Okay, interesting problem in the 71 Demon. I have five Auto Meter gauges in my panel - voltage, oil press, fuel level, speedo, water temp. No problems with water temp, speedo, fuel level. But some interesting silliness with the oil pressure and voltage.

You see, everything works out beautifully when the car starts and runs. All gauges show correct. But when I pull the light switch to either parking lamps or headlights, my oil pressure and voltage cease to work. Oil pressure buries itself past 100 and my voltage zeros out. No instrument lamps go on in these gauges either. Additionally, the dimmer switch on the light switch can be used to change the values of these gauges. At this point you're probably saying, "haha, this problem's easy! You have it on the instrument lamp circuit!"

But that's where I can't figure this out. You see, my instrument lamps and my +12V leads to the gauges are on completely different circuits. Further, and here's the kicker, all the instrument lamps are in parallel with each other, as well as the +12V leads on a separate wire(see photo 1).



So you'd assume that if two were having problems, the other three would be, too, because they get their power from the same place.

So I decided to trace it - maybe I wired it wrong somewhere? I traced it back, and all of my +12V are going to the Dark Blue with tracer wire (DB* on the chart in photo 2).



So not sure what's going on. Maybe a bad ground? Maybe something else. Any ideas as to how to test this?