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MG Midget and Sprite Technical - Fuses (or lack of them)

From postings on the MGA thread I discovered there is a danger of a burnt out loom or very much worse as the car's lighting circuit is not fused.
Reading the "stop lights" post by Nick it made me realise the same applies to the Frog. A short in the wiring could be a disaster.
Has any one fused that circuit? I am wondering where the best place for access is. From the wiring diagram it looks like the Blue wire leads to front lights, red leads to rear lights, and a second red leads to panel lights. Thanks
Graham M V

If you run into the back of someone and smash your headlamps then there is a bright, incandescent (sp?) element burning away. Gotta hope you didn't rupture their petrol tank!!

I've always put a high load fuse and relay system in my 'modded' cars. Thick wiring to cope with the 130W main / 90 watt dipped bulbs and at least a 60 amp fuse.

On the left of this pic are the 3 relays and the MG Metro fusebox to take the Maxi fuses. The relays work so that if the switch fails then the lamps revert to low beam. They are effectively 'spring loaded' so that any failure will not leave you without lights, unlike the standard dip switch if it gets dirty contacts. You can also see the size of the wiring across the bottom left corner and the wiring protection covering. Big wire cause less voltage drop and thus give more light.

I've not fused the rear lights but have added a small buld that glows red and can be seen via the rear view mirror. This will alert me if the switch fails because it won't light up.

Paranoid? Possibly.


rob multi-sheds thomas

Hi Graham

IIRC only 2 circuits fused on 50s cars; and we have 4 fuses in the midget (5 if you count the wiper motor on later cars).

Circuits that aren't fused include headlights and ignition (coil and fuel pump).

At some point (could be next 1, 2, 5, 10 years) I will fit a later fuse box (blade fuses, maybe 8 or 10) and fuse all circuits... having said that, like many, BRB's been around for some years in current form.

A
Anthony Cutler

The 2 fuse set-up persisted right through the '60's. Unfused circuits were same as the 4 fuse set-up, plus side & tail lights. Fused circuits include brake lights, indicators, horn, instruments, wipers.

I always figured the theory must have been that you couldn't fuse the ignition or lights because if the fuse blew at the wrong place then you couldn't see/stopped in the outside lane/etc. (Which of course happens anyway, but if we want to be dull and boring we can buy something modern) They must have thought it is much better to carry on with a smoking loom!

That's my favourite conspiracy theory on it, who else has one?!
Paul Walbran

Paul

I always thought the burning loom would light the way.

Re headlights: I thought the other reason might be that the old glass tube fuses weren't/aren't reliable - that is the connection between the fuse and the holder is subject to corrosion and so on, so that the connection can fail. This happens to me every now and again - a fuse needs a good twist and/or clean to re-establish contact. You don't need this kind of unreliability on the headlights or ignition.

Always open to a better idea... cost?! ...

A
Anthony Cutler

I dont fancy having a smoking loom. I will have a good look around and work out best place to site a fuse beyond the ignition switch so independent of coil. I was just being lazy and hoping someone had already done the brain work.
Any idea what fuse rating required for front & rear lights (excl brake) on the frog?
Graham V

Watss = Volts x Amps, so, each 21 watt bulb at 12 volts needs nearly 2 amps to run, so, a brakelight system ought to be fine on a 10 amp fuse. Double the amps required and add a bit for grandma.
You can get some nice fancy fuseboxes these days that are waterproof and even fuses that will self illuminate if they have blown!!!!!!! Hard to miss which one is dead. GM also sold/used fuses that self-reset after a cooling-off period. Good for getting you home.

If you are rewiring the whole car, run the red-with-white-tracer dashboard illumintion light wires outside of the main loom since they always seem to be the buggers that melt the whole loom out. Also add lots of extra (black) eart wires and join them frequently.

