A kill switch is required by the rules. A tidy installation takes time and effort, but it’s rewarding and worthwhile.

This is a story about how to install a kill switch in a Miata, but it’s also something of an ode to two guys named Dave, who factored heavily into this job.

The first is Dave Wheeler, who was the owner of Advanced Autosports, a Spec Miata specialty shop in Beloit, Wisc. Wheeler unfortunately passed away in December 2018, but his shop lives on. Wheeler devised the kill switch kit I used for this story. In fact, I’ve used the same kit a few times. It’s the simplest way to add a kill switch to a Miata, which is what makes it so brilliant.

The other Dave was the founder of a company called Dave’s Custom Boats, DCB for short. In a previous life, I edited a high-performance boating magazine, and one of our jobs was performance testing and inspecting, typically, around 70 high-performance boats per year. We looked where most people never would, under compartments, in the bilge and behind the dash. High-performance boats are hand built, and you never knew what you might find behind a dash. It could either be a rat’s nest of wires or custom rigged perfection, which is what you found on every DCB.

Start by removing the positive cable from the battery. Clip off the terminal end because you’re going to install an eyelet on this cable to be connected to the kill switch.

That kind of rigging takes time and effort, and I always admired the workmanship that went into it, and when I do “rigging” work on my racecars, that second Dave is always high on my mind. I doubt he did the wiring work himself, but he hired someone who could do it to his standards, which were high. I wish I had photos to show you. Some of the work was breathtaking, and it’s clear to me that fastidious rigging enhances reliability.

I wouldn’t go so far as to call this kill-switch installation breathtaking. But by taking one’s time, using the Advanced Autosports kit, paying attention to the details, I think it will enhance reliability, something we all value in a racecar.

This is the connector for the backup lights, which is part of the harness attached to the positive battery cable. Disconnect it so you can reroute it.

What I learned from looking at all those boats is that cushion clamps, zip ties and a critical eye are key to building something that looks decent when you’re done. This job takes a couple of hours anyway. Why not make it three hours and turn it into something worth looking at?

And that brings us back to Dave Wheeler and the kill-switch kit that makes the job easier for people who don’t have an ASE certification in automotive electrical systems. It comes with everything needed to install the switch, and it works for all first- and second-generation Miatas. What drew me to it at first was that it didn’t involve cutting or splicing any of the factory harnesses. The instructions that come with the kit list two ways to do the job, the easy way and the way Dave did it. Dave’s way takes more time, but it’s worth the effort in terms of finished appearance and the reliability of the system.

Cut the tape off the harness behind the right rear wheel (remove the plastic splash guard first) to expose the wiring. Save the white plastic collar and the black plastic “router” for routing the negative cable.

To give a synopsis on how the system works, you’re essentially putting the kill switch between the battery and the starter and connecting the alternator output to the on-off function of the kill switch, which is what shuts the car off when you throw the switch. I’ve never had one fail tech at the required 2,000 rpm.

To get started, remove the positive cable from the battery, cut off the battery terminal and feed it through the hole the floor pan above the rear axle. When you do that, you’ll find it’s taped and loomed to the negative cable to the point where it grounds to a subframe. Remove the tape and the loom and separate the two cables. You’ll also have to move some thinner gauge wiring for the backup lights and one device in the evaporative emission system.

Get ready to do a lot of this. You’ll be separating the positive battery cable from its conduit and two other harnesses for the evaporative system and the backup lights. It’s messy.

With the positive and negative cables separated, you can remove the positive cable and the reverse-light, and emissions wiring from the power plant frame that connects the transmission to the differential. You will remove all the conduit and tape up to the transmission, then unwrap the wiring for the connectors to the transmission. This lets you separate the battery cable from the trans and evaporative system wiring. Remove the tape and conduit to the point where you can get the battery cable and small wires through the hole in the transmission tunnel where the evaporator case drain is.

Before you do that, take a minute to clean the positive battery cable of all grime and tape goo. I used Acetone, which takes no prisoners. I also wrapped the small wires in new conduit where it passes through the interior.

That little white plastic collar clips together in the middle so you can remove it from the factory harness and clip it back around the harness after you’ve installed the kill switch. This will protect the wiring from the hard steel edge in the floor pan.

This is where you learn the hole isn’t big enough to get the reverse light and evap wiring connectors through. You’ll need to enlarge it with a step drill to 1 inch. Put the small wiring connectors through before the battery cable. They won’t fit otherwise. I found out the hard way, of course.

Cut one side the evaporator case drain grommet and reinstall it in the hole around the wires to protect them from the hard edge of the hole in the transmission tunnel. You also might be able to push the connectors through the grommet, then reinstall it in the enlarged hole without having to cut it.

Pop the white plastic clips off the power plant frame so you can reroute the battery cable and harness inside the car.

Pull the cable and wiring through and route it along the rocker panel and up to the kill switch, which gets mounted on a plate on the roll cage about where the passenger’s shoulder would be. The theory is that the driver and a corner worker can reach it at that location.

