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MG MGB Technical - Extra Fuses needed?

I am rebuilding my loom and fitting uprated lights.
I will be fitting relays and two seperate fuses in both of the Main and Dip beams.
I have noticed that the gauge illumination bulbs and fuel pump are not fused at all.
They both come from the switched brown, one via Ignition Switch and one via Light Switch.
Is there any reason why I should not add fuses to protect them?
D M Tetlow

DM. A friend who is a professional mechanic recently recommended such to me. He also recommended fitting a fuse into the fuel pump circuit.

I do not know about fusing the headlights. In the past, the reason that was given for their being an unfused circuit was that they were "safety related". Not sure why the brake lights and turn signals were fused and the head lights were not--all are, to me, "safety related". I would be interested in those more knowledgeable about things electrical have to say.

Les
Les Bengtson

DM,

Relais and additional fuses are allways a good idea as long as you are not after orginality.
If you have a look upon the wiring diagram of a modern upmarket car, you will get an idea upon the curcits that should have a fuse inset.

On one of my cars i also added 10 further fuses to the four fixed and two flying ones it came with. All switches have been upgraded with relais, even for the brake lights.
Since i made this modifications, the car has bright headlights, a got working heater motor, the switches do not become hot anymore and ther have not been any problems with the electrics as it was before.

Ralph
Ralph

You don't need brake lights or parking lights to avoid the drop on an unlit mountain road, but you sure as goodness need headlights. One fuse per filament should be fine, preferably not one per beam, or one fuse for the lot unless you have filament fuses as well. But even then if you have the presence of mind to pull on the headlamp flasher that is a separate circuit.

There is no reason why you should not fuse the gauge lights where the red/green joins the dimmer switch/rheostat, but preferably not where it comes off the main lighting switch unless you can seperate it from the feed to the parking lights.

I've fused the fuel pump on both my cars as when I got them both had obviously suffered a short at the pump at some time. Also the OD, especially if you have the manual switch on the gear lever, which is prone to chafing and shorting. These fuses can be installed very easily where the main harness joins the rear and gearbox harnesses by the main fusebox, by using in-line fuses with bullets on the tails, and is just as easily reversible if someone was desperate for originality. Use cylindrical fuse holders of the type that take the same fuses as in the fusebox, as the fusebox should be holding two spares, and the standard 17 amp rated 35 amp blow is perfectly adequate for protecting this wiring as well.
Paul Hunt 2

Thanks Guys
I will put an ammeter in the fuel pump line set to max and see what size fuse I need.
Regards
Dave
D M Tetlow

Don't bother, just use a standard fuse, that way you already have the spares. To use different fuses just means you have to carry more spares. The fuse is there to protect the wiring, and all the wiring is the same size, hence only needs the same fusing.

An ammeter in series is unlikely to give you the current anyway as it only ticks, and neither an analogue or a digital instrument (unless pretty sophisticated) won't give you an accurate reading. If you really wanted to know the fuel pump current then measure the pump resistance and use Ohms Law at 14.5v to calculate the current.
Paul Hunt 2

Hi

I installed headlight relays a few years ago and fuse's for the head lights.
I recently was driving at night and my lights went out. Thankfully I was driving at a very slow speed. If that had been highway driving the result would have been much worse.
This winter I am deleting the fuse's.
If it came to that, I can replace the car. At least I would be alive to do so.

FWIW

Bruce
Bruce Mills

Bruce what rating bulbs do you have, what rating fuses did you use, and where in the circuit were the fuses?
Paul Hunt 2

My one experience with unfused lighting failure was in a Triumph TR250. The dip switch developed a short, car filled with smoke, lights went out and I spent the next three days repairing the harness. An unfused circuit will not insure your lights will work if there is a short. I was driving into the garage and the only damage was the wiring.

Clifton
Clifton Gordon

When the electrical layout of the B was designed, there had been very few limits from the registraion authorities. It aught to be cheap and relyable for a cars life span of ~8 years. So we are talking about a life span bonus of at least 27 ./. 8 = 19 years now.
Not that bad, i think.
For getting an idea what can be done to modify the wiring and add fuses and relays, circuit diagrams of modern cars are a very helpfull adwise. Just look how the wiring is made in a Ford Modeo for example, and you will find all the questions for upgrding MGB electrics answered there.

Hope this helps

Ralph
Ralph

I am going to fuse the outputs from each relay to each filament as close as possible to the relays, and there will be no fuses before the relays. I will then be no more likely to have a complete failure than on a std setup.
Dave
D M Tetlow

Paul
The headlights are 100/75 with ancillaries lights and the fuse's are before the relays.

This set up worked well for a few years until I put in new gauges (remember from last summer my thread "VDO Gauges")?

I know I was drawing too much power but it was pretty scarey at 20 mph to suddenly have no lights at all. I can imagine what it would be like at 60+. My own fault, I agree, but maybe that is why the factory wired the lights that way?


Bruce
Bruce Mills

There's never anything wrong with adding protection to your cars electrical system. Adding fuses to any circuit is a great idea. I'm leaning on using more circuit breakers instead of fuses in the future. They cost a bit more initially but the convience of being resetable more than makes up for that in my opinion. When you do have a problem it's often difficult to locate and you wind up using several fuses during trouble shooting until you find the cause of the short. Try this in the dark alonside the road with a limited number of fuses in the boot and you'll soon get the idea of using circuit breakers. There are a number of types of automotive breakers available for almost any type fuse block, modern or older glass fuse style. The only downside is the cover for the original Lucas fuse block won't fit after a circuit breaker is installed. http://www.electerm.com/breaker.html shows some of the types available. Another idea would be to keep a couple of circuit breakers of appropriate sizes in the boot and use them when troubleshooting. When you find the problem then you can install the proper type glass fuse and cover and save the circuit breaker for future use.
Bill Young

DM Tetlow,
As long as you want to preserve your installation by fusing it, have the fuses between the hot source and relays. That sounds better.
R.G.
Renou

I agree Renou, except for the main lights where total failure would be dangerous. They will be fused singularly after the relays.
Dave
D M Tetlow

This thread was discussed between 22/10/2007 and 26/10/2007

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