far as I'm aware no MGB came out of the factory with the fuel pump fused, but it could be something that was added to Aussie-built cars. Certainly needed, both mine came to me having suffered a short at the pump which damaged main and rear harnesses, as well as several I have worked on. The best place to fit one is in that mass of connectors where the white wire in the rear harness joins the main harness, and in-line fuse with bullets at each end is all that is needed.
paulh4

Too true Paul--had forgotten the fuel pump ran off the white unfused circuit.
I think---Mike uses his car in a bit of competition and jt's best to keep things as simple as possible on any racer
Easiest/simplest place for the relay for the new Reversing light would be down the back close to where the light is--- so
with the new light mounted out the back then i'd mount the relay on the inside of the back panel near one of the original reversing lights out of the way there somewhere and wire it up with the shortest run cables possible(just to keep it simple-)
Relay wiring would be-
pin
86-connect to original rev,light power wire green/brown
85-connect to the console switch-and switch to earth
87- to new reversing light
30- Power supply from either the fuel pump power supply which you should be able to pick up from at the rh front corner of the boot floor--can't remember what model your GT is but if it's a later one the end of the fuel pump will be poking through a grommet into the inside of the car just there with the power wire on the end but if it's an earlier car the wire will be there on top of the floor going through to underneath through a grommet.---OR--- if your car is a 68 onwards it'll have a hatchlight switch and you could tap into the fused purple supply wire at the switch. Either or would work. but yeah if you're going to use the fuel pump power a fuse would be a good idea-
willy
William Revit

Hatch light 12v supply has a bullet connector down by the offside rear light cluster and revering light (shared with the HRW feed), but although it's fused it's also live all the time.

On that basis the fuel pump might be better being only powered with the ignition on, but if that feed is fused in the engine compartment a fault in that circuit would blow the fuse and cut power to the pump. Unless there was an additional fuse between the pump and the relay.




paulh4

Thanks both. I do have existing reversing lights ( 1973 BGT) and have fitted LEDS to them. They are still not bright enough to be honest for what we want. I have seen some comments on how powerful a rear facing light can be but cannot find anything definitive. I am now planning on a 48W bulb

My intention is that the switch disables the rear spot so it will remain MOT compliant

Thanks for the advice on the wiring. My fuel pump is fused - courtesy of Hammer and Spanner ( thank Paul!) but I agree another will be required it I go down that route.

I can see advantages in the approaches you both suggest. I have just used one similar to Paul's suggested approach for my front spots, tapping into the main beam warning light to activate the relay. Just running an earth wire through car has its attractions though.

I will have a ponder and another look at where all the connectors are, I do try an minimise the amount I actually cut into the existing loom within reason - it keeps it simple and reversible.

This has been a great help in looking at the options and explaining where the connectors all are - you have saved me a lot of time and head scratching.

Thanks

Mike

Mike Dixon

Mike - just a thought - If you could find a suitable LED flood/spot light to use as a reversing light you wouldn't need a relay setup. If you could find one with two wires, which most are, that doesn't rely on a case earth---The power wire could connect straight to the existing reverse light supply (green/brown) which would give you power when in reverse only -then the other wire-earth- could be a single earth wire up to the console switch to switch to earth---simple and safe, no extra fuses or relay needed either.

willy
William Revit

Thanks Willy


I am a bit if a fan of relays to take the load off the switch - I am using an illuminated 10 amp toggle but to be honest that is enough for 48W anyway


The light I am looking at is the Lucas WLR576:

https://www.s-v-c.co.uk/products/wlr576p-reproduction-wlr576-rear-mounted-spot-lamp-with-plain-finial#pic1


Odd wiring now I look at it. Red and Yellow is power for a spot so connect to he green brown or power from the relay as to the green - earth? I may give them a call!