You will have extra positive cable, so cut off the excess and crimp on the copper lug that comes with the kit. That attaches to one side of the kill switch.

Once you have the harness separated from the PPF, cut the tape that holds the white clips to the conduit. Pop off the two catches with a small screwdriver.

The Advanced kit comes with a new positive battery cable that leads down from the other terminal on the kill switch and is routed rearward to the battery in the trunk. It also comes with a length of red wire that goes from that post on the kill switch to the post on the alternator, the one attached with the 12 mm nut.

The existing connector on the post on the alternator, which is a white wire, gets separated from the other alternator connector so you can run it to the battery terminal on the starter.

The little details make a difference. Clean the battery cable before routing it inside the car. I used acetone to get all the tape goo off.

Separating the alternator wiring is, well, a PITA, but you have to separate them because one will go back to the starter and the other is the multiwire connector that plugs into the alternator. The factory loom is held on with electrical tape but there’s also a black plastic loom that needs to be cut off so the harness can be routed in different directions. It’s difficult, so take extra care not to damage the insulation on the wires. I used scissors rather than an open blade.

Use a step drill to enlarge the whole to 1 inch so you can get the connectors through.

You’ll need to cut off the eyelet that connected the white wire to the alternator and crimp or solder on the supplied eyelet that fits over the positive post on the starter.

That describes the essence of the job, but for the gritty details, refer to the photos and captions.

When you remove the small wires for the backup lights and evaporative canister, you push them through the hole in the transmission tunnel, then feed the battery cable through.
Before you start on the interior work, wash your hands so you don’t gum up the works. I like to wrap the small wires in conduit so they’re not flopping around.
I also like to install the plastic sill plates for a more finished look. For this installation, I drilled holes in the sill plates for zip ties to hold the cable and wires in place.
Leave the zip ties loose until the job is done. This way you can adjust the wiring to sit just right.
The small red wire attaches to the post with the supplied cable that leads back to the positive battery terminal. The red wire runs along the rocker into the engine compartment and to the post on the alternator.
Route the alternator wire through the same zip ties as the other wiring.
The alternator wire can go through one of the open holes in the firewall normally occupied by A/C tubing.
Mazda supplies plugs for the holes in the firewall. I drilled out one and used it as a grommet for the alternator wire, which works great.
Take the factory alternator wire from the post and remove the multiwire harness and connector plugged into the back and separate them from each other.
Here you can see the white wire that the factory routes to the alternator post and the smaller wires for the multiwire connector that plugs in. The black plastic loom is a bugger to cut, so be careful you don’t slice the wire insulators or your fingers!
This eyelet isn’t factory, but it used to attach to the alternator. Cut off the eyelet and install the supplied eyelet that will attach to the positive post on the starter.
This is the red wire from the kill switch now connected to the alternator.
Crimp on or solder the new eyelet on the white wire that used to attach to the post on the alternator.
Shrink wrap on the connector is always a good idea.
The white wire attaches to the main terminal on the starter solenoid. Once you have the two wires connected, you can go back inside the car.
Wash your hands if they’re dirty. Cut the factory positive battery cable to the right length to connect to the kill switch and crimp on the supplied eyelet.
The cable and red wire on the right connect to the positive terminal on the battery and the post on the alternator, respectively. The one on the left is the old positive battery cable that now spans between the kill switch and the starter.
The cables pass by some hard edges, so I like to wrap them with heater hose and fasten them with zip ties so they don’t rub through the insulation and short out.
The small conduit at the bottom wraps the evap and backup light wiring. The red wire leads to the alternator and the black cable goes to the starter. Miatas have some studs and bolt holes in the passenger-side floor you can use to fasten things down tight with cushion clamps.
Here you can see the zip ties cinched and snipped to hold all the wires in place en route to the kill switch.
The supplied cable connects to the positive battery terminal. A zip tie holds the cable to the roll cage.
Image courtesy of Eric Green

1 COMMENT

  1. This is a very nice, complete, well documented description of the “Dave Wheeler Single Pole Kill Switch Method”. To be sure, this is a very popular method, but I think it is fundamentally flawed, in that it connects the alternator directly to the positive terminal of the battery. The idea of a kill switch is to cut off all power in the event of a crash. I can envision a scenario where there is crash damage, and there is a hot wire in the engine compartment, even with the kill switch “off”! It also suffers from the (minor) drawback that, with the switch “off”, there is a slow current drain through the alternator regulator circuit, which will drain the battery. The solution is not to let the car sit with the switch “off”.

    I think a double pole switch is inherently safer: one pole separates the battery from all other circuits (except fire suppression), and a second pole to kill the engine, by interrupting the power to something important, such as the fuel injectors. The second pole is necessary because, with the engine running, the alternator is functioning as a “second battery”, supplying voltage to the systems that keep the engine running. Disconnecting the battery, once the engine is running, will not stop the engine, unless the alternator output is also interrupted.

    My $0.02

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