The following is the step by step procedure on how to wire a kill switch for a Spec E9X BMW by Nick Fuller.
-There are 3 main cables running from the OEM battery distribution box, two of which run underneath the car and one through the cabin along the passenger side rocker and over the rear wheel well. The cabin wire runs to the passenger side footwell fuse box. The smaller wire under the car runs to power the DME in the E-box located in the engine bay, and the larger wire to another terminal in the E-box and then to the starter and alternator. The smaller DME wire and the passenger footwell fuse box wires will be the ones used to kill power.
-A 12v solenoid is typically open until a 12v signal is sent to the S post. We are sourcing our 12v signal direct from the battery cable, and running it through two kill switches in series which are mounted on opposite sides of the dashboard.
-A new 12” (or less) battery cable will need to be made to run from the battery distribution box post where the footwell fuse cable was previously attached, to the solenoid input post. The DME power cable can be removed from the distribution box at the battery, and together with the footwell fuse cable connected to the output side of the solenoid.
-Now we should have a solenoid with input (new cable to the battery) and an output (DME power and Footwell Fuse Box).
-14 -16ga wire can be used from the new battery cable side of the solenoid (for signal) and routed through the cabin to the kill switch or switches in series and then back to the S post on the solenoid.
-Most solenoids are grounded through their body, so it is important to get a nice clean attachment to the chassis, otherwise a new ground wire can be run. In the images attached we ran a new wire to the battery chassis ground.
-A basic 12v automotive grade starter solenoid is shown. This type isn’t designed for continuous use and it is to be replaced with the part : WHITE-RODGERS DC Power Solenoid: Water Resistant, 200, 12 V DC Coil Volt DC, SPNO Continuous, 586-902
-Switches used are HELLA 002843011 2843 Series 100A Rating Battery Master Switch