Project Lexus SC300 Road Racer: Part 7 – Sealing the Things


Tom is removing the flange from the DT connector.

The flanged DT connector has a retention clip on it, but also has four fastener holes. We filed off a little of the retention clip in order to keep the through-panel hole smaller. The DT connector will simply be bolted to the panel on the cabin side. Then the panel will be bolted to the rear bulkhead. The trunk harness will be separate and have a plug at the end, which will be inserted into this flange-mounted connector from inside the trunk.

Why a separate harness inside the trunk? In theory, this makes it easier to have spare sub-harnesses in case of issues. Instead of trying to fix or troubleshoot a wiring problem at the track, just rip out a sub harness and install a replacement. If something gets damaged in a collision, rip and replace. It also makes installation and removal much easier.


Original holes in the rear bulkhead are re-purposed.

The factory hole that goes through the rear bulkhead happens to have a recessed opening, and the panel was fabricated to nestle nicely into it. Cleco pins (sounds like Atari/Coleco) are used to hold the panel in place while drilling holes. This is easier and safer than trying to climb over the roll cage, hold the panel, and drill it, without drilling holes in yourself. Epoxy will also be used before riveting the panel into place to ensure an environment-proof seal.

There is a similar hole on the other side of the rear bulkhead that has no need for a connector to go through it. Fortunately for us, Toyota made these two openings in the bulkhead identical, although mirrored. We can simply trace this panel's outer dimensions and mounting holes onto another piece of metal and cut and trim to size. That panel will simply be epoxied into place.

When the roll cage was installed, the other major holes in the rear bulkhead were already sealed, so that takes care of the back of the cabin. Now, on to the front firewall!


Tom measures one of the openings in the firewall for a panel.

The front firewall has many, many openings where factory components went through. Some will be reused, and some will be blocked off. In this case, the uppermost passenger side hole will be repurposed for three circular milspec connectors- a 32-pin and two 3-pin.

The 32-pin connector will handle all of the power circuits, some alternator signal wires, and other miscellaneous signals and sensors. One 3-pin connector will handle the front two wheel speed sensors. The final 3-pin connector will handle the remote kill switch that will be mounted on the cowl on the driver side.

You may be thinking that this seems like serious hardware to send some wires through the firewall. The engine bay is an extremely harsh environment, and the investment in milspec connectors is a wise one. If you consider that a wiring failure could sideline you for a full race weekend, and that could result in having wasted well over $1,000 in towing, fuel, hotel and entry costs, spending a few extra dollars on circular milspec environmentally sealed connectors seems like pretty decent insurance.

You might also be wondering why wiring running to the driver side of the vehicle is being routed through the passenger side firewall. On many modern cars, firewall real estate is somewhat limited. On the driver side of the vehicle you have the steering, brakes, clutch and throttle (on a cable car) all going through the firewall in more or less the same place. It doesn’t leave much room for other things. From a packaging perspective, the passenger side becomes the reasonable choice in spite of adding a little extra wire length to harnesses.

You may be tempted to use factory holes that go through the firewall and into the fender area, and then back through the frame rails (more or less) and into the engine bay. While it seems like an easy decision, don’t let that temptation get you. The fender area of a car is one of the least protected and most vulnerable to on-track incidents. Fenders are non-structural components. A minor collision with another vehicle that may not seem like it even did much damage could result in a harness being crushed, ending your weekend prematurely. In a worst-case scenario, it could result in a short circuit that causes a fire.

When you are trying to fabricate round tabs, sockets make for excellent stencils. You could buy a circle stencil, but, like Alton Brown likes to say, “that’s a uni-tasker”.
For whatever reason, the hole in the firewall is not square. It has a single differently rounded edge. You have to wonder what series of engineering decisions happened within Toyota that led to this hole not being square or perfectly circular. Strength? Manufacturing? Who knows.

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