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MG MGB Technical - Which circuits are fuse protected?

I've always sort of figured the fuse block on my 73B roadster protected all the wires connected to it. That is, both the wires coming off the back AND front of the block.

I just sort of assumed that the wires off the front of the fuse block.....like the white wires to the fuel pump......were somehow also protected by the fuse next to them via internal wiring in the plastic block.

Today I tested this theory. I removed the third fuse down from the top.....the one next to the white fuel pump wire connections. Low and behold the fuel pump still worked !! It is apparently not protected by that fuse.

So, is the fuel pump "supposed" to be protected by the 3rd fuse from the top, or is it not protected at all? If not at all, are all the wires connected to the front side of the block unprotected?

Thanks,
BH
BH Davis

BH,

You can go to www.advanceautowire.com and download the wiring diagrams in color. But the simple answer is in the color codes of the system. Brown is hot, not fused, not switched. Basically the Brown wire visits the fuse box on its way to other important duties to provide power to the fuse that protects the Purple circuits which is not switched but fused (trunk lights, horns, that kind of stuff). White is switched by the ignition switch, not fused, it goes to the fues box to provide the Green switched and fused circuit power (stuff like the brake lights, signal lights, wiper). Red with green stripe is switched by the headlamp switch and travels to the fuse box to provide power for the fused, red wires (parking and tail lights).

So, since the fuel pump is powered off the white circuit it is switched (by the ignition switch) but not fused.

Other circuits not fused are headlamps (except for the flash to pass system) overdrive and panel lights.

warmly,
dave
Dave Braun

Dave,

Thank you for clarifying.

Bottom line then is that the fuse block is indeed an "in-line" situation, with each of the rear connections being fed by the power in on the front side of that particular fuse.

It seemed so unlikely to me that items would be unprotected that I did not consider this as a realistic possibility.

On the diagrams you can obviously see power traveling from the battery to the ignition switch. Not knowing for sure what took place internally in the ignition switch, and not believing it likely that the fuel pump etc was unprotected, I didn't consider "in=line" a realistic possibility.

All is clear now.

Thanks,
BH

BH Davis

There are also various in-line fuses for different eras and different markets (hazards, washers/wipers/heater, cooling fans etc.), but again only the 'output' wire from the fuse is fused, the 'input' wire isn't.

Modern cars often have two or even three fuses of progressively lower rating in series, with various circuits tapping off at each join between fuses. There are usually 'fusible links' close to the battery meaning all wiring except the high-current circuit feeding the starter are protected, but none of this applied to the MGB.

Three out of four MGBs I have worked on have had shorts at the fuel pump which have damaged the rear and main harnesses, I have both my pump feeds fused where the rear harness joins the main. The same thing can happen with the later OD system where the manual switch is on top of the gearlever, but even though mine are the earlier dash and column switches I have fused these as well.
Paul Hunt

Ok....thanks to all of you. Now the question goes from "how" to "why".

Why in the world did they set up the wiring this way. I guess I can understand it a bit from the idea of safety.....things can't fail because of a dead fuse.....but can that be all there is to why fuel pump, headlights etc. aren't fused?

Thanks,
BH
BH Davis

BH,

Don't confuse fusing as a means to protect the equipment in the car. It is there only to protect the wiring. Absent a fuse, a dead short from a power feed to the ground side will quickly heat up a wire and cause it to burn off its insulation and probably melt the copper strands. A very expensive repair because the resulting small fire will take out the rest of the harness as well. Components rarely fail from a wiring failure, they usually fail and if you are unlucky, take out the wires!

On the technical area of my website I show how I fused the headlights (blue circuit) and panel lights (red circuit) on my TD. On that car there are two fuses, one for the horns, and one for the green circuit. Everything else is unfused. If you poke around my website you should be able to find the technical section. www.dbraun99.com

warmly,
dave
Dave Braun

Having a single fuse in the headlight supply *would* be a safety issue, which is why I recommend fitting individual filament fuses when installing headlight relays. I doubt that is the only reason though as that should mean things like fuel pump, overdrive and ignition would have been protected. I guess it's partly historical and partly cost-cutting, the UK made some pretty pared-down motors post WW2.

AS BH says the fuses are there to protect the wiring (with the possible exception of the 500mA fuse feeding the Sequential Seat Belt System, all the other being 17A ratd 35A blow) and for this reason they need to be as close as possible to the supply in order to protect the maximum amount of wiring, and not by the component that circuit feeds. I know that's not always what Abingdon/Lucas did, but it should be borne in mind when adding your own fuses.
Paul Hunt

This thread was discussed between 25/04/2009 and 27/04/2009

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