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MG MGB Technical - Headlights & Fusing - definitive answer?

I'm putting some 7" Cibies on my 71BGT. I'll (at least initially) be running standard 60/55W H4's.

I'm planning on creating a relayed sub-harness, but there seems to be debate about to fuse or not to fuse?

I WAS planning on running two fat (4.2mm) brown wires from the alternator (85 Amp Bosch) post and placing a 30 Amp inline fuse into each side, then running this into two 40 Amp 5 pin relays, from here to a 4 fuse blade-box (15 Amp for high beams, 10 Amps for low beams), then out to the lights themselves.

However - it seems to me there is debate about the safest way to fuse things. One main (albeit fat) fuse can blow, resulting in total loss. I'm trying to avoid this by running seperate high and low beam main feeds. I appreciate a 30 Amp blow is a low possibility and is designed to reduce the wiring harness from totally frying. The guy who wrote [url=http://www.britishv8.org/techhome.htm]THIS PAGE[/url] is dead against fusing headlight circuits.

I guess what would be better would be to wire things so that each main feed supplied a left and right side only - that way a mainline blow would result in loss of one side only, but I can't quite figure out how to do this without introducing uneccessary complexities and wre taps!

What are your collective thoughts??
Curtis Walker

I do not see why you would fuse the same circuit twice. Any volt drop you save will be put back in.If you used 2 relays , one driven from headlights on single pole NO and the other driven from main beam and DPCO (or a SPCO at each headlight)you could do exactly what you want, at the expense of more wiring. The 3 really approach would be less wire.
S Best

Well maintained fuses, links and relays should have a very minimal voltage drop - especially if appropriate gauge wires and fuses are used.

The fused brown supply is to protect this part of the circuit (pre relays) should it become compromised. If we believe the odds of a short/break in this part of the circuit is minial - I'd agree with not needing to fuse this section. The post relay fuses for the filaments protect the wiring further out should there be any compromise in the relatively longer wiring which is passing through bodywork (hence more likely to ground). These would seem HIGHLY desirable to me, and in any event, one blown fuse or short will not take down both headlights.

Curtis Walker

As head lamps take a constant current for an extended period I would always put a fuse in the circuit. The fuse should be rated to protect the wire rather than in relation to the light. Having said that the load requirement of the light will have contributed to the wire grade selection.

If you don't use a fuse the wire becomes the fuse with a potential melted loom as the result.

Where demand is brief and spiky I can uderstand people not using a fuse. However careful selection should result in a reliable fuse.
David Witham

In high power electrical system design, the short circuit capability of the feeder cables is a major concsideration and often influences the protecting breaker selection. It would seem wise to me to include a large fuse at some point to protect the wiring harness from frying - this may have a rating well above the normal load current.

Barry
73B
B.J. Quartermaine

I suppose he'd rather just have the wiring harness burn than to be w/o lights. Most cars, even of the 60s vintage, had fuses for the headlights.

Looking at the almost new loom which was installed in my car the wiring was swapped out probably because of a short somewhere. the grommets for the headlight wiring in the inner fenders is frequently worn out or missing and metal holes act like saw blades to wiring.
Mike MaGee

Yes, I agree - after all, if there's a bad short you'll still be without lights, and in addition, you'll need to replace multiple melted wires. I think I'm gonna plump for the fully fused enchillada (pre and post relay).

I'll be interested to keep reading others' ideas!
Curtis Walker

Oh - and I am a member here, BTW, it's just a pain to keep logging in just to post - can anyone say 'cookie'?
Curtis Walker

As was said, modern cars fuse the lights, separate from the rest of the harness. If the fuse pops (or if you don't have one and it should've) you have much bigger problems than no lights. If the main harness fuse pops you're not going anywhere anyway...

