FixBroke: Wiring the Subaru aux lights, part 6: making and installing thefuse/relay box

With this auxiliary lighting project I’m adding a bunch of fuses and relays, and these are things best left protected from the elements. There’s a reason fuse boxes have covers. In this installment, I’ll be building a box to house all of these items and fitting it to the car.

The basis for the box is this, a hard drive enclosure that has been sitting in my basement for far too long. It has been cut down by about 1" from its original length to fit in the space available.

I tried a few different places to mount the box, but the Subaru engine compartment is unfortunately pretty full. There is a big space by the air filter that was briefly under consideration, but the odd angles of the metal there didn’t agree with my rather square box. Instead, I found some space right in front of the battery and behind the left-side headlight.

There isn’t much going on in this space except for a couple of coolant hoses for the overflow reservoir. In the above picture I have lengthened both hoses by replacing them with longer pieces, allowing them to curve out of the way of the spot I have in mind.

Now on to making the box. For some time I debated how I would use this box to house my components. They fit very well in its constraints, but they also needed to be accessible if I needed to replace a fuse or relay. I thought about making it hinged, but decided against that (mostly because I didn’t have any hinges handy). I remembered how the hard drive used to be contained in this box - a small tray with all the components mounted on it slid into grooves in the box. I decided that I could mimic that design, which would allow for the components to stay in place while the box slid on and off freely when needed.

I cut off the bottom of the box, making it a 3-sided affair. I cut a piece of aluminum sheet to precisely fit the grooves in the box, making a component board and a sliding cover. The means of attaching the component board to the car would be the battery tie-down rods.

I would need a means of attaching the relays to the board. Because I was using two different kinds of relays (what I happened to have laying around) and due to the lack of space to mount the relays directly to the board, I would have to design a relay holding bracket. It was an interesting exercise to design the three-dimensional bracket and then figure out how to make it with a two-dimensional aluminum sheet.

A few cuts with some tin snips and a few bends with the vise gave me this:

I attached the fuse block and some nylon p-clamps to the board with some rivets, and test-fitted the relays. The p-clamps will hold the board to the battery tie-down rod.

This looks like it fits, but I was concerned about routing thick wires around that sharp corner on the bracket. It would be easier if it faced the other direction. A few more snips cut the black relay off the bracket, and another bracket was made for it to mount on the other side of the clear relays. These were riveted to the component board.

Notice that ring terminal attached to the fuse block? Well, as it turns out, getting a ring terminal to fit that 8mm bolt with a 4 gauge wire is next to impossible. I got some 4-gauge terminals with something like a 3/8" hole, and soldered in a washer to increase the contact area of the terminal with the 8mm bolt.

The relays will be switching the negative side of the electrical connection on the lights. I needed to attach one side of each of the relays to a common ground connection, which was done by putting a bolt through the corner of the component board, connecting all the relays to a common negative wire off the battery. It was a tight squeeze to fit those big 8-gauge wires in this tiny space.

In the above pic you can also see the lower fuse connected to all of the relays as the relay coil power source, and I have added the terminals but not yet the wires to all the other connections. Those connections would have to be made in the car.

Back to the car, the relay wires that were strung through the firewall by the brake reservoir needed to be routed to the battery area to connect to the box. I removed the intercooler so I could add protective split wire loom to the wires and zip tie them to the brake lines running along the top of the firewall.

And now, with the box mounted on the battery rod, I could attach the relay wires to the relays. Also note the big 4-gauge battery cable running through the bottom of the hard drive case.

The cable is attached to the fuse block, but not yet to the battery. A short length puts it around the battery and into the inline fuse holder (no fuse installed yet, to avoid a spark show) and attached to the battery with a ring terminal. There is a good amount of slack in the wire to allow the fuse holder to be fished out for fuse replacement.

Feeling completion was close, I put everything together for a quick test fit. Of course, I had forgotten something. The HKS “Circle Earth” system. This is a silly little engine grounding system that came installed on my car by the previous owner. It serves to provide alternate grounding points on the engine for all of the sensors and things attached there - purportedly it increases power, but realistically all it does is avoids sensor problems due to bad grounding. Regardless, it was now in the way. The bracket on the brass disk is meant to attach to the battery tie-down rod, which puts it in the way of the box.

The many cables attaching to the disc were just long enough that the whole thing could be moved slightly if the cables were doubled up on the mounting holes. A small bracket was made to act as a go-between with the battery tie-down rod and the HKS bracket.

(excuse the mud and the not-yet-blogged wires - this photo was taken after completion and thorough “testing”)

With the box completed and installed I really felt like completion was at hand. As it turned out there was still quite a lot of work to be done. Keep reading to find out more!