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Old 09-15-2009, 10:31 PM   #1 (permalink)
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MPGuino self-generating its own VSS & Inj ????

I finally plugged my guino into the civic today, and fired up the engine, and got GPH of 0.2 or so, watched it climb to 0.5 when i stepped on the gas, & fall back again.

Then, i shut off the engine, and GPH jumped to 8.15, and RPM of "240" (translates to 700 "real" rpm, i figure). It normally did this on the bench (before i installed the pullups & diodes), but I chalked it up to "ambient static" since it wasn't wired & grounded, and the numbers seemed to change if i touched it.

Now, i've unplugged the vss & inj inputs, and when it's powered on, i'm reading pulses of 1.6v at the inj lead, and 0.8v at vss. Apparently 0.8v isn't high enough to trigger VSS interrupts, but the injector sensor is going to town.

I'm using a Seeeduino 168 board, with 5.1v zeners and 100k pullups on each input. Before installing, I measured the resistors & both zeners had 0.7v forward voltage.

Does anyone know (a) what might be causing these self-generating signals, or (b) if the sensitivity can be changed in software to ignore them?

I'll add that when i was feeding it with an arduino signal generator, it would crash when using 300ohm resistors, but ran fine with 100k.

Without pullups & diodes, and without a sig-gen, my ambient readings were steady at 52.5 MPH and 5.56 GPH. Now, with pullups & diodes, and no sig-gen, i get 0.00 MPH and 8.15 GPH.

Working back from 8.15, i figure the fake pulses are 'On" about 0.34 sec out of the 1/2 sec loop. And saying that out loud, I recall rmccomiskie's "too-long-pulse" hack, and figure I should add the code.

Also, just playing with it some more, GPH follows this sequence when i change the LCD brightness: 8.15, 0.00, 0.7, 8.15, and stays at 8.15...
A minute ago, it behaved the same no matter what brightness i had (0-3), but now, brightness==0 has GPH at 0.00, and now brightness 1 has GPH fluttering around at 0.00, 0.7, 0.25, 0.15, 0.00. Brightness 2 & 3 have higher GPH, but are no longer steady 8.15 like before. Almost all my benchwork was at "1" so as not to blind me.

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Old 09-17-2009, 06:00 PM   #2 (permalink)
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Fixed.

The culprit? a 2-pin 100-mil jumper header i clipped out of an old ethernet card to plug my VSS & Inj inputs into. You'd think the 2 contacts would be "open", but it turns out there were about 700k ohms connecting them. No es bueno, as they say in Spain.

No more spurious Inj readings. RPM is off by x2. Let's roll this puppy!


If anyone's interested, i added a "Throwaway Too-Big Pulses" feature to processInjOpen/Closed in attempt to hack a software fix. It helped a little, though fixing my wiring was ultimately necessary. I'll post the code on the Workspace One page.
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Old 09-17-2009, 06:44 PM   #3 (permalink)
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I was going to answer this but forgot, you found what I would've suggested, anyway

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Old 09-21-2009, 11:10 PM   #4 (permalink)
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Well, i'm almost there. Took it for a test drive today to check the mph/miles, The result: crap. Mph was severely low & inconsistent. Overall 8.9 miles on the odo were 2.46 mi on the guino.

I jacked the front wheel up & checked the voltage on the vss line. 4.73v with guino disconnected. Connected the guino, and got 4.62v before the 100k resistor & 2.44v after. I figured the voltage was being pulled down too far, making it marginal for the atmega to pickup. I spun the wheel by hand & watched the pulses on a voltmeter - about 30 pulses per 13 rev. Spun it 13 again & ticked off 59 vss on the guino. OK. Spun it 13 revs again - but a little faster - and the guino hit 52/60. 13 more, faster, and 46/60. 13 more the slow way, 60/60.

So I rerouted the vss through a 300-ohm resistor, and now i'm getting 3.6v instead of 2.44. Spun it as fast as possible - 180/180 after 39 spins. Looks good. Took another test drive, and mph looks steady & reasonably accurate. Odo vs Guino was about 2.95 vs 3.00 mi, so that looks decent.


The bad news? Now my speedo has 3-5mph bounce when i get over 25mph -- even though Instant MPH is rock steady on the guino. This bounce may also account for the 0.05 mile discrepancy on the odo.

I guess i'll have to play with different resistors, unless anyone has some better advice ? Is the zener diode supposed to prevent vss from grounding in the guino, or is it there to limit voltage to 5.1v ?

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