Project Lexus SC300 Road Racer: Part 6 – Practical Panels

Project Lexus SC300 Road Racer: Part 6 – Practical Panels

by Erik Jacobs

When we left our project SC300 we had just installed the XS Power S925 battery. As we said during the parts selection, the battery is a critical component to ensuring solid starting and good electronic component performance overall. For safety reasons, every race car requires some kind of cut-off mechanism to make certain the car dies when the lever is thrown. This helps completely kill the car in an emergency. In many cases, the easiest way to accomplish this is to ensure that battery stops being a part of the party at the right time.

When you start adding up all of the current (amperage) that flows through all of the electronics in a race car, and remember that all of it essentially comes from the battery, you’ll quickly realize that you need quite a big, heavy-duty switch to be able to just cut off this current. And, in fact, many race cars employ just that — a very large heavy-duty rotary switch that has big heavy battery cables running to it to help disconnect things.

Some of you might be thinking “what about the alternator?” That's a very good thought. In an over-simplification, when the engine is spinning, the alternator is generating voltage and current to recharge the car’s battery. It is part of the charging circuit. Even without a battery, if the engine is spinning, the alternator can generate sufficient voltage and current to power the vehicle. If you started your car and disconnected the battery, it could likely keep running depending on how the car was wired. Many poorly wired race cars have a battery cut off that doesn’t isolate the car’s electrical system from the alternator and the battery. This results in a car that doesn’t go off when the kill switch is thrown. That, my friends, is no good. How can we ensure that the car is properly killed? Read on.

Since we are going high-end on this project, we decided to use something a little bit fancier than a big honkin’ switch: a contactor. According to the universal source of knowledge, Wikipedia, “A contactor is an electrically controlled switch used for switching an electrical power circuit, similar to a relay except with higher current ratings. A contactor is controlled by a circuit which has a much lower power level than the switched circuit.”

This sounds like the perfect thing for our needs. We want to be able to use a very low-power switch to turn a big heavy-duty circuit on and off. If you look around for race car cut off switches, you may see them (somewhat improperly) referred to as solenoids. While the contactor does have a coil, and a relay uses electromagnetism to move a contact similar to how a solenoid can convert electrical energy into motion, it’s not really a solenoid in the classical sense.

So, where does one find such a contactor? This is where some time and research pays off. In many cases every-day kinds of things have “race car” slapped on them and their price is doubled. A bunch of spelunking on the internet led me to find the Trombetta Defender family of contactors. This is the exact same unit that some racing equipment places sell for twice the price. It is rated for harsh conditions, has a nice Deutsch form-factor environmentally sealed connector for its controls, and will support switching up to 200 amps, which should be more than enough for our race car.


So, what do you do once you have your contactor? You drill giant holes in your car, of course. And, as Tom says, “there’s nothing I like more than drilling big holes in perfectly good cars.” Thanks, Tom. At least there was very little perfectly good about this car before we started drilling holes in it. Anyway!
The contactor ideally is located close to the battery, hence installing it on the transmission tunnel. Our good friend the nutsert (or rivnut, depending on how you want to describe it) comes to the rescue and provides a convenient threaded hole for a bolt to go through.
The final product is clean and affords good clearance to the transmission itself. It also means you don’t have to find your way underneath the car to hold a nut when installing the contactor, and also is very unlikely to back out since it’s affixed to the sheet metal like a rivet.
Here’s the result. The positive battery terminal will attach with 0-gauge XS Power battery cable to the contactor, and then the switched side of the contactor will continue on to supply the rest of the car. One side of the Deutsch form-factor connector will attach to battery positive, and through the kill switch circuit (detailed later in the series) will connect to ground, which will energize the contactor.

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