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Can't speak to the wire routing as I have not put one on my Saab yet either . I have not had it very long but you simply tie into the signal wire from the O2 sensor and provide a ground. The ground can change the signal somewhat. I have seen people complain about the accuracy when using a DVM(digital volt meter) as they will compare ground. Generally it's best to use the same ground as the ECU as that is what the car is running off of. Signal voltage being less than 1 Volt can vary up to .1 volt depending on grounds but is usually much less.
The Autometer is another variation of the standard Lambda guage. Most use a LM3914 which is a comparator/driver. Any time the voltage goes up another .1 volt the chip triggers another LED. I have bench tested a few meters like the Cyberdyne and the Autometer and they are accurate to within .002 volts. The Autometer uses the chips ability to be stacked so it uses two chips and gives you twice the resolution or it toggles the next LED every .05Volts. These types can be made cheaply but by the time you get them into a package I don't believe them worth it.
Beware the guages that tell you A/F ratio in ratio form. If your tying into the stock O2 sensor, they will not be accurate. Alot depends on how accurate your O2 sensor is to begin with. Your guage can be very accuate but if the sending device is not than why bother. All I am saying is recognise that those meters that have ratio numbers are a guide only. Find out at what voltage level they are triggering that particular ratio. Once you have one of these in your car and watch it for awhile you'll understand why it's just and appoximation and only WOT readings become the truly important items.
Thus stated their are Wide band O2 sensors and meters but they are prohibitively expensive($800-1500 last I checked) which can be true tuning guides. If you do some time on the dyno buy the option of using the sniffer and verify your readings off you A/F guage. Locally it's $15 an hour for this option.
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