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Old 11-26-2008, 02:37 PM   #5 (permalink)
Coyote X
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Join Date: Dec 2007
Location: Southen West Virginia
Posts: 654

Metro XFi - '93 Geo Metro XFi Convertible
90 day: 62.17 mpg (US)

DR650SE - '07 Suzuki DR650SE
90 day: 55.26 mpg (US)
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I am using the latest firmware, I copied it off the project page when I reloaded the AVR.

The pin could be configured to pullup 5V but that is on the wrong side of the zener and current limiting resistor so it would maybe not have that much of an effect on reducing the jitter coming off the 10-20 year old reed switches. Adding a pullup resistor from the 12V line in to the VSS and one from ground to VSS on the 7805 would probably clean up any Metro as good as doing it on the back of the cluster.

Most cars probably use a better setup than a Metro to generate the VSS pulse so I am not sure if adding it to the standard board would be a good idea. It might confuse some ECUs possibly. I guess the first step would be a standard 1K debounce resistor from VSS to ground. And if that doesn't clear up the signal then adding the 12V to VSS resistor. But so far it looks like the Metros are really the only ones having issues and adding both resistors won't hurt a Metro so it seems ok to try.

I followed the VSS wire in the diagram and it goes to the door interlock, swift automatic seat belts, cruise control, and the ECU. My car doesn't have any of those things anymore so that is why my wire was not getting the full 12V. The audiovox cc that is in it now was slightly putting some voltage out on the line and that was what was letting my MPGuino read anything but I suspect that the 12V pullup on that VSS wire actually comes from the ECU and not the door lock module like I originally thought so anyone running the stock ECU will probably only need the VSS to ground debounce resistor. But it will take a car that is a bit less modified to test out what is actually required.

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