A View to a Kill (Switch)

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Update by Scott Lear to the Mini Cooper S Club Racer project car
Jun 22, 2009

The MINI instantly sputtered and died, just as intended.

Every sanctioning body that we can think of requires the presence of a master disconnect switch (often called a kill switch) on any race car competing under their rule set. The kill switch has a simple function: It turns off the car. While this might seem like a redundant feature in a car that still has its factory keyed ignition, the kill switch has one more important function: It completely disconnects the battery from its various target systems as well.

With the hood open, we decided to take a short detour before tackling the kill switch; our car’s hood pin posts were simply gross. They were welded into place well enough, but for whatever reason they never got a coat of paint, so the bare metal had lots of ugly surface rust. A wire wheel on a Dremel made short work of the rust. Then we masked off the area and blasted it with a couple coats of leftover Rust-Oleum from our cage painting.

Our parts list for the kill switch job was pretty short. The main component was a QuickCar Master Disconnect Switch, which we bought for $34 at Daytona’s Action Performance Speed and Custom. This switch is designed for use in race cars, and has four posts on the back, a pair of big posts for the large battery cables, and a pair of smaller posts for the ignition. We also went through some zip ties, a little bit of solder, and approximately 20 inches of 12-gauge wire.

The first step in wiring up a kill switch was to figure out exactly where the various battery connection cables were routed. In our 2005 MINI Cooper S, this turned out to be a multistep process, as the rear-mounted battery has two large positive cables: one for the starter and the other for the underhood fuse block. The fuse block cable is routed along the driver’s side doorsill in the interior of the car. The starter cable follows a similar path, but once it leaves the battery it follows the sill on the outside of the chassis.

Our MINI came to us with a simple disconnect switch already installed inline with the grounding cable from the battery. However, since it wouldn’t turn off the car once it was running—the alternator provides power when the engine is spinning, so the battery isn’t necessary—it wasn’t the right kind of switch for the job. Also, because it was mounted inside a latched hatch, neither the driver nor any corner workers could get to the switch in a moment of need. It was totally useless, so we removed it.

We planned to mount our proper four-pole kill switch in the circular hole where the leftmost dashboard vent was situated, and we needed both of the positive cables to meet up at the same kill switch. We decided to re-route the outside cable along the same path as the interior cable; this ended up giving us a couple feet of extra cable, too. Disconnecting the exterior cable was a simple matter of clipping a bunch of wire ties and undoing some plastic screws to gain access to the left-front fender area behind the plastic shield. In the back, we had to temporarily remove a few heat shields near the battery well and exhaust cans.

With the fatter of the two cables free of its position, we rerouted it on the inside of the car and used zip-ties to hold it in place. We snuck the engine bay side of the cable through an available grommet in the firewall and reconnected it to the jump-start post that ultimately leads to the starter.

We spent some time freeing the wiring loom around the interior cable until we had enough free play for both of our cables to reach our intended kill switch location. Then, we cut both the cables and used two pairs of crimp-style battery cable lugs so we could bolt our freshly cut cables to the kill switch.

We knew that the battery was fully disconnected from the car’s electrical systems at this point, so now we had to find a way to cut the ignition. We put the switch inline with our rerouted cables to confirm that the switch was indeed cutting battery power, and then broke out our digital multimeter to do some exploring in the neighborhood of the wiring harness at the back end of the ignition cylinder.

NASA racer Rich Peterson, who campaigns a 2004 MINI Cooper S in the US Touring Car Challenge series, found our query post on northamericanmotoring.com and suggested we go hunting for switched 12v power in one of the two larger-diameter red wires coming from the harness. We actually found switched power in both of the wires, so we did like any proper bomb-diffusing action hero would and simply guessed which wire to cut. We guessed right, but the worst-case scenario would have involved splicing a cut wire and snipping the other one. However, it’s possible that cutting either red wire would have had the same end result.

We soldered in an extra length of wire to each end of our cut ignition wire and routed them to the smaller poles on the kill switch. With the switch in the on position, we fired the MINI right up, let it idle for about 30 seconds, and then flipped the kill switch to the off position. The MINI instantly sputtered and died, just as intended. Better still, it started again after we put the switch back to the on position and cranked the key.

To give our kill switch a functional and reasonably attractive home, we removed the dash element surrounding the wheel and performed a bit of surgery on the area around the leftmost vent. The switch is designed to mount to a hole drilled in flat steel, so we cut an approximately 4x4-inch square of 16-gauge steel and drilled the appropriate-sized hole in the center. Using a Dremel with a cut-off wheel, we careful cut several slits into the plastic structure on the back side of the vent housing so that the metal plate could slide in snugly.

Because we needed to mount a metal plate firmly to a plastic housing, we decided to break out our good friend JB Weld. We mixed a good dose of this magical binary epoxy and smeared it all around the front and back of the plate, locking it firmly into place; as the JB Weld dried, we situated the on/off plate so that it would also be Welded to the assembly. Then we masked off the surrounding areas and spray painted the whole lot black.

With our kill switch in place and functioning properly, it’s time to get our personal safety gear properly bolted up.

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