Alternator & MFB Wiring

[BigDanSubaru]

For the alternator, I plan to run 8 gauge wire to an 80 amp fuse then to the + post on the starter. I am using the supplied FFR wire running from the + starter post up to the battery.

I see that my main fuse box (MFB) has a large wire with a ring terminal on the end, intended to hook to the battery +. Also on the MFB, there are some larger white wires. Two of these white wires are paired together with a ring terminal on the end that used to route to the alternator. Since I am running from my alternator to the starter now, what do I do with these paired white wires with the ring terminal on the end?

I saw on another post that someone just put that ring terminal with the paired white wires onto the battery +. Is that even necessary if there is already that one larger wire feeding power from battery + to the MFB?

If my understanding of the OEM circuit is correct, it was Alternator====MFB-----Battery+ (I am using BLUE to indicate the double white wires)

I want to change it to Alternator----80Afuse-----Starter--------------Battery+------MFB

I don't see any reason for Alternator----80Afuse-----Starter--------------Battery+------MFB=====Battery+

Is this correct?

[RPGs818SNA]

Hey Big Dan,

You are pretty close. The red FFR wire from the battery to the starter is 4 gauge and is rated for 135 amps continuous over short distances. It will work fine for short durations from the battery to the starter. It is not normally fused, so be careful to cover it anywhere it might rub.

The 2007 WRX has a 120 amp slow blow fuse in the main fuse box between the battery and those white wires to the alternator. If you run directly to the starter, I recommend using a 120 amp slow blow inline fuse near the starter. To carry up to 120 amps, I recommend using a 4 gauge wire so the wire doesn’t limit the alternator current. I used 4 gauge welding wire because it’s more flexible.

On my 2006 Impreza, those white wires after the 120 amp fuse in the main fuse box also went back into the main fuse box for the ABS and the radiator fans, as I recall. I tied the 2 large white wires and the 4 smaller white wires together. I recommend you check all the white wires on the back of the main fuse box to see how they are connected to power.

The white alternator wires should not be connected to the battery as that would bypass the SBF-1 fuse. See the wiring diagram below, and a photo of my 6 white wires being connected together.

RPG

I'll add the photos later, as the forum isn't letting me right now.

[BigDanSubaru]

Thanks for the clarification RPG. So it sounds like my 8 gauge wire with 80 amp inline fuse is quite a bit undersized and I should upsize to 4 gauge wire with 120 amp inline SBF from the alternator to the starter.

I did some digging around online but I am not having a lot of luck finding a 120A inline SBF. Most are meant for plugging into a fuse box. Do you know of one that would work for this application?

As for the white alternator wires from the MFB, I guess I should just trace them back to their junction and remove them. There are other white wires that come from the MFB that serve some other separate small fuse/relay block. I will keep those and just remove the double white wires that used to serve the alternator.

[BigDanSubaru]

Another question I have is if I can use what I have already started piecing together. I have 8 gauge wire and several inline fuses 80 and 40 amp.

If I run two 8 gauge wires of the same length in parallel, one with an 80A inline fuse and one with a 40A inline fuse, isn't that basically the same as one 4 gauge wire with a 120A inline fuse?

Only thing is that the fuses that I have are not "slow blow fuses". Is that a deal breaker? It would be nice to use what I already own, but not if it isn't going to work, or isn't safe.

Thanks for all the help!

[RPGs818SNA]

I used a Littelfuse 02980900TXN fuse holder and a Littelfuse 0MEG125.XP 125 amp slow blow fuse. The best prices seem to be at the auto parts stores.

I installed mine between the ground and frame to protect my starter and alternator wires and the battery itself. It’s unconventional, but it gave me a convenient place to put it. I also put a cutoff switch between the battery + and everything else to prevent parasitic battery drain. However, putting one between the alternator and starter provides just as much protection as your donor wiring.

20201018_132929.jpg

I also removed my double white wires to the alternator. I then realized all those other white wires were connected, so I cut them off close to the main fuse box and joined them together with a torch and lots of solder.

20200827_185445.jpg

Here's the schematic for your donor showing the white wires going to SBF-8 (ABS) and MB-1 (radiator fans).

2007 power supply page 2.jpg

Best of luck with your wiring,
RPG

[RPGs818SNA]

8 gauge wire has 8.37 square mm of copper, 5 gauge has 16.8, and 4 gauge has 21.1. So two 8 gauge wires is equivalent to 5 gauge, but the two 8 gauge can carry as much current as a 4 gauge since the two wires can dissipate heat better than one wire. Two 8’s will carry more than your alternator puts out.

The 80+40 combo would first blow the 40 with 80 total amps, and then blow the 80, so that's not quite the same as a single 120.

Slow blow fuses help with motors starting up, such as your ABS and radiator fans. It’s unlikely that they would start at the same time, but they theoretically could. You can protect the wiring by fusing both with 80 amp fast blow fuses, and are unlikely to have problems, but I’d use slow blow if you can find them.

All the best,
RPG

[BigDanSubaru]

RPG you are the MAN. Seriously, I really appreciate the detail and clarity in your explanations. The pictures and diagrams are also super helpful. I am going to keep progressing on this wiring stage which has proven to be challenging and time consuming. I have a feeling it will be very rewarding though if/when this car fires up.

[RPGs818SNA]

Thanks Dan! You’ve come a long way since September. Just keep at it. You'll get there. First startup validates a lot of your work. I found it quite satisfying.
RPG

[BigDanSubaru]

If you are reading this thread, here is what I ended up with. 4 gauge stranded wire to a 125A slow-blow fuse then another 4 gauge wire to the starter positive. The power then travels down the FFR provided starter wire up to the battery. I mounted my fuse box to the frame rail near the starter by tapping M6 holes in the frame and using black button head fasteners.

alt fuse to starter wire 1.jpg alt fuse to starter wire 2.jpg alt wire routing.jpg alt fuse wiring.jpg alt fuse no cover.jpg alt fuse with cover.jpg