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MG Midget and Sprite Technical - Replacing Fuse Box

I'm considering replacing the original Lucas Fuse Box for a modern Blade type (Practical Reasons)
But have my reservations.
The existing box has 2 x 17.5/35 Amp fuses one for IGN and one for LIVE circuits.
I could just replace and wire up the same way with 2 x Blade fuses, I know that I would have to compromise and drop down to a 15A blade as that is the closest to a 17.5A Glass Fuse.
Has anyone done this or has anyone seperated out the individual IGN and LIVE circuits so that you end up running approx 8 fuses. If so how would you calculate the correct Blade fuse rating for each circuit e.g Lights, Brakes, Heater Fan, Wipers etc.
Thanks
Ed
Ed H

Fuse ratings are not an issue; fuses are top protect wiring, and all the wires can take 17.5, as any one circuit could be the only one operational on the feed. Generally, the headlamp circuit could pull a bit over 10A, so use a 15, and an electric fan likewise. Everything else will do with 10 or 15s. If a circuit known to be working blows the fuse, go up to 15A or even 20.

But the light circuits are not fused as OE, a serious fault in my view. So you have to decide how far into it you want to go. Simplest is to use two common feed 4 fuse blocks and use one for IGN W-G circuits, and the other for Hot N-P circuits.

If you use the easier to find separate feed blocks, you have to jumper the feed side. The IGN one only needs at most, as many fuses as there are G wires. And the Hot one as many as there are P wires. I leave at least one fuse not on common feed. and run the fuel pump W through that, as the pump wire is especially vulnerable. Ditto for OD on cars with that.

I usually take the N wires (except the ALT feed) off the solenoid, and run them all through the Hot block, which gives basic protection to every circuit. When I do most cars for minimal basic protection, I wind up with the OE 4 fuse block for IGN W-G circuits, with #4 as the pump fuse, plus a common feed 6 fuse block for all constant hot N-P circuits, including 2 for headlamp relays. All fuses except headlamps (15A) are 10A.

FRM
FR Millmore

Hey ed,

Ive contempaleted this back and forth, if i goba modern day fuse box then ill do a company called "painless wiring " here in the states...but its every man to his own opinions

Prop
Prop and the Blackhole Midget

Ed
P=VI power=volts*amps
you know V=12 you then need the power consumed to work out the fuse rating which as you know is the amps. This is easy for the lights e.g 55/65w (watts is power). Therefore for the dip lights 2x55w/12=8.33amps.
Most major items will have a power rating on them eg. wiper, heater etc but if not you could always put an ammeter in the circuit and measure the current. Use the 10A setting on a multimeter and step down to the 2amp range if appropriate.
Remember fuses are there to protect the wires when something goes wrong in the circuit and you should step up to the next available fuse (particularly for motor based circuits which can draw a surge current on start-up.

MGmike
M McAndrew

Mike-
Trouble is the condition at ratings are not so simple. Lights are rated for output at 12.8V generally, but it is never stated at what voltage the actual rated wattage is. I assume at the same 12.8V, but who knows? So for a 65W lamp pair at 12.8V, A is 5.1x2 = 10.2 total load. But a good Alt with good wires will deliver 14.5V at the lamp, so A = 5.1x(14.5/12.8) = 5.8 x2 = 11.6A total. Plus, lamps have a high cold start surge, so 15A is about minimum, and by experience, effective.

The best realistic way to protect existing circuits is by wire size, which is implicit in my first post above; tables can be found at British Wiring.com for American and old Lucas, metric is your problem!

FRM
FR Millmore

I replaced the 4 fuses on BRB with 2x 4 blade fuses; some of the wires on the fused side (e.g. the purple, always live; red; green) had 2 terminals on the old box, hence easiest is to provide 2 separate fuses. You therefore need to connect across the some of the terminals on the 'input' side, e.g. for the lower fuse box, ignition connection across all 3 right-hand fuses.

I also took the opportunity to replace the in-line wiper fuse.

Hence 4 fuses became 8, with none spare.

I think all wires are ~standard, except for the lower box where the red goes to the Emerald ECU, and the green/orange is for the engine cooling fan and washer motor relays. The green/white is the wiper.

As noted earlier, I measured the current and added around 50% to fix the values. I think 10A is the max value I used (red colour fuse).

HTH

Anthony
Anthony Cutler

I have got about 15 blade fuses in 2 boxes in addition to a box of relay type things. Whilst I know that I have fuses for amongst others: lights, heated screen, heat seats, ignition etc etc I have no idea how they are wired in.

This is because vehicle wiring is witchcraft.

One useful comment: my fuse boxes are marine quality with stainless steel posts etc. whilst I don't understand wiring I do know the effects of bad wiring.

Johnathan (No help at all)
J Frame

Johnathan
Like you say it's a bit like alchemy, after all Joseph Lucas was known as the 'Prince of Darkness' something to do with his electrical products not always performing as intended !!
Ed
Ed H

This untidy, unholy mess was mine before I tidied up the wiring.

The 6way fuseholders are used for the services used by the two fuses in the original fuseholder.
.
One has the battery "live all the time" systems, lights and radio etc.

The other has all the ignition powered circuits off it.

The fuse holders have each had a busbar soldered beneath them to link each fuse in the "block" with any other fuse on the system, thus replicating the two fuse system safely.

