Project Lexus SC300 Road Racer: Part 9 – Begin Rewiring at the… Back?


Electrical current flowing through a wire generates heat. If you’ve ever seen a traditional light bulb in action, you know that this can be quite a lot of heat! In fact, this is the very principle on which fuses work- the current generates heat and the fuse is precisely tuned to “burn up” (fail) above a specific current level. For the sake of simplicity, we can more or less safely assume that the 22759 milspec wire in 20AWG can handle about 9 amps. We know our pump may draw 14 A, so we still need two wires.

Since we need two wires anyway, you might wonder why we didn’t just go with the DT connector in the first place. Using the DT connector here provides huge overhead in current capacity, and provides some future proofing. While we are not designing this harness to have additional device support (there are no “spare” wires in it), we could add another wire to the connector in the future to power something like a cool suit pump or other higher power device.

Looking at the diagram you will notice four wires on the pressure pump — two green and two black. Devices almost always have power and ground wires, and pumps are no exception. If you think about electricity, it flows a lot like water. So, for every unit of electricity (amps) that goes “in” to something, it’s probably coming out. That “out” is the electricity that goes to ground. So if there’s 14A being used by the pump, all of that has to find its way to ground, too. If we need two wires “in” to handle the current, we need two wires “out” (for the ground).

That takes care of the overall ground design, the fuel level, and the pumps. What about the lights?


Project SC300's tail lamps.

At a minimum, almost all racing and track day / HPDE sanctioning bodies require functional brake lamps. If we ever do any enduros with this car, we also will want functional tail lamps. And, since I’m stupid, I also want functional blinkers and reverse lamps. This creates a few electrical considerations.

When the brake pedal is depressed, we need the brake lamps to come on. But there are two lamps — left and right. Electrically speaking, there are only two ways to get one “signal” to go to two places. You either have two wires, or you have one wire that splits.

For the purposes of both lightening (marginally) and simplifying the trunk harness, I opted to split the wire. But, that doesn’t mean I used a razor blade and cut it in half; it means a splice. There are proper motorsports electrical splices, and then there are the other methods. A proper motorsports electrical splice is a great and useful thing.

What it isn’t? Solder. Wire nuts. Butt connectors. Wires just twisted together and electrical taped. With the exception of solder and a proper parallel splice, nothing is suitable in an automotive environment. In a motorsports environment, my personal opinion is that solder has almost no place or use. We are not installing a stereo here. Yes, aerospace “rules” say that proper solder joints are allowable in certain situations. But doing them properly takes time, and parallel splices are much easier and are just as robust when done properly.

You’ll learn how to make a proper motorsports parallel splice very soon, because we need one for the brake lamps. We also need another splice for the tail lamps. When we turn on the headlights, both tail lamps need to come on as on a street car. And, lastly, we need a splice for the reverse lamps, since both should come on simultaneously when the reverse switch on the transmission is activated.

In the trunk we have two wires for the blinkers, because they are actuated separately. The left blinker comes on for left, and the right blinker comes on for right. Yes, they both come on if we flip the hazard switch, but that is controlled by the Racepak. It will turn on both outputs (left and right) when the hazard switch is active.

In hindsight as we write this, we wonder if we should have opted for two circuits for the brake lamps. Electrically speaking, it means that I have an extra layer of redundancy in case of a failure. In other words, right now if something goes wrong with the Racepak output channel we will lose both brake lamps. If something goes wrong in the wiring, the same. We think the odds of this are pretty minimal, but we have a tendency to sweat the small stuff (as Tom will often complain about).

OK, now that you understand the plan for the trunk harness and all of its circuits, it’s time to assemble the harness! Finally! We kind of want to happy cry right now. Hey, if it is a member of your preferred sexual counterpart or has tires it will make you cry, right (we’re very PC here, I think…)?


Pulling wires of varying colors to length.

If you remember from Part 7, Arcadia (the software suite we used) is capable of producing varying reports, one of which is a cut list for wires. We simply printed out the cut list for the trunk harness and then got to snipping.

As many of the wires were of identical lengths, we simply pulled them all at the same time and cut them. You might be wondering what the colors mean. It depends on your preference, but it’s good to adhere to some loose industry standards, especially if you are not the final owner of the wiring loom.


  • Red is an always hot +12V (essentially the battery voltage).
  • Black is chassis ground.
  • Brown is signal ground.
  • Orange is a 5v sensor power / reference.

MoTeC and other companies have other color conventions they use. Other than the above, though, it’s up to you. The conventions we kinda sorta of adopted were:

  • Green for high power circuits.
  • Blue for switches (on/off type signals).
  • Purple for sensor values.

But I didn’t always adhere. The reality is that a lot of folks build the entire car’s wiring using only one color wire. It would mean more work in labeling and a lot of time with the multi meter during construction, but a lot of people do just that. Outside of these general conventions I just tried to cycle through the wire colors to make life a little easier when pinning out connectors.

It really is a personal preference at the end of the day. But, remember one thing- someone might have to fix or change it later. And, that someone is probably you if you are doing this yourself or if you are building this for a customer. Which means both a good plan (we did it!) and mostly adhering to that plan so that it matches expectations later are important (we’re working on that second bit!).


Beginning laying up wire on the car.

Because we didn’t trust ourselves completely just yet, we started to lay some of the wire out on the car to double check our measuring and routing. And when I say “we” I mean that Tom didn’t trust that I had done anything right, which was a fair worry I might add.

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