Fuel gauge only going to 3/4 when grounded?

RedFish said

I've never seen a panel that had both warning lamp and gauge for oil. Warning lamp does operate on 12 volts. Only way that could work is with two separate senders and wires gto them. Anyway...

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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.