Adding injector cutoff to a Scion xA

[Niner]

This directly applies to the 04/05 xA, but may work similarly with the '06 or gen 1 xB.

The fuse that protects the injectors in the xA also protects a lot of other things that I don't want to turn off when I kill the engine for EOC. Luckily, I found a very good wiring manual for the xA that traces the path of the wire from the ignition switch. There was a good location to find the wire I needed in a harness plug directly behind the glovebox. I wanted to just run a normally-closed switch across this wire, but couldn't find something rated high enough for my tastes at the local Radio Shack, so I went with a relay.

Button on front of shifter - one end goes to ground, one to the relay:


Behind the glovebox of the xA:


The appropriate connection - the injector power line is the black wire with the red stripe on the far right:


My small relay board - the always-hot side of the injector power goes on the bottom screw terminal, otherwise the relay will oscillate:


And... the day I installed this and used it, I got a CEL. My first guess is that it senses the injectors "failing", although the CEL was gone this morning and I did use EOC once. It's possible I underrated the relay I needed, so if the CEL comes back I'm going to get it decoded at the auto parts store. In case of relay failure, I have a crimping tool and some butt connectors in my car anyways.

Hopefully this helps someone else.

[Niner]

And just like that, the CEL went off. I drove to & from work one day after posting this, and the next time I was in the car it was off. I've done plenty of EOC using the switch. Maybe it had to adjust to a slightly higher resistance (due to the "switch" in the relay)? Or the relay had to be "broken in" to get good contacts?

Works well in general, except for the one time I bumped it sitting at a stoplight with the engine warmed up. I idle at 500 RPM, so from there it's only a short drop to dead. Under normal speeds, tapping it just causes a little stutter in the engine.

[BLSTIC]

It may have been because the ECU periodically measures the resistance across the injectors to see if they are electrically functioning. It certainly does this on other cars...

The technically correct method would be to switch in a resistor for each injector (or bank of injectors) when you want to do this (easily achieved with the right relay). However your method appears to work fine.

[SentraSE-R]

Niner, any updates on the CEL issue? Specifications for the relay? TIA.

[SentraSE-R]

I built my own kill switch following Niner's instructions, with some modifications and caveats. The wiring diagram for my '06 xB also shows a common red/black wire for my fuel injectors, so I started my build with the confidence that Niner had done my homework for me..

There are three black and red wires in the wiring bundle behind the glovebox. I didn't pay attention to detail, and cut the wrong wire the first time, and the second. Finally I paid attention to detail, and saw the wire Niner used is the one attached to the connector plug in his photo. I cut it, and the engine wouldn't run - confirming I'd located the correct wire.

I found a good relay web page, and designed my own wiring setup. I bought a Radio Shack 12V 10 amp single pole double throw relay and a 1N4004 switching rectifier, as well as a momentary pushbutton switch (normally open).

I ran +12V DC from the center pole of the cigarette lighter (a handy fused circuit) to one side of the pushbutton switch, and the other side to one of the contacts of the relay trigger circuit. I ran a ground wire from the side contact of the cigarette lighter to the other contact of the relay trigger circuit.

I wired my rectifier diode in parallel with the trigger circuit contacts, with the striped end of the diode to the positive wire coming from my pushbutton switch. This prevents inductive kickback or back EMF, from damaging the relay's contacts.

I wired the two ends of my cut common fuel injector red/black wire to the NC (normally closed) high voltage contacts of the relay. This is pretty fail-safe. When other parts of my circuit fail, the fuel injectors run normally.

While I was into the fuel injector circuit, I wired in a toggle switch for theft prevention. When I switch the toggle to open the circuit, the fuel injectors are disabled. Toggle the switch closed, and the car runs normally.

Push the pushbutton momentary switch for a second or two with the engine running, and the trigger circuit activates the relay's electromagnet, moving the contacts from the NC position to the NO (normally open) position, killing the engine. It does this without rebooting the ECU, traction control, airbags, and vehicle stability control systems.

[SentraSE-R]

The new kill switch is working wonderfully. Using the key caused multiple misfire trouble codes P0300, P0301, P0302, P0303, P0304 when I turned the key on the first time each day . I theorized it's because the ECU saw 0 engine speed, but various vehicle speeds. Anyway, I haven't seen the problem since I installed the kill switch.

Now, instead of fiddling with the key, I press my button and a second later, I'm in a silent glide.