I've just customized a harness for another (non-MG) project, and given that experience I'd recommend you visit the wreckers and grab a main fuse block from a donor. Cut the wires long so you can splice them in to the MG harness. Easier (and safer IMO) than inline fuses. You'll get a bunch handy relays, modern fuses and maybe even resetable fuses all laid out nicely and mounted in a protected box.
Mike Polan

There are two camps here, fuse and no fuse. After some consideration I decided to fuse my headlights through two relays. I have one fuse per relay. I also renewed the grounds for the headlights and all of the connections from the relays to the headlights. The odds of losing both high and low beam at once is low and good wiring/grounds further reduces the risk.
Jim Lema

There is no definitive answer, just as there is no definitive answer as to the value of insurance. If you knew what the future would bring, such decisions would be ever so much easier. If I knew when I was going to die, I'd wait until just before that to buy life insurance, and I'd buy a LOT of it! As with many things, there are trade-offs involved, and each has to make his or her own choices, whether it be for a modification to your own car, or as an engineer/designer for a new car. Here's my rationale for the decision I have made:

The only time you'll ever blow a fuse in any circuit is when a live wire comes into contact with ground. The only time the headlught wires are live is when the headlights are on. The only time a wire will move to make contact with ground is when the car is moving. The faster the car is moving, the bumpier the roads, the more curves involved, and the higher the speed, the more likely it is for wiring to move.

In other words, the most likely time for a headlight fuse to blow is when you're driving at night, in a spirited manner on a bumpy, twisty, winding country road - the absolute last time you need to lose your headlights. Without headlights, you can wind up in a ditch or up against a big oak tree before your eyes can readjust to the dark. As much as I'd hate to see a wiring meltdown, I'd a whole lot less like to have my wife and family attend my funeral.

With a dead short in the wiring, you'll probably lose the lights any way, fused or not, but under the situation described above, the short to ground is likely to be of a momentary nature, and the lights might momentarily dim, but you won't lose them.

Some folks have argued that if you keep your wiring in good condition and well routed, you won't have a short in the wiring. If you can be sure of that, why do you need fuses at all?

What about when you're working on your car? Sometimes in moving wires around to get to something for repair, if the wiring is in bad shape you might cause a short to ground. Two answers to that - 1) ALWAYS disconnect the battery ground before doing any work that might involve moving wiring, and 2) ALWAYS have a fire extinguisher close by.

In fact, if you carry a fire extinguisher in your car (and you DO carry one, don't you?), you should be able to pull over and put out any electical fire before too much damage is done to your car, certainly before the car is a total loss.

With fuses, you run the risk of dying; without fuses, you run the risk of an electrical fire. The choice is yours.

On the other hand, you can minimize both risks by using four fuses, one for each of the beams on each side of the car - two for high beams and two for low beams. I would NEVER, under any circumstance, place a single fuse where it's loss could take out all the lights. If you do use four fuses, it would be a good idea to make frequent checks to make sure all four beams work. If one beam is out and you lose the other - well, not a pretty sight. And, you might want to make a mental note that if you should ever lose your lights, immediately switch to the other beam as fast as you can.

My company, Advance Auto-Wire, sells headlight relay kits with four fuses. We do this because this is what most people want. My personal preference is for no fuses, but we have to meet customers needs to stay in business. Our power block kit has no headlight fuses.

I hope this has been somewhat informative, but the final decision is up to the individual.
Dan Masters

Why not add self resetting circuit breakers in line in place of fuses. This will be the best of both worlds. Chevrolets used to have a circuit breaker built into there HL switch. Thats how I would go. Bob Thompson
RHT Thompson

That's what I was gonna say, Bob. But it still doesn't solve the whole problem. It might get you home, tho. And that can be real convenient. Just my opinion from reading all of your responses, clean all your connections occasionally and inspect where your wires go through sheetmetal (good grommets). And hope your control box doesn't stick its points. I haven't yet had a catastrophic failure from any Lucas components, but have had warning signs in use and evidence of same from previous owners.

Do what makes you feel safe. Are you ever?
Tom

This thread was discussed between 28/07/2005 and 29/07/2005

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