I've been doing some EOC and also turning off the engine at long stops, and until now, have been doing it with the key. The drawbacks to this are twofold:
- There's always the possibility of overshooting the ACC position, thus locking the steering column.
- If I want to keep the engine off with the clutch engaged, I have to leave the key in the ACC position. That means no instruments (e.g. speedometer) and no air bags.
A kill switch removes the need to use the key, and allows me to cut off fuel to the engine even when the clutch is engaged without compromising safety.
In my car, there's a single 10A fuse that provides power to the fuel injectors, and only the fuel injectors, so this seemed like an ideal place to wire in a kill switch. I did this using a pair of "Add-a-Circuit" modules, and a high-quality relay with a set of normally-closed (NC) contacts. Here's the schematic/wiring diagram:
The grey boxes with dashed edges are each one "Add-a-Circuit" module. The one shown in lighter grey is plugged into the fuse box in place of the fuel injector (INJ) fuse. The darker grey one is plugged into the primary (A) circuit part of the lighter one, in the opposite direction. 10A fuses are installed into the secondary (B) circuits of both. You can see in the diagram how the current to the injectors flows through both 10A fuses, the NC relay contacts, and the red wires (which are pre-installed on the Add-a-Circuits). Two fuses are of course redundant, and F2 could be replaced with a 20A or 30A unit, since its only purpose is to be a jumper.
A separate 0.5A fuse (shown as 1A in the diagram) provides power to one side of the relay coil. The small fuse is there so if there's a short in the coil circuit (or the wire to terminal 85 comes off and touches something grounded), that fuse will blow instead of F1, which would cut off power to the injectors.
The other side of the coil is connected to a long wire (green) which goes to a pushbutton switch on my centre console beside the shifter. Pushing this switch energizes the relay, opening the NC contacts, and cutting off power to the fuel injectors.
I gave careful thought to the design of the switch so that in the event that it fails, it's likely to fail in a way that the engine can run. If the relay fails to work, the cut-off just won't work. If the coil shorts out, F3 will blow instead of F1. There is still the possibility of course of the relay itself getting stuck open, or double the possibility of a nuisance blow because both F1 and F2 are in the circuit. However, if anything does fail, I just need to pop the hood, open the fuse box, pull the Add-a-Circuit out of the INJ fuse socket, and put a fuse back in its place.