Mike
Mike Dixon

Requirement here is that as well as auto-off when not in reverse, the light beam is aligned as per a dipped beam. Makes sense, high beam reverse light alignment might be useful for high speed reversing but would blind anyone approaching.
So might be worth checking if you have a similar requirement.
Paul Walbran

Looks like you can get that light with a LED globe
If you went that way then there'd hardly be any current draw and you could go without a relay and just connect one wire to the green/brown and the other to the earthing toggle switch --simple and less stuff to go wrong----

Them little switches usually have Blue-Red-Yellow wires
Normally as a 12v Power switch---
Blue---12v in
Red----12v out to whatever-(spotlight)
Yellow-Earth- for the globe in the switch

With the toggle with a warning light, if you're using it as an earthing switch all you have to do is put 12v for the switch illumination onto the terminal marked earth instead of earth (yellow) then earth onto the power terminal(blue) and the other wire goes off to the spotlight as normal but will be switching to earth instead of power
---Some switches won't light up though like this ,it just depends on if the telltale light is a normal little globe or led ---some leds work both ways and some don't
William Revit

Thanks I will have a play with switch to see what does what before I cut anything. I quite like the simplicity of a single earth wire up to a switch

I have a look and see what it is like cutting into the green/brown in the boot area to power the spot - it should be enough to power a 45w bulb, especially as my existing reversing lights are LED. I will put a 5 amp fuse on it. All I can see green/brown is 2 wires joined together like this, courtesy of MGB Stuff, second picture down:

http://www.mgb-stuff.org.uk/lights4.htm


I could always cut those , put bullets on and use a 4 way bullet connector to power the spot . Looks like that could be fun in that space. I will unwrap a bit of the loom to see what is going on in there.

Mike

Mike Dixon

There are no connectors in the reversing light circuit all the way from the bullets in the engine compartment to the 2-pin plug at the light units themselves.

You could just use a Scotchlok to pick up the connection, but they aren't always reliable. Or these Wago 221 connectors which should be more reliable but mean cutting one of the three wires.


paulh4

Those three wires joined together are the reversing light power you're after, There is no other joiner- One is the power from the reversing switch and the other two feed a reversing light each---The two (reverse light power) just get taped into the loom and come out again just below each reversing light paired up with there respective earth wires----To make it easier you could tap into one of these near one of the original reversing lights
William Revit

Paul---must have been typing at the same time----not a fan of scotchlocks, specially for a rally car, crimps are best.or an old fashioned screw on connector.
Cheers
willy
William Revit

Sorry to be labouring this but what is the joiner mentioned in note 2 on the above link:


Note 2: The junction in the green/brown for the two reversing lights is done with a sealed connection (arrowed below) under the off-side light, and not with bullet connectors.

Can I not prise that apart and use a 4 way there?

Mike Dixon

You would have to cut it off, the wires are in a thick soldered crimp as in the attached for brown wires, there are several of those round the car.


paulh4

Still think it'd be a whole lot easier to just join onto the wire near a reversing light where it's easy to get to----cut back some insulation and crimp the new wire onto it there------or if you have to, solder it if you haven't got access to crimps---if you're going to solder make sure you twist the new wire tightly around the original a couple of times first then minimum solder so that it doesn't wick along the wire


William Revit

Thanks for the photo, I don't fancy taking that apart! Splicing in near a light looks a better bet.

I like the look of that crimp, but have not seen one before - what is it called? I am obviously not getting the magic word on my google search and that EFI site is huge!

Mike
Mike Dixon

we just call them crimps
Their title would be-
Open barrel splice crimp
You get about 1000 of them for a dollar
If you went to your local auto electrician or rally/race car joint they'd be using them all the time and i'm sure they'd be happy to part with a few
You'll need crimping pliers to get a good job
If you haven't got any crimpers, maybe if you had everything ready drive there and get them to crimp on while you're there





William Revit

Thanks Willy, found them now I know what they are called. My normal supplier does not seem to have them for some reason but as you say they are not expensive!

I prefer the un-insulated crimps anyway so have the correct crimper.


Mike
Mike Dixon

This thread was discussed between 21/08/2023 and 03/09/2023

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