E30 Kill Switch
Quick background on myself – I started attending track days in 2012 with my E46 M3 and eventually purchased an E30 with the goal of building a Spec E30 race car. As part of the build process a number of safety items are required, including a kill switch. Working at a shop like Electron Speed, I couldn’t settle for a basic mechanical kill switch. I knew I had to reach for something a little more robust, something a real motorsports outfit might install in their race car. Using methods learned around the Electron Speed shop, I was able to implement a safe and effective kill switch.
The obvious goal of any kill switch is to kill the engine and disconnect the battery. There are a number of ways to accomplish this goal with some methods having advantages over others. Most DIY’ers will use a mechanical kill switch due to its price and simplicity. With a little knowledge a safer, lighter, and more reliable kill switch can be created.
The first issue with a mechanical kill switch is that it must be located in a position that a safety worker can access it. This typically means it’s near the driver or hood area. Consequently, the large battery cable, which in the case of my E30 was an enormous 0 AWG, will have to be run through the switch wherever it happens to be located. When the switch is flipped and the car is killed, you’ll still have a large length of wire running to the switch that remains powered, which in the event of a crash can be a bad thing.
Our particular method solves this issue by using a solenoid in the trunk placed close to the battery. When the kill switch is flipped, the only powered portion of the battery cable is the ~6in from the battery to the solenoid.
The second issue with a mechanical switch is that you are limited in your mounting options due to the large gauge of wire required. By using the solenoid to handle the amperage, we can use cheaper and lighter weight 22 AWG wire which can be located virtually anywhere.
So you’ve killed the battery by severing the connection with the solenoid, but you still need to interrupt the alternator power in order to stop the engine. When you place the ignition in run, you are supplying a 12v “run” signal to the DME. This means when you stop supplying 12v volts to the DME, the DME thinks the ignition has been turned off. An elegant way to accomplish this is to supply a 12v run signal in a way that our kill switch will interrupt power to the DME. Switching off the 12v supply to the DME kills the engine and flips the solenoid to cut the connection to the battery.
You may have also noticed that we added a 5A breaker. In our setup, we have the battery positive almost directly connected to the kill switches and going toward the ECU. If any part of that line were to be shorted to ground, it will pop the breaker. If the breaker were not in place and a short occurred, the wires would be melted or the operation of the kill relay would become intermittent.