I bought the two fuse boxes from AES at http://www.autoelectricsupplies.co.uk/
in Tenbury Wells.

They have a £4 minimum posting cost so if you are looking to do any other work it's a good idea to check online for what you may need and get it all in one posting, I had new bullet ends and connector blocks as well as a tube of their non corrosive battery terminal gel.



ps

It looks a good bit tidier now ;-)


Bill1

I found it.

This is the fuse box i want to go with, they have several differant sizes

But as you can see, everything is pre wired and labeled...just install and cut the old wiring and splce to the new box... It even has built in relays for head lights and fans

Maybe a bit expensive but looks like its thought out and you cN buy wiring kits to go with it where all the wiring is is marked every few inches as to where it goes... Aka... Horn, left rear turn signal, fuel gauge, head light, ect ect

But this the direction im eventually going....its from painless
Id do the 12 circuit, but 18 circuit would look cool esp with a flaux capicor plugged into it...hahaha

Prop

http://www.painlesswiring.com/webcatalog/largeview.php?SearchField=30001
Prop and the Blackhole Midget

I used a similar setup but mounted it in the cabin to keep it dry. I soldered the terminals together with copper wire to form the busbar. I drew the wiring diagram on AutoCAD somewhere. Fuse ratings were an educated guess at the time - luckily all's well a couple of years later. It's all under a plastic cover now to protect it from stray fingers.


Andrew Dunn

Bill
Somebody has stolen your pedal box and master cylinders!!!!!!!!!!!!!!!!!
I Pickering

Actcherlley




Now I remember where I put it!


still, car's lighter without it and quite quick

:)
Bill1

On my 1500 I replaced the original fuse box for the blade type, but did not separate circuits or expand # of fuses used. You can see in the pic the fuse ratings used: nothing melted, no fuses blown. As for calculating correct fuse size, if it blows it's too small (assuming your circuit is healthy to begin with). Overall it probably can't hurt to increase the complexity of the fusing system, as the originals were pretty basic.


Richard Reeves

Hi Boys and girls
I rewired my 1500 with a fusebox from a 1984 Nissan hatch. 12 fuses, used a relay to take the load off the ignition switch, 5 other relays for hibeam, low beam, horn, engine fan and electronic ignition (to be added when I build it). Works superbly, all circuits are separated and rated appropriately e.g. P=VI, excellent advice that.
cheers
R W Bowers

To add to richards thought.....

" """As for calculating correct fuse size, if it blows it's too small (assuming your circuit is healthy to begin with)."""""

If the wiring catches fire and blows up the car ... Then the fuse rating was probably to large... :(

Hahaha

Prop
Prop and the Blackhole Midget

In my MGB GT I have an Advance Auto Wire wiring harness (http://www.advanceautowire.com/), as designed by Dan Masters in the USA. It is a superb piece of kit, with a power board containing fuses and relays in one compact unit. The point is he does NOT advocate fuses on the headlights. His manual says this:

Headlight fuses: If you’ll notice, there are none! I am
STRONGLY opposed to the use of fuses in the headlight circuit. Having the headlights go out while doing some spirited driving on winding back road on a moonless night because a wire momentarily bounced into contact with ground can be fatal! Having your car burn is only a financial loss. Some might disagree with me, but the choice is clear in my mind.

He also has only eight fuses:

Why eight fuses? Because eight are enough. Unless you are adding extra, high current draw loads, eight fuses adequately protect your wiring. The 12, 18, or 21 fuse panels offered to the "street Rod" folks have fuses that you don't need in an MGB or a Triumph TR. If you wish to add extra, hight powered loads, you'll want to consider one of our auxiliary add-on systems - either the extra four-fuse auxilary fuse block, or the driving/fog light add-on relay/fuse block.

Just a different point of view.
Mike Howlett

I quite agree with what Dan Masters says.

My boxes have six terminals but they are not used for all of them in either case.

I have used one to allow power to my relays at pin 30 off the battery live one and there is spare capacity at the ignition live circuit too.

Well I hope to have the car for a long time, who knows what I will need to power later? Megasquirt soon certainly...
Bill1

Strongly disagree with Dan.
I've had headlights go out on unfused circuits - it's OK because the fire helps EMT find the crash. I've had headlamps go out on circuit breaker protected cars - 47 Ford, and fused cars - 39 Chevrolet. For about 50 years the standard US GM switch had a fuse in the switch itself.
All my more modern vehicles have/had both main fuses or fusible links (they SUCK!) AND downstream headlamp fuses - Dodge truck, two Ford Tauruses, 7 Mazdas -and so does anything I rewire - never blew a fuse yet.
Bad fixes of poor engineering, and bad maintenance, are not a good reason to leave out safety items. And fuses make it so much easier to diagnose electrical problems.

FRM
FR Millmore

Guys
Thanks for all the advice decided to go with 2 x 4 blade fuse boxes (because I had some) soldered a busbar to one for the IGN and linked it to the second. It leaves me with 1 spare fuse slot and I left the Lights alone. So IGN has fused, Indicators, Heater Fan, Fuel Gauge, Wipers, Brake Lights & Rad Fan. HOT fuse has just the Horn. All mounted to the original hole in the mud guard.
Thanks
Ed


Ed H

This thread was discussed between 30/08/2012 and 13/09/2012

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