How does the fuse box get power?

[reporter000001]

Ive looked over the wiring for months now, i have several books with diagrams, seems the entire box is powered by the 12red splice?

New fuse boxes have a main hot bar like in a houses circut breker box,but these boxes tend to just all be an inline fuse holder basically...

correct me if im wrong, but is everything powered by the 12red splice??? If so thats a lot of current for a 12g.

UPDATE: yes i know this 12red snakes to ign switch in 2 spots and the 1 to "horn relay" and other to headlight sw.

Im just confused please someone explain, when i disconnect this circuit off firewall block i will loose power to entire truck.

EXAMPLE: how does blower motor get power, dome, brake??? this has to be a main branch in the box somewhere to where all spider web off other circuit..... i dont know now im just rambling...

[greg64]

I had similar confusion with my fuse block (77). It sure look like there are two 12 gauge wires feeding the fuse block through the firewall. But on my truck, only one of those two wires actually powers anything. It feeds a bus inside the fuse block, which then feeds all the fuses. That then feeds the blower motor, radio, etc. The headlights are unfused at the fuse block.

[reporter000001]

ahh man thats what I was afraid of, back to the drawing board. Thank you for your info. You know your $h!t

Originally Posted by greg64

I had similar confusion with my fuse block (77). It sure look like there are two 12 gauge wires feeding the fuse block through the firewall. But on my truck, only one of those two wires actually powers anything. It feeds a bus inside the fuse block, which then feeds all the fuses. That then feeds the blower motor, radio, etc. The headlights are unfused at the fuse block.

[greg64]

Thanks. My Dad, who has been retired for 20 years, taught automotive electrical systems at our local community college.

I agree that one 12 gauge wire seems pretty puny. So on my truck, a ran a second power feed wire into a second fuse block near the glovebox. I use this one to power my electric rad fans and electric fuel pump. These are both loads with relays, so I don't need the ignition switch to power up the main feed.

[enaberif]

You know it was weird I was checking my fuse panel underneath my truck and discovered that I have dead spots in my fuse panel. I'm not sure why but I have power to some parts but not all.

So I'm thinking there is definitely two wires feeding the fuse box.

[Slammed84]

Im not certain what year your truck is but mine is an 84' and I just went through this exact problem.
There are two wires from the engine harness that supply power to the fuse box.
One comes directly from the battery, passes through the starter lug and then into the fuse block. This wire only powers the horn, and the lighting circuits. As you know these are always hot.
The second wire comes from the battery and hits the junction block on the firewall. this junction block also has the wires that feed the lug on the alternator, but ultimately it is the power source for the ignition switch. Also the power wire for the heater relay thing that mounts on the A/C box hits the other side of the junction block.

when the truck is in the off position you should have power to lights, horn, and the input of the ignition switch. everything else i suppose would not be hot until the ignition is set to acc or run position.

i can post pics if you would like to see my factory wiring

[enaberif]

Well on my truck I also have the ammeter and disconnected that wiring and wondering if that may have something to do with it. Or perhaps the dead spots on the fuse block are only live when the truck is running.

We'll see.

[greg64]

There are certainly parts of the fuse block that only have power when the ignition switch is on.

[enaberif]

Originally Posted by greg64

There are certainly parts of the fuse block that only have power when the ignition switch is on.

Yeah I turned the key to the run spot but the fuse block was still no power.

[Slammed84]

Originally Posted by enaberif

Yeah I turned the key to the run spot but the fuse block was still no power.

What spots dont have power? I can double check mine later.

[reporter000001]

stop lpstraffic, taill lps ctsy, accfused should b hot all the time, they are on a 3 bar linked off the 12red circut. the male tabs are also linked to those fuses as well, everything else should be switched

[enaberif]

Whether the truck is running or not running. Or the key is off or in the run position the two in the circle never have power. The only thing I have done recently was remove the two black and white wires with inline fuses from the distribution block. I'm thinking that these two wires provide power to this area of the fuse block and will be doing some tests to confirm and if so I'll be rewiring with 14g feed wires and new terminal ends.

[ray_mcavoy]

Originally Posted by enaberif

This is from a 74.

Whether the truck is running or not running. Or the key is off or in the run position the two in the circle never have power. The only thing I have done recently was remove the two black and white wires with inline fuses from the distribution block. I'm thinking that these two wires provide power to this area of the fuse block and will be doing some tests to confirm and if so I'll be rewiring with 14g feed wires and new terminal ends.

Those two terminals you have circled should be receiving power from the RADIO TCS fuse that's directly below them. And that fuse should be receiving power from a 12ga brown wire (circuit #4 on the factory diagrams) that leads back to the ACC terminal on the ignition switch. That terminal should be powered when the key is in the ACC and RUN positions.

That 12ga brown wire from the ACC terminal on the ignition switch also supplies power to the W/S WIPER fuse and to the turn signal flasher.

The black and white wires with the inline fuses that you removed sound like they would have been for the ammeter. Those wires went directly to the printed circuit traces on the cluster and from there to the meter. They didn't power any other circuits.

[enaberif]

Originally Posted by ray_mcavoy

Those two terminals you have circled should be receiving power from the RADIO TCS fuse that's directly below them. And that fuse should be receiving power from a 12ga brown wire (circuit #4 on the factory diagrams) that leads back to the ACC terminal on the ignition switch. That terminal should be powered when the key is in the ACC and RUN positions.

That 12ga brown wire from the ACC terminal on the ignition switch also supplies power to the W/S WIPER fuse and to the turn signal flasher.

The black and white wires with the inline fuses that you removed sound like they would have been for the ammeter. Those wires went directly to the printed circuit traces on the cluster and from there to the meter. They didn't power any other circuits.

Well zooming in on the fuse that is in the RADIO TCS looks like its popped. So I'll replace that and try again.

Thanks!

[motornut]

may want to swap/move the fuses around confirm a good contact.
that box (post 12) looks like my 78s

[reporter000001]

so this is a 78?

EDIT: this circut should be powered by the brown wire off your ignition switch......the newer trucks diagrams in this book arent that accurate, you can get a voltage tester/multimeter and test it to ground, thatll tell ya... but the 75 and 77 say the wiper and radio are on the same bar, so if u have wipers, you should have that circut as well.

[enaberif]

This is from a 74. I have wipers but no power there. I've tried to look at diagrams but they are so crude and confusing.

[Slammed84]

Yeah.... im out. Your wiring is completely different from the mid eighties models. Although its probably more simple in design.