Just my personal opinion.
rob multi-sheds thomas

Sorry, Graham. Just re-reread your post above.
Headlights ought to be approx 65 watt main and 40 watt dipped, so, call it 100 watts per side on flash (if you have that facility) which means 200 watts plus maybe one for the blue light in the speedo.
201 watts at 12 volts is 20-ish amps. A 40 amp fuse ought to be plenty since you'll not be driving all night on flash. Bear in mind that you will have that 20 watts running from the battery to the switch (blue wire) and then breaking off to the main (Blue /white) and dipped (blue/red) wires so they will share the amps. The dipped will see 80 watts normally so would run at 7 watts (how about a 10 or a 15 amp fuse?) and the main will see more.
One extra thought. What if it blows a fuse at night? Darkness!!! If you have a 40 amp fuse up near the battery end it will protect pretty much everything, but then add a pair of 15 amp fuses near the bulbs. You'll have one headlamp still working if the other bulb pops since that blown fuse will have been a weaker link in the chain that that understressed 40 amp fuse. Does this make sense?

Personally I make all my wiring looms from scratch and have the front lights etc all on quality relays.


rob multi-sheds thomas

Alternatively, buy lots of spare....


rob multi-sheds thomas

hmm never worried about it
since we do't see them going up in flames by the bushes

so my point don't worry to mutch
Onno Könemann

Fusing the head lights would be my first choice as they take the largest amount of current. On a Frog with a foot mounted dip switch I'd probably fuse off those outputs, one fuse for low beam and one for high. You could easily mount a second 4 fuse block from a later Spridget or MGB on the firewall just above the dip switch. Personally I'd probably also install a couple of relays in the two circuits as well, but the fuses there will protect the main harness from shorts out under the bonnet or wings. Similar connections for a later Spridget with the column mounted dip switch, just extend the wires to the fuse block on the firewall and then back to the main harness near the connector. Either way, theres nothing under the bonnet that would look modified for show purposes and the fuse block is out of the weather as well.
The remaing two fuses would be there for the tail lamps and brake lamps if you wished to fuse those circuits as well.
Bill Young

I spent 18 months off work recovering after a car crash, hence my overzelous attitude to fuses, rollcages and fire extinguishers.
These cars were made down to a price and designed to last only f or a decade or so. Outdated and underprotective in the event of a crash or even a splash.
Whilst I agree with Onno that we don't see many going up in flames byt he bushes, there are still some that do. For £20 of wiring, fuses and relays you can makes it last longer and be safer.

£2000 on a paintjob can be protected by £20 on the wiring.

http://www.halmatferello.com/journal/photography/burnt-out-mg/ Fortunately it was only a 1500 :o)
rob multi-sheds thomas

Before we consider designing the electrical circuits we are going to use in a vehicle and its attendant wiring loom, fuse-box and relay requirements we need to look at some (basic) definitions:

Energy is defined as the work done when a force acts on any system. A force of one Newton acting over one Metre produces one Newton-Metre of work, or one Joule of energy.

Power is defined as the rate of change of energy with time. One Watt = One Joule per second.

The Coulomb is a measure of electric charge and is approximately 6 ×1018 electrons.

Current is a measure of the displacement of electric charge. One Ampere represents the rate of one Coulomb of charge per second.

The Volt is defined as the potential difference across a conductor when a current of one ampere dissipates 1 joule of energy per coulomb of charge.
Resistance is a measure of the degree to which an object opposes an electric current passing through it. Assuming a uniform current density, an object's electrical resistance is a function of both its physical geometry and the resistivity of the material from which it is made.
R = (L x r) / A
Where: L is length, A is cross-sectional area and r is the resistivity of the material.



It therefore follows that if one Amp is a Coulomb per second and one Volt occurs when one Joule of energy is dissipated per Coulomb, then the power generated is one Watt.

Ie: P = I x V

Where P is Power, I is current, V is voltage (the easy way to remember this is Watts = Amps x Volts).


Ohm's law states that, in any electrical circuit, the current passing between two points through a conductor is directly proportional to the potential difference (voltage) across the two points, and inversely proportional to the resistance between them.
The mathematical equation that describes this relationship is:

V = I x R

Where V is voltage, I is current, R is resistance.

Using these 2 equations we can always work out the current requirement(s) for any given electrical component in a car. If we know the current requirement we can use the appropriate grade wiring, fuses, and/or relays.

This way, we can wire the car in such a fashion that we don’t:

1. Use over graded wiring (which is a weight penalty.
2. Use under graded wiring (which will lead to excessive current flow, overheating and, eventually an electrical fire.

As stated earlier, in order to prevent wiring looms becoming overloaded (which eventually leads to a fire) it is essential that the current capacity of the wiring and fuses matches (or exceeds) the current requirement of the electrical component in question. Another thing that is essential in my view is to ensure that the current flow through any switches is minimal.

