521 Fuse Blocks.

[datzenmike]

The 521 should have a fusible link between the positive battery terminal and the fuse box making the 25 amp fuse redundant. I would say bridge the fuse but there's obviously over 25 amps flowing and the wires to the fuse may not like that.

 

Where does the power to that fuse come from? Should go back to the positive terminal.

[Socalman]

On 1/2/2015 at 9:45 PM, Charlie69 said:

I have noticed that there have been a few requests for fuse blocks for the 521.

 

As far as I know there are 2 types,

http://i477.photobucket.com/albums/rr132/Charlie69_Datsun/521/Datsun521FuseBoxBacks2Types.jpg

 

I have spent a couple hours searching for a good replacement and this is what I came up with.

The Blue Sea 5035

http://www.powerwerx.com/fuses-circuit-protection/blue-sea-5035-6-independent-circuit-fuse-block.html

 

http://i477.photobucket.com/albums/rr132/Charlie69_Datsun/521/BS5035_xlrg.jpg

 

http://i477.photobucket.com/albums/rr132/Charlie69_Datsun/521/Independent_Sourced_CircuitST_Blade_FuseBlock-page-0.jpg

 

By using these Blue Sea 9217 jumpers this fuse block can be jumper to match either 521 OEM fuse block configuration.

https://www.bluesea.com/products/9217/Terminal_Block_Jumper_for_30A_Terminal_Blocks

http://i477.photobucket.com/albums/rr132/Charlie69_Datsun/521/9217.png

Blue Sea 9217

Is there a write up on this upgrade?

[Charlie69]

i did this upgrade on friends 521 that I sold him and if you purchase and us what I suggested above, it is basically switch the wires and corresponding fuse sizes.  I suggest Mounting the box first taking note of the shortest wire fitment.  I did not extend any wires. If I remember correctly, I am getting old and remembering things is a challenge, You need to purchase a set of the Blue Sea Jumpers.  I believe there are five to a package and you only need two or three depending on which fuse block you have in you truck.  The jumpers are to replicate what wire terminals are bridged on the OEM fuse block. I will look at the two oem styles of the fuse boxes I have and will add information I think of in this post so all the information I provide is together in one place.  I will also fix the broken pictures if I can figure out what pictures these are I should have the original pictures in my PC.

I have made more doing repairs on Mauro's 521 than I sold it to him for.  LOL  Those days are gone I only work on my truck now.

My 66 520 will have a heavily modded 1980 720 harness in it.

If you do this mod and have questions please feel free to pm or email me.  I will be having neck surgery in late April or May.  I have something seriously wrong in my neck.

my email is in the information that I believe you can click on my name icon and you will see the email address as I made it public.

[Charlie69]

On 3/16/2024 at 9:00 PM, Thirty said:

Hey everyone,

 

I have a 1970 Datsun 521. It has a modern fuse box conversion and it’s vertical in engine bay. Second to last fuse is a 25amp and it keeps melting. It’s the main power fuse for the Datsun. So when it burns out the entire Datsun loses all power and cannot drive anymore until replaced or jump wired. What should the amp be? And any ideas on cause?

 

-Thirty

The 25 amp fuse is too small amperage for the entire truck electrical so the fuse is doing what is intended burning out to save the wiring.  Need to know what gauge of wire is providing power to the block and also what gauge of wire is coming out of the fuse.  Lengths of these 2 wire runs would be helpful also. I will check back in a few days to see if you respond or if you do not I will think you have already solved the problem. 

I know this is and old post but if he/she still needs help maybe I can help them.  Daniel C has a vast knowledge of the 521 wiring.

Edited March 29 by Charlie69

[Socalman]

Once the items come in I’ll take photos and ask for assistance. Thank you 

[Socalman]

On 3/29/2026 at 3:19 PM, Charlie69 said:

i did this upgrade on friends 521 that I sold him and if you purchase and us what I suggested above, it is basically switch the wires and corresponding fuse sizes.  I suggest Mounting the box first taking note of the shortest wire fitment.  I did not extend any wires. If I remember correctly, I am getting old and remembering things is a challenge, You need to purchase a set of the Blue Sea Jumpers.  I believe there are five to a package and you only need two or three depending on which fuse block you have in you truck.  The jumpers are to replicate what wire terminals are bridged on the OEM fuse block. I will look at the two oem styles of the fuse boxes I have and will add information I think of in this post so all the information I provide is together in one place.  I will also fix the broken pictures if I can figure out what pictures these are I should have the original pictures in my PC.

I have made more doing repairs on Mauro's 521 than I sold it to him for.  LOL  Those days are gone I only work on my truck now.

My 66 520 will have a heavily modded 1980 720 harness in it.

If you do this mod and have questions please feel free to pm or email me.  I will be having neck surgery in late April or May.  I have something seriously wrong in my neck.

my email is in the information that I believe you can click on my name icon and you will see the email address as I made it public.

so this is what fuse box I have, so I assume that the 3 lower wires are switched hot? so there's no jumper block needed? the single hot wire on the lower right, its always hot I assume, so it would need a jumper block on each side so the the 3 top wires would be always hot? 

[Socalman]

I figured it out, you will need 4 jumpers 3 for always hot and one for ignition hot. 

[Socalman]

I need one more 15a and 2-20s, I just threw in what I had for testing.

[Charlie69]

Looking at the back of your old fuse block will show you what jumpers are needed.  

It has been 3 years plus since I upgraded my friends 521 with the Blue Sea System Fuse Block and jumpers and he has not had any wiring problems since.  At the same time I did the fuse box I also installed a new old stock combination switch.

Edited April 23 by Charlie69