Fuel gauge only going to 3/4 when grounded?

I glossed right over this when I first read your post, sorry. I'm guessing they used the same board for gauge or lamp, so it would only have one or the other installed. The speed nuts (on the gauge studs) are too close together for the lamp socket to fit into the board easily, but it can be done, if one is really intent on it.

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Well now I have seen it all. Well... almost, I would like to see a larger view of that circuit board trace paths. how they kept 12 volts away from gauge posts and 5 volts is too strange. I know lack of a bulb and socket would interrupt a connection but... They didn't throw similar curve balls at their own service techs often. At the same time... knowing what penny pinchers they were, one circuit board for two different assemblies doesn't surprise me. I'm glad you fingered it out. Happy moparing