Unfortunately 60’s and 70’s cars weren’t really built with this in mind. For example in the headlight circuit the master lighting switch was placed in series with the lights. Now, this wasn’t too bad with respect to the lights of the day; sealed beam units were generally rated at 45/40W. Running a pair of headlights at full beam would give a current flow of (45/12) x 2 = 7.5A.
This is all well and good given that standard 14/0.30mm.1mm² cable has a current rating of 8.75A (although I believe 7.5A through a grotty Lucas switch is a poor idea).

However, what happens when the vehicle owner decides that sealed beam headlights offer too little illumination and replaces them with halogens?

Generally speaking, halogen H4 bulbs are rated at 60/55W. In this case if we run a pair of headlights at full beam we get a current flow of (60/12) x 2 = 10A! This is well above the current capacity of the wiring and will cause it to overheat, the insulation to break down and the system to fail. However it gets worse than that; running on dipped beam gives a current flow of (55/12) x 2 = 9.17A which still overloads the wiring!

I’ve lost count of the number of cars of Spitfire/Midget/G15 era where the behind dash wiring is heat damaged.

So how do we stop this occurring, and how do we keep the current flow through any switches to a minimum?


The answer is to use a relay, which is basically an electromagnetic switch. Anytime you want to switch a device, which draws more current than is provided by an output of a switch or component, you need to use a relay. A relay is basically an electromagnet switch that that uses a coil (when energised) to connect two contacts together to allow a current flow that is independent of the switching circuit. The coil of most automotive relays draws very little current (less than 200 milliamps), whereas the amount of current that you can pass through a relay's common, normally closed, and normally open contacts will take up to 30A or 40A.

There are, fundamentally two types of relay

1. SPST Relay (Single Pole Single Throw).
2. SPDT relay (Single Pole Double Throw).

For most basic vehicle applications all you need is a SPST relay.

The SPST relay consists of a coil (terminals 85 & 86), 1 common terminal (30), and one normally open terminal (87). Thus when current flows through terminals 85 and 86 then the coil energises and current can flow from terminal 30 to the output (terminal 87).

Using a SPST relay we can ensure that the high current to an item, eg headlamps, is independent to the switching circuit. Generally speaking, when wiring a relay like this I take a common feed from a dedicated fuse to both terminals 30 and 85 and I put the switch in the earth line to terminal 86. That way there is commonality in the wiring of every relay.
Deborah Evans

But it gets worse. There are things to consider such as the material for making the switching contacts. Silver seems to be the most common, I think, but some materials don't like sparks going through them.

Same with those Sparkrite-type electronic ignitions. The chuffing great big spark lancing off a pair of Lucas distributor point can burn off any oil but if you change so that the points are merely the trigger for a 'remote relay operated spark' in some fancy box then you might find the ignition dying from oil overload. BTDT. Same with floor mounted dipswitches.

Good relays are sealed waterproof-ish units rather than the cheapo "Let them fail so we can sell the punter some spare ones" that Rover used on the Metro. Remember those yellow ones with the loose bodies that could come apart?
rob multi-sheds thomas

Don't know if Debs is an electrical engineer or good with google, but speaking as an electrical engineer this is how we tend to work in industry.

Both my MGA and Midget have been rewired with new multiway fuse boxes. It is very important to have numerous fused circuits so that fault finding becomes very simple and a fault with one item does not stop other items from working. This is important in industry and so is equally as important in our hobby cars. You should try to have where possible one fuse per item. Cost more money and more weight :-) but is extremely desirable for us hobbiests.
Equally in industry we tend to use wire (and other items) that are just good enough, why? because it keeps costs down! I suppose from Debbies point of view the difference between .75 Sq/mm and 1 Sq/mm weight wise must be worthy of consideration but us mortals it is nothing. Obviously cost is important and again the smaller the wire the cheaper it is, however simply doing our car it makes more sense to buy 1 or 2 reels of wire and over rate the wiring rather that buying numerous reels of the cheapest wire possible and end up wasting 3/4 of the reel.

