MGB & GT Forum

Hello all,
After describing what the car is doing, I will be open to any humor that you see in my predicament...I'm prepared...
Switched out the old fuse box to a 12-position blade type box.
As it stands now:
Engine only turns over and starts if light switch is on.
Turn on light switch, fuel pump starts.
turn lights off, turn key to ON position, fuel pump starts.
Turn key to start, nothing except fuel pump.
Switch on light switch, car starts.
Turn lights off, engine still runs but dash gauges shut down.
I've been tracing wires to see if I connected two (some) of them wrong. I had to double up the circuits because of the 12 fuse connections.
Also, I was told the new box needed to have a circuit "jumped" which I do not know what that accomplishes.
With engine off and light switch on, every fuse post has power.
Noticeable with new fuse box, brighter lights, more responsive blinka's.
I know it might be hard to follow my description but if i can get guidance for the correct connections, I appreciate it!
Car is drivable as long as parking light are on...everything else electric functions fine.
I think I need to start from square one...
Thanks All,
Bill
(Wire nuts are temp.,)

Walk around the car counter-clockwise three times, starting at the drivers door, kicking each tire as you pass them, wipe the headlights on each pass. After three times around, sit down and have a brew. Then try the electrics. If they still don't work right, repeat.


GMc

“ Also, I was told the new box needed to have a circuit "jumped" which I do not know what that accomplishes.
With engine off and light switch on, every fuse post has power.”

On your original fuse box, the top two fuses are linked and powered when the light switch is on. The 3rd fuse is powered by ignition and the 4th is always hot.

It sounds like your light switch is powering all fuses. That’s where the issue is.

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1978 MGB
When diagnosing issues related to running and stalling, always check to see if you have gas in the tank first.

This might sound silly, but when I changed to a blade fuse box, I installed it upside down… things that shouldn’t work with the ignition off would work!! I turned it around, reconnected the wires, and,,,, viola!! All was right!!

Bill,
Fuses protect wires. No engine running, (example) LT to ignition is never fused. A blown fuse will never prevent the car to start, turn over and run.
You are aware that a Brown colour wire is 100% 12v, or direct from the battery. Bob above mentions a good place to start, there is a Brown colour wire feed to the main lighting switch.

In any event you need to 'engineer' in some common sense. You should/want/need to have some circuits switched and others live. Re think what you have done.
Looking at what you have done in your photo, I can't deduce much, with that secondary fuse box and aftermarket wires different wire gauges/colours joined in house nuts? Yikes!!

Be methodical and draw out on paper what the result wants to be. Check one circuit at a time as you assemble it.


B

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Life's most persistent and urgent question is, "What are you doing for others?"



Edited 2 time(s). Last edit at 2024-04-07 04:27 PM by riley1489.

Bill, I see the problem...you forgot something..

The original MGB fuse box is a power-in power-out through each individual fuse. The top and the next below fuse are connected together on the left side.Not sure what you have but, the original does not have a common + or a common - feed.

Oh boy. Where to start. There is a lot going on in that picture and much of it is concerning. Household electrical wiring nuts, butt splices, wires going every which way to mention a few. You really need to break out the wiring diagram and sort through that methodically. Why the switch from the original, simple fuse block?

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I was addicted to the hokey pokey but I turned myself around.

The problem with doing nothing is not knowing when you’re finished.

You went from a 4 fuse box to a 12 fuse box.

The extra fuses will allow you to individually protect more individual devices and their wiring, that’s fine.

However as B points out, in the original fuse configuration, the brown wires are connected to constant voltage from the battery, unswitched, with one brown wire from the battery usually via the starter post. You need to wire your new box such that you can replicate those connections.

Likewise, Originally a white wire from the ignition switch is powered when the ign switch is in the On/run and start position and additional white wires feed and power more devices. Again these connections need to be replicated.

So essentially your new fuse block needs two independent busses that will each feed a bank of fuses.
One buss is to be fed from a brown wire from the battery, the other buss from the white wire from the ign switch.

Then follow the original wire diagram of the original fuse block to add the remaining original wires!

I assume you have the wire diagrams but if not see the attached!

Take Bruce’s advice and draw yourself a wiring diagram of your new fuse block and assign the necessary feed wires (brown or white), to the busses, and the outgoing wires to the assigned fuse positions. Use the original fuse block diagram to identify the wires to attach.

Include any wire color changes from the original harness due to splicing and/or use wire numbering!!



Edited 2 time(s). Last edit at 2024-04-07 02:58 PM by MG14611.

Bill you are using that box in an incorrect way. With the original fuse box power goes in one side and out the other side. On your new box power goes in the the big terminal at the top marked with + sign and power comes out at the side terminals after going through the fuse. Note if you connect the brown wire to that terminal everything will be live, and will probably not do your car much good. Study the wiring diagrams to avoid disaster.



Edited 1 time(s). Last edit at 2024-04-07 04:12 PM by Crispin.

A diagram or schematic of the fire block would clarify a lot!

Robert P, great post!!

Curious about “So essentially your new fuse block needs two independent busses that will each feed a bank of fuses.
One buss is to be fed from a brown wire from the battery, the other buss from the white wire from the ign switch.”
I believe that would be 3 buses, the two that you mentioned plus the feed from the light switch for the top 2 fuses in the original.

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1978 MGB
When diagnosing issues related to running and stalling, always check to see if you have gas in the tank first.

The upside to all this is: Once you’ve sorted this all out, Bill, you're going to have a better than average understanding of the mgb electrical system.

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It is easier to fight for one's principles than to live up to them.

When you worship power, compassion and mercy will look like sins.



Edited 1 time(s). Last edit at 2024-04-07 04:56 PM by rocannon.

The OP has installed a distribution buss, not a fuse block. And in order to replicate original function there needs to be two distribution busses instead of one, because that's how the original functioned, brown being one source, white the other. Since brown is always hot but white is switched, you can't make a single distribution buss replicate original function.


GMc

Can't think of anything positive to say; I'll go back to my corner.