The vehicle already has a starter motor interlock (starter cut relay). Adding a second hidden mechanical switch into that circuit is relatively easy which would give you the same functionality as having the clutch interlock switch button drop out. It is easy for a knowledgeable thief to by-pass this by jumping the starter motor directly.
If you want a second level of interlock and you want to have the dash warning lights operational during power up when the security interlock is enabled the ECU has to power up normally. That leaves you with two options, separately disabling the ignition or the fuel supply. Personally, I like the idea of an interlock in the fuel supply because cranking the engine until the cows come home with a dead ignition system just fills the engine with unburnt gasoline which is much worse than any risk associated with operating the injectors dry (that is not a problem that I have ever heard of).
The fuel pump intercept is also relatively easy to do. The main FI relay is located inside the cabin on the back firewall and is moderately easy to access. The main FI relay is two relays in one. One relay powers up the ECU and the other relay powers up the fuel pump. This is the diagram for the main relay circuit.
View attachment 173062
The fuel pump portion of the main relay powers up after it gets a ground enable signal from the ECU on the green/black wire. If you put a switch in this circuit you can permanently disable the ground enable circuit preventing the fuel pump from running even though the ECU has powered up. I like the intercept in this circuit because it is low current ( probably less than 0.050 amps). You could also put the switch in the main supply to the fuel pump (black/yellow wire); but, this is a high current circuit (6 -10 amps) and requires a much more robust switch plus you are adding additional wiring (and resistance) into the fuel pump supply which is never a good thing.
Note that the relay in the diagram confusingly called the fuel pump relay is not the fuel pump control relay. The fuel pump relay is really the fuel pump resistor by-pass relay. Intercepting the ground on this relay with a switch will not disable the fuel pump and could cause problems with high load engine operation if it failed open.
If you want to add additional sophistication you could add Drew's TK-103a into control the circuit so that you can shut off the fuel pump.
Thanks for the info
@Old Guy &
@drew, and especially the wiring diagram. I'm starting to understand how the "main relay" (or should it be main relays?) works in order to add a kill switch to stop fuel delivery, but the fuel pump relay with two supplies to the coil is confusing.
So, how does the "main relay" work? I can see it contains an ECU relay and a fuel pump relay. (As you noted, the relay labelled fuel pump relay should actually be labeled fuel pump resistor bypass relay.) Since the main relay has multiple same-coloured wires, I'll call them, starting clockwise from the top left: 1-YEL/BLU, 2-BLK/YEL, 3-BLK/YEL, 4-BLK/WHT, 5-YEL/BLK, 6-GRN/BLK, 7-BLK/YEL, 8-BLK.
This is the issue: I don't want just simple on/off switch that I need to remember to turn off when I leave the car. I want to use a relay that is activated with a momentary contact but that shuts off each time the ignition is turned off. (Can do this with a diode from the switched lead of the relay back to the coil, which then keeps the relay activated after the momentary switch is released until power to the relay is terminated by shutting off the car. I'll probably use a small PLC controlled relay so I can disable it when visiting the dealer, but this can only supply ground or hot based on supply, not connect the black/green wire so I'd need another relay. See timers.shop) I'm thinking of installing it inside the main relay.
As a result, if I interrupt the fuel pump with my PLC relay, the fuel pump 12V will power my relay. I have to be sure that that supply won't get interupted by something else (like the ignition/fuel getting cut by the rev-limiter) which I suspect could be the case for the main cabin harness running to the fuel pump. Thus, the rev-limiter cutout won't reactivate the fuel pump until I press the secret switch again. So interrupt prior to "main relay"?
Guessing: 2-BLK/YEL (switched on by the ignition key?), activates the PGM/FI relay coil, providing voltage from (always hot?) 1-YEL/BLUE to 5-YEL/BLK which supplies the PGM FI ECU. I assume that the other side of that relay coil 8-BLK goes to a fixed ground which also seems to ground the ECU (and the AT controller)? Internally, the 5-YEL/BLK also powers the coil of the fuel pump relay although as you noted the ECU controls that relay by controlling whether 6-GRN/BLK is grounded. Interestingly, the Fuel pump relay coil can also receive voltage from the 4-BLK/WHT which I can't figure out, or BLK/WHT from the ECU, which I also can't figure out. (The ECU grounds the red wire of the fuel pump resistor bypass relay to raise the voltage to the fuel pump - I agree I definitely don't want to affect that and potentially overpower or underpower the fuel pump.)
Does the ECU rev limiter "unground" the 6-GRN/BLK to cut power to the fuel pump as well as stopping the fuel injectors via their individual ground controls and presumably the ignition too? That seems to be the only reason to not just have the 6-GRN/BLK not just go straight to ground via 8-BLK. Or is it monitoring the fuel pump relay voltage?
I ask because, in that case, I need to interrupt 3-BLK/YEL, not 7-BLK/YEL, because if the ECU cuts power to the fuel pump this way and I've interrupted 7-BLK/YEL, my kill switch relay will also shut off and have to be reactivated each time the rev limiter is activated.
I was considering interrupting 1-YEL/BLU, but I don't think that would work if either a) it's always hot so won't reset the relay when the ignition is shut off, or b) the fuel pump relay (and the ECU) still be powered by the WHT/BLK.
Thanks for any insight,