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Discussion Starter #1
Hi BB

I sat down with Papajam's excellent wiring diagram for my 73 and (I hope this is okay) modified it a bit to show how I might use a modern blade 10 position fuse box.

1. If I understand the factory wiring correctly, all the always-hot lines are bussed @ fuses 1-4. Since I won't be able to find a box that does this, I propose to use a power block to distribute these inputs to terminals 1-4*

*Well, 1-3 actually since so far as I can tell, #4 isn't used on a 73 as there is no interlock device. This would let me spread out some of the connectors and use terminal 4. See #5 below.

2. Am I correct that all of the inputs from the power block to the fuse box need to be 4mm/AWG10? If I used 2.5/AWG12 would I be creating problems as there is a 4mm wire "upstream" ?

3. If I understand the fuse box correctly, the only ignition switched loads are #6 & 9? In other words, you can run the wipers, headlights, defroster etc with no key? I bought my car as a project so I don't know.

4. I'm able to do away with the 2 "extra" fuses that the factory used by running my headlights thru relays, freeing up positions 9 & 10 and eliminating the need to bus terminals 7-8, 9-10.

5. I'll need a fusebox that accommodates three and four ganged "piggyback" connectors. This might be a bit trickier than it seems.

First image is the modified box, second the factory configuration.
 

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I'm surprised Jim hasn't weighed in, this is really his area of expertise. Maybe ping him?
Andrew
 

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Discussion Starter #4 (Edited)
I've given this some more thought and have another idea. Since I'll be running my H4 headlights thru relays I'll need some way to fuse them in the engine bay. Why not fuse the foglights (I'll be running H4 low beam only for 'foglights') and headlights with a small ganged box in the engine bay.

Is it prudent to NOT fuse the signal wire to the relays? I presume it is, as the switching amperage is extremely small and this is the way the factory runs the foglight switch. This will let me simplify the cabin fuse box and use the box I found below, which has jumpers that allow selective ganging.

I like this box because its the only one I've found that has a pre-engineered selective ganging system and its outside dims are nearly the same as the factory box. The orientation of the inputs/outputs are not the same, that would take a little bit of wire management.
 

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The original headlight power wire, now the trigger wire, is already fused unless you have changed things up.
Yes, a separate engine bay fused power distribution block works well and simplifies the necessary re-purposing of wires in the under hood area where this is required. I ran 8 ga. AWG wire from the back of the alternator to supply this block. You will of course have to create a new harness from the relays forward to the headlights, and there are some definite rules of thumb to observe here. Here is where you should spend some time studying the recommendations of someone like Daniel Stern Lighting, an invaluable resource IMO.
I have some shots of my installation if I can just figure out how to attach them in this new format...:frown2:
 

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Discussion Starter #6
You are correct that the original headlight power is fused, actually each filament gets its own fuse. I am trying to free up space in the cabin fuse block and will be fusing the headlights and foglights in the engine bay so I am soliciting opinions on the safety of letting the low amp trigger wire run unfused from the stalk to relay.

For sure i will be making a 12ga harness from the relays to the h4s. I have some good ceramic sockets on order. I did in fact contact daniel stern, what a knowledgeable and responsive guy! Ill be taking many of his recommendations.

The original headlight power wire, now the trigger wire, is already fused unless you have changed things up.
Yes, a separate engine bay fused power distribution block works well and simplifies the necessary re-purposing of wires in the under hood area where this is required. I ran 8 ga. AWG wire from the back of the alternator to supply this block. You will of course have to create a new harness from the relays forward to the headlights, and there are some definite rules of thumb to observe here. Here is where you should spend some time studying the recommendations of someone like Daniel Stern Lighting, an invaluable resource IMO.
I have some shots of my installation if I can just figure out how to attach them in this new format...:frown2:
 

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I place my headlight relays before the fuses to protect the relays. I do it all the time to protect the switch. It's a Plug in system with no mods to the existing system. If there is enough relay sockets in the fuse box, I install them in there. Did this on my 82 Spider and GiGi
 

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Why not fuse the foglights (I'll be running H4 low beam only for 'foglights') and headlights with a small ganged box in the engine bay.
Yea, that's the way to do it. Mount a small fusebox near the front of the car that powers the lights, horn, and electric fan (if you have one). You can tap the power for these fuses from the alternator or battery which are close-by. With this configuration, you aren't sending high-current wires back to the dash, and then forward to the lights/horn/fan.

Is it prudent to NOT fuse the signal wire to the relays?
No, it isn't prudent to have any wire unfused. But it is easy to overcome this problem. Just fuse the power going TO the headlight switch - that fuse will protect the parking & tail lights, as well as what are now the signal leads to the headlight relays.
 

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Discussion Starter #9
No, it isn't prudent to have any wire unfused. But it is easy to overcome this problem. Just fuse the power going TO the headlight switch - that fuse will protect the parking & tail lights, as well as what are now the signal leads to the headlight relays.
Good idea. I can achieve this by pulling power to the headlight switch from the fused side of 1, 2 or 3 in my diagram. This will add load to that fuse. Is the load of the relays and yellow wire (headlight on lamp?) acceptable to add, or should the fuse be upgraded?
 

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Is the load of the relays and yellow wire (headlight on lamp?) acceptable to add, or should the fuse be upgraded?
If you are going to be modifying the wiring of your Alfa, you are going to have to learn how to answer questions like that yourself. I would never trust some unknown person on the Internet to size fuses for me.

Just hook it all up, connect an ammeter across the fuse leads, turn everything on, and measure the current draw. Add a few amps as a safety factory, plug in a fuse with that rating, and you're done.
 

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Discussion Starter #11
If you are going to be modifying the wiring of your Alfa, you are going to have to learn how to answer questions like that yourself. I would never trust some unknown person on the Internet to size fuses for me.
Well said.
 
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