MGA Inertial Shut Off Kill-Switch

May/June 2024 V49/N05

Often, after hearing about a car accident, I ponder on ways that I might improve the safety of my MGA. After ensuring all five tires are properly inflated, the brakes are in good working order, the lighting works as designed, and carrying a fire extinguisher, the best safety improvements I have yet to install are three-point seat belts and a roll bar…the latter being a non-starter. However, have you ever considered the safety of your MGA after an accident? Specifically, how to automatically shut off the engine and fuel pump in the event of a collision or rollover?

There are exceptions, but most of today’s petrol-powered cars and trucks use an electric fuel pump to provide fuel to their fuel-injected engines. Modern petrol-fueled vehicles also have an inertial force switch installed that opens the fuel pump and engine ignition circuits during collision or rollover. Without question, securing the fuel pump and engine becomes critical in minimizing the possibility of a post-accident fire; especially if the occupant(s) is trapped or unconscious. In my way of thinking, our MGAs should be afforded the same type of post-collision/rollover fire protection.

In the following paragraphs and surrounding pictures and diagrams, I will attempt to provide you with an understanding of the steps that I took to install a collision/rollover fuel pump and ignition kill-switch in my MGA. I am not endorsing that you install a kill-switch but socializing an idea for improving the safety of our beloved MGAs. Also, I don’t recommend this installation for those that never crawl under their dash or are uncomfortable cutting-n-splicing and installing wire connectors. Lastly, even though a kill-switch may be installed by simply cutting, twisting and e-taping wires, I would not consider doing this install without using properly shielded bullet or blade connectors.

Purchasing a kill-switch is relatively easy. If you enter “Inertia Fuel Pump Cutoff Switch” into any browser, you will find many of the same switches (picture 1) ranging in price from $10-$70 USD. Unfortunately, I couldn’t determine the wire size used on any of the available kill-switches, so I purchased a used Ford Taurus switch on eBay, and it arrived with rather stubby 14 AWG lead wires. While researching switches, I also discovered electronic switches have an “Ingress Protection Rating” (IP). The IP two-digit rating system basically rates the switch’s ability to protect against dust and water intrusion. The first number refers to the protection against solids and is rated on a scale from 0 (no protection) to 6 (no dust ingress). The second number rates the enclosure’s protection against liquids from 0 (no protection) to 8 (long periods of immersion under pressure). For me, I was comfortable with a kill-switch rated at 54. If interested, type “Ingress Protection Rating for electric switches” into a browser and check it out.

Once you have decided-on inertial cut-off switch in-hand, I recommend you test it thoroughly. If you need to learn a bit more about inertia kill-switch functionality, type “Inertia Safety Switch Explained #INERSW” into Google or YouTube Search and watch the video.

Please be advised that the following suggested ‘steps’ describe my personal requirements for testing and installing an inertial kill-switch. I am fully cognizant that there are a number of easier and harder ways to accomplish the installation.

Step One: Test the switch mechanically by pressing down on the center of the rubber cap to ensure the metal ball underneath is seated (closedcircuit). There is tactile feedback when the metal ball seats…hard to explain but very intuitive. Then shake the switch about in a fashion you might expect under normal driving conditions. You should not feel or hear the metal ball become dislodged. Next, with the switch still in closed-circuit position (ball seated), give the switch a sharp rap with your hand. You should feel the metal ball become dislodged (open circuit) and hear it rattle about under the cap. The next test follows the same procedures but incorporates an electrical circuit. After reseating the metal ball (closed-circuit), I used my continuity tester attached to two leads with alligator clips to conduct the closed/open circuit shake tests.

After connecting my continuity tester leads to the two leads coming from the kill switch (closed-circuit), I heard continuity tone. Then, I wiggled the switch about checking that the tone remained uninterrupted. After confirming a good closed-circuit test, I gave the switch a sharp rap on the side and my continuity tester became quiet. I reseated the metal ball, heard the tone again, and then repeated the electrical test several times with the same results. Good-to-go! All kill-switch installation and testing may be accomplished using a continuity tester and two 4’ leads with alligator clips. Hence, I recommend you disconnect your battery before starting the installation.

Step Two: Where and how do you mount the kill switch? It is of vital importance to remember that the kill-switch must be mounted vertically with the rubber reset cup at the top (picture 2 & 3). Since I don’t have a radio installed, I elected to mount my kill switch using the radio blanking plate mounting pegs and tensioned retaining straps (pictures 2 & 3). I fabricated a small mounting strap to attach the switch to the back of the radio blanking plate (pictures 2 & 4). Since the switch is fairly light, any bolts that fit through the switch’s mounting holes should work fine (i.e. 10-32 x 3/4”). Please notice in picture four (4) that I cut slots in the switch mounting strap. I did so because it allowed me to slide the switch strap down and onto the blanking plate pegs and sandwich it between the blanking plate’s tensioned mounting straps and its two push-on fasteners (picture 3).

During my installation testing, I attached two 4’ lead wires to each end of my audible continuity tester for hands-free use and easier access to confined areas. Using a continuity tester to determine which white wire to install the kill-switch is fairly straight forward.

Step Three: Establish yourself under the dash of your MGA. Locate the back of the ignition switch and remove only one of the blade-connected white wires from the ignition switch. Connect one of the continuity tester lead wires to the disconnected white wire and connect the other lead wire to the Fuse Block “A3” screw (picture 5). If you have continuity, you have identified the wire that provides power to the fuel pump and ignition circuits. If you don’t have continuity, you are likely holding the white wire that goes to the ignition lamp. Simply check continuity from the other white wire, still connected to the ignition switch, to “A3” before reconnecting the wire you removed. Once continuity is confirmed, mark the white wire with continuity to “A3”.

Step Four: If not already completed, attach your shielded wire connectors to the two wires coming from the kill-switch. It’s helpful to leave as much wire as possible protruding from the kill-switch so you don’t have to splice-in additional white wire to reach the kill-switch once it’s mounted. You can mount a kill-switch where I did or, if you have a radio, identify another location, and mount it. The kill-switch can literally be mounted anywhere on the vehicle, but I preferred behind the dash for a quick reset if ever needed; but it has never been needed. Next, identify a good location on the fuel pump and ignition circuit wire
that will easily reach the kill-switch connectors, and mark it. Then cut the white wire and install the appropriate shielded connectors. Plug the white wire connectors into the kill-switch connectors (picture 6) and ensure the kill-switch is in the closed-circuit position (ball seated). It’s always a good idea to take a few moments to look over your efforts and ensure all the connectors and wires are where they should be.

Final Step: Now it’s time to reconnect the battery. Once power is restored to the battery circuit, turn the ignition on and confirm the fuel pump starts running. Then, rap on the radio blanking plate or the side of the kill-switch to confirm the kill-switch will open the fuel pump circuit causing the fuel pump to stop running. Reset the kill-switch and listen for the fuel pump to start working again. Next, start the engine and let it warm up a bit. Then, as before, rap on the dash or the side of the kill-switch and the engine should stop running and the fuel pump should be silent. Turn the key to the off position, reset the kill- switch, turn the ignition switch back on, listen for the fuel pump, and then restart the engine. If all works as it should, you have successfully installed an inertial kill-switch. Hopefully, you never find yourself in the circumstances that require its use.

Note: I retest the kill-switch every time I change the oil; to date, it has passed all tests. Please be advised that the white wire powering the fuel pump and ignition circuits also powers the fused circuits that power the heater motor, windshield wipers, fuel gauge, turn signals and stop lamp switch

Written by John Casey, January 2024.