With that in mind we use some rules of thumb for wiring sizing

6 Sq/mm around 32amps
2.5 Sq/mm around 15amps
1 Sq/mm around 6 amps

These 3 sizes are very common and can be bought cheaply, if you use the formula "Watss = Volts x Amps" as stated by Rob then you can decide on the wire size then fuse to the wire size. (Fuses protect the wiring)So if you use 1 Sq mm wire then a 6 amp fuse should be used. Simple

I am not a big fan of relays. They make a job complicated and expensive. I personally have had very little problem with lucas switches over many years, but I guess the choice is yours. What I can't understand Debbie is why do you worry about the weight of a bit of wire yet increase the weight and complexity of circuits 2 or 3 fold by adding relays?
Robert (Bob) Midget Turbo

Nobody with the flaming car story's mentioned halogens!!!

i've always counted on and used normal bulbs in normal reflectors
if those are in good working order (good reflector and good contacts)they produce enough light for daily use

for rally driving i have fitted 2 lucas 700 spots and those are fused and relais operated.

don't mess with the std loom!
aditions should be properly exicuted
Onno Könemann

Last winter I added a couple of relays and a dozen fuses instead of the 2 original ones. I also draw some new fresh leads from battary to the front to accomodate the lamps from the relays on.

What a difference. There is a lot of energy lost in corroded connections too thin wires and 45 year old switches. All electrical devices operated better, stronger headlights, quicker wipers, faster blower etc. My car is a 1963 Midget MK I but with a 8 years old loom so it should not be too old and maybe even a tad better then the ones made in the early sixties.

Bas
Bas Timmermans

All the input is much appreciated. But now I am even more confused. My frog is hardly the pretiest in the pond & not an everyday car. So not really wanting to rewire everything just want to protect me and the loom from something ugly (incidentally, brilliant pictures Rob but I guess sad memories).
So just wanted a short quick fix. Guess I will just put a small fusebox behind dash to protect light feeds.
Graham M V

I use standard harness for my halogens. I found that the worst part of the system was the brown feed from the solenoid, so I re-made this joint with a new spade, and cleaned up the tang. (Actually, any solenoid that's not been cleaned for 10+ years would benefit from being dismanted and having the posts, washers, nuts etc cleaned and re-assembled).

Lights were noticably brighter after this.

BTW you can also dismantle and clean the light switch - at least the rocker kind.

I take the view that it might be safer to make the mods suggested below, adding fuses and relays. But I'm always wary of subs-standard wiring being less reliable/safe compared with the original.

A
Anthony Cutler

Graham,

Adding fuses and relays isnt a big job. Can be done in 2 hours and without cutting your existing loom. Just adding new feeds from the battary. The relay will be as close to the device as possible so the new fresh powerlead is close to the device. The old circuit is used for the switching only. Realy simpel.

Your last report tells me you are considering only some extra fuses and in this thread its told thats not enough because you want lets say 10 or 20 amp fuses and that can be too much for the old wires.

Consider 3 relays 2 for lightning and one for the current thats going though the ignition switch. Put in as much fuses as you want they are cheap and come in banks of 4 or 6.
Bas Timmermans

Bas,
That makes good sense. At least I think I understand! The existing lighting cables can simply feed relays. But, for example, should I be putting a small bank of relays in the far right hand side of bonnet to feed the side lamp, dip and beam, with same again in left side?

If so, do I still need to fuse the original wiring which is now just a switching circuit as well as fusing the new wiring that powers the lights?
Thanks for the help
Graham M V

Whilst I am not in favour of relays it is your car and your decision so good luck. When it comes to your question here. ""do I still need to fuse the original wiring which is now just a switching circuit""

The answer is YES you do. As I stated the fuse protects the wiring incase of a short circuit. No matter what the circuit is doing if you have a wire from the battery that short circuits to earth without a fuse then you will have a burnt out loom or worse still a car on fire.
It is my opinion fusing is far more inportant that a few relays so must be the priority.
Robert (Bob) Midget Turbo

Absolutely true! I not only put some relays into the circuit I also put 12 instead of the 2 original fuses. I not only fused the new circuits but also some of the switching circuits.

Graham if you want some pictures I can take them tomorrow and post them here.

I have some of the stuff mounted on or close to the bulkhead and some on the inner wing to accomodate the lighting circuit.

