[dcb]

Glad the Mrs. is doing ok. It soulds like your display issues are an order of operations thing and not a hardware glitch per-se. Like you might need to bounce the power to the whole unit after you program it. Shouldn't affect in-car operation as long as you adjust your setup so the LCD is powered and connected to the duino before or at the same time the duino is powered up.

So about the metro vss thing, Keep me honest here:

I tried an experiment where I do a bunch of analogReads on the vss pin while at the end of the loop and keep track of the min and max reading each half second.

I was getting min=0 and max=~630 which translates into 0 and 3.08 volts respectively, a solid LOW and HIGH in atmega land if I am not mistaken. So this leads me to believe that we have a 9 volt drop on the 100k resistor and are running 90 microamps through the pin. I don't know if it cares about current or voltage or what, only saw voltage where below .2 is LOW and above .7 is HIGH on an input pin on the datacheet. I also saw that they reccomend max of 50k for a pullup resistor. But our 100k is attached to 12-0 volts, not 5, so there ya go.

I have not seen the behavior on the saturn, but I believe the saturn uses a hall efect->ecu->speedo to make a nice signal the speedo (and we) can use. I believe the metro actually uses a physical switch in the speedometer head. So it may be addressable by adding some debouncing to the vss signal.

To confound (or possibly explain) this, when I ran my "do a bunch of analog reads" trick, I never observed the half speed displayed issue. I am wondering if the analog reads add just enough capacitance to the input signal to clean it up or what?