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MG MGB Technical - Basic fuse block question

Took me by surprise, but it seems the fuse block terminal with the double white wires (one to the fuel pump) is a 12v+ RETURN to the fuse block, and the fuse passes the power straight through to the green wire grouping directly across the block.

It also seems that power TO the the fuel pump is from the double brown pair (one to the starter relay, the other to the ignition switch etc). That way the fuel pump gets no power unless the ignition switch is on.

Correct? If so, strange fuse block!

BH Davis

Wiring diagrams are available at click on stock schematics. Make yourself an enlarged copy. Follow the white wire circuit and note how the branches supply power to various components.

White wire circuit is switched, unfused power. The green circuit is switched, fused power and gets its power from the white circuit.

Depending upon the year of your car, one of the white wires going to the fuse block is supplying power to the fuse block and the other white wire is taking power from the unfused side of the fuse block and supplying power to the ignition system.

The fuel pump is powered by the white wire circuit. It's not part of the fused circuits in the car.

A relay is an electrically controlled switch used to reduce the electrical loads on the ignition switch.

The starter relay has a constant power supply, brown wire. When the ignition switch is in the start position, 12 volts is supplied to the relay through the white/red wire causing the switch to close inside the relay allowing power from the brown wire to the white/brown wire.

I don't understand what it is that you find odd.

BH. I am not sure what you are saying or what your intended point is.

I am assuming that your first paragraph deals with the third fuse from the bottom which has a single white/brown wire providing input, through the ignition relay, to that point. From that point, two white/brown wires provide current to the distributor system and the radiator fans. Power is also supplied, through a fuse, to the green wire circuits.

Really not sure what you mean by "the double brown pair" of wires. Brown wires are the source of all power, being a direct connection to the battery/alternator. They have the marker colors changed, based on function, as they go through various switches, connections, fuses, etc.

Power to the ignition switch is from a brown wire. When the ignition switch is turned to the "run" position, the power flows through the switch into the "white wire" circuit which is unfused, switch "brown wire power". Part of the power now flowing goes to the ignition relay and activates the relay. When the ignition relay, a form of electric switch, is activated, it provides direct power, from a brown wire, through the relay to the front of the third fuse from the top (power input side) through the white/brown wire. The white brown input wire is connected to two other white brown wires, providing power to the radiator fan fuse (where the wire color changes at the output side of the fuse) and the Opus (Lucas 45DE4) electronic distributor system.

Brown wires are the source of all power input. Power always flows, through various other devices and colored wires, from the brown wire circuit to a ground point. If you trace the circuits with that in mind, you should be able to figure out the power flow through the system.

Les Bengtson

I didn't mean to evoke such an intense response....sorry about that. I also could have been a good deal more clear regarding my statement that the brown wire appeared to feed the fuel pump. I believe it does......but through the ignition switch or course, and then via a white wire.

I have multiple wiring diagrams for my 73B roadster (including the one from the Advance Auto Wire site)and am very experienced in electrical work.

It just seems odd to me to have a power source coming "to" the fuse box as versus power going away from it through the fuses. I am far more experienced in AC wiring with it's fuse (breaker) panels where you have a main power source feeding all circuits in the panel. I'm beginning to see that it works somewhat differently in auto wiring (at lease MG/Lucas auto wiring)....where I am somewhat unexperienced.

I don't know how the fuse block is wired internally.....if at all.... and that is part of the problem.

So, question is: are there any internal cross-overs imbedded in the plastic block, or do all 4 fuses simply pass power from the spade connectors on one side, through the fuse, to the spade connectors on the opposite side of that fuse.

I understand that the brown wire from the starter solenoid brings unfused power up to the forward/bottom spade connectors on the fuse block. If this is only feeding the bottom fuse then it's sending power through the fuse to the purple wire accessory items cirucit.

Also, I understand Kimberly's explanation regarding the fused green circuits being supplied,through the second fuse up from the bottom, the unfused white wires.

Based on Kimberly's statement that the white wire brings power to the fuse block there must be internal connections within the block. If there are, do you know where I can find a diagram of THAT? All the wiring diagram shows is a permanent connection between the first and second from the top connectors on the front side of the block (red/green wire on the top terminal).

I've spent a good bit more time thinking about and trying to explain this question than I did first time around. The detailed response supplied above deserved a better explanation of my question.

BH Davis

The fuse box in your car is no different than the fuse panel/breaker box in your home.

The way your home is wired;
electricity goes into the fuse panel from the utility company and goes out through the various breakers or fuses. The breakers or fuses are used to divide your home's wiring into various circuits.
If there is a power failure or your utility company shuts the power off to your neighborhood, you have no power going into the fuse/breaker box and therefore you will not have any power coming out. Ultimately all the circuits in your home are fused.

Not all electrical circuits in cars are fused. That is why you don't see the positive cable from the battery going to a fuse block and then branching off to the various circuits. Fused circuits are sub circuits.

The terminals on the left side of the fuse where the white wires connect is a three-way splice, (1)one wire coming in from the ignition switch, (2)the left side of the fuse, and (3)a white wire going to the coil.

A fuse or breaker is an intended weak link in an electrical circuit. If there is a problem with a circuit the breaker or fuse will fail preventing damage to components and wiring or an electrical fire.

The fuse is what connects (splices) the white wire to the green wire. There is no wiring in the fuse block to bypass the fuse.

The top fuse goes to the lighting on one side of the car and the second fuse goes to the lighting on the other side of the car. The power source for the top two fuses is the red/green wire from the headlight switch. There is an internal bridge in the fuse block instead of using a jumper wire to branch the power source to each side of the car.

What are the sources of the wiring diagrams you have for your car? I find that the Hayne's versions have too many model years or model variations combined into one diagram and therefore not always the best. I always try to use the factory wiring diagram when possible. I like Dan's (Advance Auto Wire) schematics because they are based on the factory versions.

Are you having a particular problem with your car? Or are you just trying to get a better understanding of automotive electrical systems? If the latter, Rick Astley published a book last year about the MGB electrical system. You can also try your local public library for books about automotive electrical systems.

Do you have any more questions? If so, ask.


I understand the system now. The key point is that power goes into each individual fuse from the front, then through the fuse to the circuits connected on the opposite side of each individual fuse. Plain and simple.

It's not really like an A/C panel though where you have your power main coming into the central buss bars, and circuits take off through breakers on both sides.

Here it's really just a stack of inline fuses.

I finally spent a bit of time tracking circuits on the schematic (AdvanceAutoWire download) and see just how it all ties together. I've also realized that all the various 72/73 and 73/74 diagrams I printed out months ago were unnecessary as Scematic #17 appears to be my vehicle.

To answer your question there is no problem. I just wanted to understand the circuitry better. I had wondered if perhaps the brown power side wires somehow fed a hidden "buss bar", and spade connectors on both sides of the block were somehow fused through burried interconnection terminals.

Leave it to me to make something about 10 times more complex then it really is!


BH Davis

Funnily enough there *is* a 'bus-bar' of sorts in the MGB fuseblock - the front of the top two fuses is connected together on the back so there is only one red/green wire coming in for two fuses, but four red wires (two on each of the fuses) going out. One fuse feeds one sides parking lights and the other fuse the other side.
Paul Hunt 2


Yeah, that is part of what was throwing me the curve here. Once I looked really closely at the schematic and saw that I began to figure out what was going on.

It's just so weird (from my perspective.....not anecessarily automotive wiring perspective) to have power coming to the block from multiple (3) sources.

Anyway, I understand what's going on so am a happy camper.

BH Davis

This thread was discussed between 29/09/2007 and 02/10/2007

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