Bas
Bas Timmermans

The early Midgets had no protection for the rear lights, sometime in the late '60's early '70's two inline fuses were added to the red wire(s) going to the back lights. This is a very easy fix as the rear harness and the main harness meet under the dash on the right side of the car. buy some aftermarket in line fuses, solder some bullets on and unplug the harness and put in the inline fuses. BMC/BL realized this was a problem and did the same fix. I've had more that one car with melted rear wires but that was years ago. Relays are definitely the way to go for high amperage headlights. Saves our old switches.
J Bubela

Although it still looks very untidy and wasnt done by the greatest electricien im very pleased with the 12 fusebox(with them modern blade fuses easy to get at any fuelstation) instead of the 4(oldfashioned glas ones) i had.
If something goes wrong its just that fuse that will pop so other functions on the car still operate.

Hopefully this winter i can tidy-up the electrics and make a box for the batery in the passenger footwell.
Always have things to do on the car dont we? :)


Arie de Best

"make a box for the batery in the passenger footwell"

You finaly gave up on women?

Bas Timmermans

Bas
women gave up on Arie and he finaly got it ;P
Onno Könemann

Thanks for all the advice. Like Arie, this will be my project (or one of them) for the winter.
Graham M V

Making fun of the single guy hé?!
You b*st*rds!
Onno, you better watch out with what you eat next week... LOL

But actualy Bas, your almost there, you see, being single makes that you dont have any "deadweight" on the passengerside so as a substitude ill have my battery on the passenger side.




Btw Bas, can you give me a call?
Arie de Best

I can recommend vehiclewiringproducts.co.uk in the UK for wiring parts. I bought bags and bags of wiring terminals off them years ago so that I never need to fart about resoldering dodgy wires.
Another good idea is to visit a scrapyard and slice a loom out of a moden car. Thinwall wiring weighs less and is less fragile.

How about one of these?
http://www.vehicle-wiring-products.eu/VWP-onlinestore/fuses/photo/BDB1.jpg
rob multi-sheds thomas

Arie, I tried but no luck. I am painting my house this week so free and I'll try to get in touch. You can come up north to help....... BBQ is in good working order
Bas Timmermans

Arie
i've just turned 26 (yes it is my birthday) you would not hur a man in his prime!!!
And wouda won't even be there to take care of me (sauna day with girlfrinds sh forgot about!)
Onno Könemann

Reading above again, I wanted to ask one other question please. I have always found crimping the terminals a bit "hit or miss". Sometimes the connector will grip tight, but sometimes with just a small tug, it comes out.
Should I always solder the cable first (I have never soldered them) and then just crimp it for a "belt and braces" approach.
I only have a cheap crimping tool and I see that they can be quite expensive, is that likely to be my problem? Thanks
Graham M V

For wiring loom terminals it is not such a great idea to use solder. It makes the wire end brittle and you are more likely to get the wire breaking off. Maybe a t-i-n-y dab of solder onto the terminal after crimping, just to tack it into place but don't let the solder wick up the wire( which it is quite likely to do). I bought a 250g roll of super-dooper thin solder that has lasted years. Much easier to drop a teensy blob in the right place.
Vehiclewiringproducts sell a fairly good crimp device that has lasted me for years (MP71 £8.95)
rob multi-sheds thomas

Graham. Re your earlier post, you only need to have relays for high load items like the main and dipped beam. One relay for each of them is enough. Pic enclosed.


rob multi-sheds thomas

Graham
NO SOLDER BEFORE CRIMPING!!!!!

sorry for shouting

But soldering and then crimping leads to a bad contact (due to the solder oxidising) witch can work lose resultinhg in intermitent faults verry difficult to trace

if you want to solder doe it after crimping so the solder ataches to wire and terminal
Onno Könemann

Hi Rob

The coil in the relays in your diag must have an earth connection on its 2nd terminal? (2 pins for actuating coil and 2 pins for the switched terminals.)

A
Anthony Cutler

True, but it took me 30 minutes to get that far with the drawing!
rob multi-sheds thomas

fair enough! assume Graham can figure it.

I know how long a 'quick' PPT drawing or equiv can take.

A
Anthony Cutler

Yup. I understand that clear enough. Many thanks
Graham M V

This thread was discussed between 19/08/2009 and 25/08/2009

MG Midget and Sprite Technical index

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