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Old 03-18-2009, 10:42 AM   #1 (permalink)
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Quick question about EOCing with the MPGuino

I'm pretty sure this has been asked before, but I couldn't find it, so here it is:

When you turn the car off, does the MPGuino record mileage for the time between turning the key to off and then to run? I know it's only like a second, but a lot of them will add up to a bit of mileage, which might affect the calibration.

And also, does the car's odometer behave the same as the MPGuino? So like, if the MPGuino can't record mileage for that second, does that also mean the odometer can't record the mileage either? I'm thinking that's the case, because they both work off the same source, but I just wanted to make sure.

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Old 03-18-2009, 10:50 AM   #2 (permalink)
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I can't speak for the mpguino, but the Tercel's odometer is fully mechanical. It will record distance traveled with the engine off.
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Old 03-18-2009, 11:00 AM   #3 (permalink)
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Right, that makes sense. The speedo works off a signal generated by the odometer though, right? And that same signal is what drives the MPGuino, I think.

So, if it generates that signal during the transition between turning the key to 'off', and then to 'run', then it should be accurate. Otherwise, it would be a little bit off.
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Old 03-18-2009, 11:07 AM   #4 (permalink)
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There is a speed and gph reading on the main (custom) screen, if speed goes to zero while the car is moving, it does not have a speed signal.

The kill switch died in my saturn, so I have to use the key for now. I turn it off just long enough to stop the engine, but turn the ignition right back on so I get the speed signal back and can track distance a little more accurately. I should just fix the kill switch on the shifter though
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Old 03-18-2009, 11:07 AM   #5 (permalink)
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I'm pretty sure even the speedometer is mechanical since it doesn't drop when you turn the car off. The speed sensor on our cars is really only used by the ECU for fueling purposes and/or abs. So, I'd imagine that it turns off for that second or so when you key off. Just another reason to get a engine kill switch that kills fuel injector signal when you're in there wiring up the mpguino. Electronics all stay on the whole time then.
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Old 03-18-2009, 11:15 AM   #6 (permalink)
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I guess I'll have to look into the injector cutoff switch then.

I always forget to look at the MPGuino when I'm turning the key, so I'll have to look today to see if the speed drops.

Oh, also, there's been a few times where I turned the key back to on too soon, and the engine starts back up again. Did I just not give the injectors enough time to fully shut off? Or is there some kind of safety thing built in to try to stop accidental turning of the key?
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Old 03-18-2009, 11:15 AM   #7 (permalink)
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Hi, Sorry for overloading the thread, but this is related: I have seen thee/four methods for a kill switch - fuel pump / ignition coil / injector etc., and there was an endless discussion about which is the best... Is there a definitive conclusion as to which method of killing is the most humane for the engine, and won't result in a disaster or random codes being thrown?

I would appreciate if someone points to this definitive source or otherwise indicate the correct method to use.

PS: Related in the sense, ordered my MPGuino today, and am anxious about its use including this 2-sec interval where I have to keep ignition off because the engine keeps spinning and picks up if ignition is put to ON too soon.
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Old 03-18-2009, 11:25 AM   #8 (permalink)
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" Did I just not give the injectors enough time to fully shut off"
It happens to me a couple times a day. I just figure I didn't give the engine enough time to come to a stop, in my rush to get the speed signal back. With the kill switch I can hold it in longer and make sure the engine stopped and not lose distance tracking.


"Is there a definitive conclusion"
I don't think so. I put the switch in series with an "FI" fuse in the saturn and in series with the cam sensor on a metro. I lean towards shutting off both ignition and fuel if possible (I believe the cam sensor interrupt does that on a metro), but just shutting off ignition will give a smoother shutdown than just shutting off the fuel injectors, as shutting off fuel while leaving ignition on makes the engine stumble on the fuel coating the manifold, and will result in more inadvertent relights.
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Old 03-18-2009, 11:30 AM   #9 (permalink)
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If your engine restarts, you didn't leave it off long enough. The engine was still spinning when you turned it back on, so it injected fuel again and fired it back up.

IMO the best way to do a kill switch is with normally closed relays that cut signal to the fuel injectors. Use any ol switch to control as many relays as you need to cut fuel from the injectors. The normally closed relays protect you in case of a failure (of the switch or relay), your injectors are still getting signal and you can drive home and fix it vs being stranded.
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Old 03-18-2009, 11:39 AM   #10 (permalink)
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Interesting note on the engine stumbling dcb. I forgot that some engines use throttle body injection.

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