DIY engine kill switch (via Crankshaft Position Sensor interrupt)

[Loren]

Gotta have a diagram to keep my head straight.



I'm not going to put the switch on the shifter. When I'm NOT hypermiling, I don't want to accidentally bump the kill switch. Plus, it wouldn't work with my solid steel knob.

On the Miata, I put the switch right next to the shifter. So, I'd pop it into neutral and bash the switch. It worked, but there was a bit of a lag because I was doing it all with my right hand. So, here, I think I'll put the switch in the stock start button location (there's a blanking plate there now) if it's close enough that I can hit it with a finger from the steering wheel. I'll have to go look at the car a little closer before I commit.

Another possible location would be below the AC controls, directly in front of the shifter.

Or... I guess I could mount the switch on the stick a little below the knob. (my shifter boot leaves a couple inches of the stick exposed due to the knob I'm using) That could work. And if I do a bypass switch... that would allow me to disable the kill switch when I want to be sure I don't accidentally hit it.

[Loren]

Another idea. Requires slightly more wiring.

Instead of a bypass switch, use a relay. Wire the relay to be normally closed, so it's default condition is always to be "bypassed".

Then just use the push button to enable the relay and break the circuit.

Takes away the "security switch" feature, but it's already been noted... ain't nobody wanna steal your Mirage, anyway!

Otherwise... as far as fail-safe, it should be more reliable than having a mechanical switch directly in the ignition switching circuit. If the pushbutton switch fails, the relay stays off, the car still runs. If the relay fails... it will probably fail and stay closed, the car still runs. If the supply wiring or the wiring to the switch fails, the car still runs. As long as the wiring to the relay contacts is good, the car will run.

Aside from requiring a relay and 12v source, the wiring is actually simpler and more elegant, I think.

Should look something like this:

[MetroMPG]

I'm not quite sure why, but I've got the itch to finally do this mod.

I know why!

Because you find hypermiling a fun challenge, and you want the extra 5-10% MPG boost that this mod will give you!

I'm not going to put the switch on the shifter. When I'm NOT hypermiling, I don't want to accidentally bump the kill switch.

Or just mount it low on the shifter so it's nowhere near where you naturally place your hand. Or put a collar around the button so you have to deliberately press "inside" it. Mine's located so it's a stretch from my thumb's natural position. I can say I've never accidentally activated it in over a decade of using this mod in multiple vehicles. Shifter location makes the most sense.

Or... I guess I could mount the switch on the stick a little below the knob. (my shifter boot leaves a couple inches of the stick exposed due to the knob I'm using)

Yup, good idea.

I like your relay idea. Better failsafe design. I'm too lazy to implement it though.

The only two kill switch problems I can remember experiencing in the past decade:

1) a loose wire caused a brief open circuit over bad bumps (engine would stumble);
2) I once had a slightly sticky switch situation where the car wouldn't start because the circuit was stuck open. Took me a minute to figure it out. The relay wouldn't have prevented either of those.

FYI, I've owned my junky Miata for 3 years and haven't put a switch in it yet - I just use the key.

[Loren]

Yeah, the one installation I did, the switch failed in a matter of months. Probably a fluke, but I need better reliability that that. A good quality automotive relay is pretty reliable. I wonder if I can find one in the garage?

[Loren]

Went for a drive a while ago. Mounting a soft-touch push-button underneath the HVAC controls would be perfect! Grab neutral, extend fingertips to smack the button.

And if I do the relay trick, I don't need a fancy and expensive normally closed switch. I can use something as simple as a doorbell button. And being up under the lip of the center pod, it would be almost invisible.

[Loren]

Okay, pulled the trigger on some parts. I'm sure I could have done this for $0 with crap I had laying out in the garage (if I spent enough time rooting around in the garage to find it) and a bunch of cutting and splicing and all that. I'd have made it work, and it would have been some variation of a hack job and not "exactly" what I wanted it to be.

The way I'm doing it, the only wire I have to cut is the CPS signal wire. Assuming I can find a ground screw somewhere.

What I bought was a pretty big doorbell switch. It's like 3" long by maybe 3/4" wide. Surface mount. So, all I need is a little hole for the wires, and two screw holes. And where I'm mounting it, if I were to remove it, nobody would even notice the holes. (it's UNDER the HVAC panel) The switch is big enough that I don't have to think too much about where it is. If I slap in that general direction, I'm going to hit it!

And I bought a fuse adapter (low-profile mini, if you were wondering) that I should be able to plug into one of the unused fog lamp fuse slots.

And I bought a standard relay with a socket.

Of course... everything is "cheaper by the dozen". So I actually bought 10 fuse adapters (with fuses) and 6 relays (with sockets). All of this cost me about $27.

Or, if I'm in full justification mode:
$6 for the switch
1/10 of $9 ($0.90) for the fuse holder
1/6 of $12 ($2.00) for the relay

So, about $9. And I have a bunch of left over garage fodder. (hey, I'm ready for NINE more Mirage projects!)

If this nets me 10% better fuel economy... lately, I'm not driving more than 6,000 miles per year in the Mirage. So, 10% of that would be 600 miles. If I'm getting around 60 mpg, that's 10 gallons. At $2.20/gallon, that's $22.

Even if I have to amortize the full $27... it would pay for itself in under 2 years. Fair enough.

Good thing I'm just doing this for sport!

Parts if anyone cares:
Switch: https://www.amazon.com/gp/product/B08FRV4MKJ
Fuse Kit: https://www.amazon.com/gp/product/B07T8LRSH7
Relay Kit: https://www.amazon.com/gp/product/B01KVZ2MU4

[Loren]

Did it!

The last time I made a short "hypermiling" trip, it was 2pm, no traffic on 35-40 mph roads. Pretty much ideal conditions, and I netted 56 mpg without doing anything fancy. (never shut off the engine, just general eco driving and neutral coasting when possible)

Today, I made the same drive at almost 5pm on a Friday. There was some traffic, but it wasn't too bad. Still, I got "pushed" by traffic a little bit, and caught by some situations that made me use the brakes a little. But, I made liberal use of my kill switch and engine-off coasted whenever I could. I'd accelerate up to maybe 5-7 mph over the limit, and if there was nobody close behind, I'd kill it and coast down to 5 under... less if there was NOBODY behind me. Just pulled into the driveway with 64.2 mpg! I saw 66 for a while, but got bit by a traffic light near the end.

15% improvement just from installing a kill switch and knowing how and when to use it. I didn't even sit with the engine off when I was stopped. I idled at the traffic light. So, there's even more to be had if I wanted to use the starter more. As it was, I never had to use the starter on this trip.

Now I'm inspired to seek out 70 mpg...

[MetroMPG]

Only 70!? Slacker.

[Loren]

Baby steps.

[Space Wolf]

I'd kill it and coast down to 5 under...

Did you decouple the engine during coasting? And restart it by releasing the kill switch and simply coupling the engine again?