DIY engine kill switch (via Crankshaft Position Sensor interrupt)

[MetroMPG]

I did a test installation of an engine kill switch today, and it works just fine.




(Not my actual kill switch - this pic is from a DIY/project thread on EcoModder.)

FYI, for those not familiar with this mod, shifter-mounted kill switches are commonly used by manual transmission ecomodders to improve MPG. 5-10% gains aren't unusual (non-freeway).

Of course, you could just use the key to do the same thing, but it screws up the odometer & fuel use readings slightly, and adds a little extra wear to the ignition switch. In cars with steering wheel locks (ie. not the Mirage), it's also arguably less safe if the driver is bone-headed enough to turn the key all the way off to stop the engine.

You could also use this as an anti-theft measure, though you'd have to incorporate a single pole single throw switch instead of (or in addition to) the shifter-mounted momentary switch.


All the usual warnings apply:

EG. you will likely die because of this mod and/or ruin your car.

1) Steering: You lose electrically assisted power steering when the engine stops, so if your puny arms can't handle the Mirage's steering in manual mode, don't do this.

Or... modify your EPS system to stay on when you stop the engine.

2) Brakes: You typically only have 2-3 full brake pedal applications with the engine off before the vacuum reservoir is depleted and you lose power assist there as well. Then you have manual brakes which require significantly more pedal effort for a given amount of stopping power.

Or... modify your braking system: increase your vacuum reserve (add volume) / add an electric vacuum pump.

It goes without saying: If you've never done this kind of mod before, do not use it on public roads until you're very familiar with how it affects your car.


Check engine light?

Tim and I were debating whether this would work without throwing a code/CEL. But I've been modifying the same circuit on many of my cars for years, and it's never done anything other than improve fuel economy.


Short version:

I'll add details / schematics to this post later, but the short version is:

1) I used the Russian site electrical circuit diagrams to locate the appropriate Joint Connector plug (behind the cabin fuse panel/junction block) that connects the engine harness (including the Crankshaft Position Sensor wire) to the ECM harness. (So you don't have to mess with splicing wires under the hood and running wires into the cabin.)

2) I cut the Crankshaft Position Sensor wire on the ECM side of the connector and confirmed breaking the circuit stops the engine. Then I spliced in wiring with a normally closed momentary switch to mount on the shifter.

3) Static testing: started engine, opened the circuit and the engine immediately stopped.

4) On road testing: Installed the shifter-mounted switch and did a ~10 minute drive with about a dozen engine kills and nothing unusual happened. No warning lights. No creaks, bangs or funny smells. No CEL.


Last disclaimer:

I'm no technician/engineer. I'm a hacker. If you don't die doing this mod, at the very least you will probably ruin your car.


Pics/details:



Above: the blue connector is the one you want.

Note: it also contains other potentially useful wires from the engine harness: MAP, ECT, Speed Sensors.



Above: schematic shows wire colours. Red for the Crankshaft Position Sensor signals. I cut the one in pin #4 that goes to the ECM, rather than the wire coming in from the CPS, because it appears that connection goes to the Camshaft Position Sensor as well as to the ECM.



Pinouts of the connector. I used the wire on pin 4 of C-219. (Right-click & View Image to be able to read image labels.)