Just a note on that power adapter. It is truly an AC power adapter, of the type that provides an AC output, not an AC adapter to provide DC as tends to be the common form of wall wart in the current era. It is simply a transformer.

Some have given me feedback on this, saying it is hard to find one. I'm afraid my junk collection is older than most. And others have success using a DC power supply that has no filtering, such as one to charge small batteries. The nine-volt voltage isn't critical, it is an AC signal above one volt needed. The input circuit of the gauge does a pretty good job of limiting higher input voltages, but that doesn't mean you want to put 30+ VAC in.

The advantage of injecting the signal back by the diff is you can troubleshoot the connector, ground, and power path provided in the car.

Better yet is using the tone generator app as described in the other link. I think this is more accessible to the 21st generation. And you can use your 12V supply to power the gauge:






PS, yes I've read Dave's remarks and some of the feedback from his customers. He has a nicely organized web presence.
--
Art Benstein near Baltimore

What's the fastest liquid on earth? Milk, because it's pasteurized before you see it.