1979 d-100 225.
In an effort to diagnose my "dead" gas gauge,I grounded the hot wire that was plugged into the sending unit stud.The gauge went to full ,as it should.
Question: When I put a test light to the female plug of this hot wire, my NAPA test light flashes on and off. What does this mean and is it normal?
THANKS !!

You're seeing the instrument cluster voltage regulator making and breaking to maintain an average of 5v over time. It's normal and correct. Sounds like probably a bad sender (@ the fuel tank) or sender wire (from fuel tank to gauge) is causing your problem.

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Too many people who were born on third base actually believe they've hit a triple.

The wiring from the dash to the tank is OK, and the gauge and IVR are fine. the problem is either the sender in the tank or the ground at the tank.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Sounds like this is the "S" post of the gauge, it is really a controlled ground.

The true "hot" side of the gauge is positive voltage coming out of the voltage limiter. You don't want to go to ground with that post.
DD

To "get at" the sender on a pickup, I must siphon the gas and drop the gas tank? Can you check/replace the sending unit ground wire without dropping the tank?