Cutting power to the Megasquirt
 

Hey guys!

I want to add a "manual immobilizer" (aka, hidden switch) to my car, but I have no idea where. I had this previous idea of adding it to the battery as a ground cut-off switch. Not that much of an idea. I don't want it in the ignition barrel wires either, because it wouldn't prevent somebody from hot wiring the car and driving away!

So I figured... Why not just cut the power to the Megasquirt? The megasquirt is powered from the ignition line, so if I cut the power to it, the car wouldn't start even if hot wired. But then, I don't know if it will cause any adverse effects to the Megasquirt not to be supplied with power with the rest of the car turned on.

So, what do you guys think? Can it be done?

Regards.

I don't see a problem with that. I mean, if you remove your megasquirt and set it on a shelf for a while, then plug it back in...the car should fire right off...right? Therefore, I see no issue with a secret switch for anti theft purposes.

Well, that's not exactly the same as keeping it there, is it? I mean, my concern is that it will still receive signal from all the sensors if the car is turned on, only it will have no power. I'm afraid this might burn something inside the ECU or something like that.

Cut power to 1B

Lots of ways to go about this but the trick to doing it is having a switch that is easily accessible but not instantly figured out by someone who broke into your car. Some that have been done before is using the seat belt, lighter, dimmer switch or other circuit for momentary starter action. But like you said that is easily defeatable if given time. Adding an old style foot control hi/lo beam switch in just the right spot could be used to cut the power to the ecu like Brain mentioned at 1B. Since the ecu is either powered on or powered off, interrupting the supply wont affect it. I personally have done something a little further since after all, you could still hot wire the ecu if you knew enough about the car. On my sisters former car, an Integra GSR, I cut the crank sensor wire to the ecu and ran it to a toggle switch and hid the switch. Even if you wanted to hot wire the car, you couldnt. You would have had to duplicate the crank signal or find my kill switch. Granted a tow truck will take it regardless so there is only so much you can do. They stole her first GSR but after I did the kill switch they attempted it again with no luck. Still had to fix the ignition switch they destroyed though.

Well, it's not like they can't find a way to steal it, but I bet it would be damn hard to open all the stuff under the passenger footwell to get there...

Thanks brain! And thanks everyone else for their input too!

You will need to shut down the injectors on a normal MegaSquirt as the drivers can stay on when power is shut down.

The sensors, etc are not a problem.

Thanks for the tip Matt... Where is this injector driver? I know the Miata has an ignition driver, but the injector driver, I know nothing about. Are you refering to the injector driver inside the Megasquirt? How do I shut that down?

the injector fuse will be powered with key ON, and the MS will have the signal side grounded, so apparently you'd need ot cut the injector fuse's power as well if im understanding correctly.

You are.

or maybe two toggles for each INJ output, the MS will power up, but you'll never have fuel...but i guess you could just put a kill switch on the fuel pump relay--that would make more sense.

Aren't you overcomplicating in that case? I mean, the MS grounds the injector outputs in order to fire them, if the MS is off, then there won't be any grounding on the injectors? Or am I missing something here?

If when the MS is depowered and the INJ outputs are going to ground and the injectors get powered, they will be spraying fuel.

injectors get constant 12v from the INJ fuse. the MS rapid cycles the ground in order to inject fuel. Matt is suggesting that some MS configurations keep the INJ outputs grounded when the unit is depowered.

so with that said, if you did something to cut power to the MS but the ingnition was turned on, the injectors would be powered up through the INJ fuse and the MS would be keeping them fully locked to ground and they would be spraying fuel until the ignition was turned back off.

Oooooooh! I see!

So, how did you make this one? :rofl: MS1 PNP

is it really a black box DIYPNP?

The regular MS variants (V2.2, V3.0, V3.57) use a driver chip that, if it loses power, can put the transistors it drives in a somewhat random state. I've seen engines flooded that way.

Come to think of it, some of the other 12 volt things can be trouble - not because they can get stuck on, but because the MegaSquirt can become powered through them.

No Brain, it is one of your MS1 PNP early units, built for a 90-93 car with standalone harness!

That second issue you referred to Matt, is what I was initially worried about... That the MS may gather some power from the sensors and what not.

It can't gather power from sensors - they're powered off the MS - but it can be powered through relay control outputs.

So, what is the simplest way of installing a manual immobiliser switch to a car with a megasquirt. Is it a switch to the fuel pump power?

CAS sensor, without a doubt. The car will appear to almost catch then fail.