My son has a 1953 GMC and we put a new speedometer in from Brothers Parts. The gauge lights work but the speedometer ones do not. The speedometer has a metal case, but is heavily painted. I know the lights ground when you plug them in but is there any other way to check them? I am at a loss? Thanks
Turn on the light switch (other dash lights are on). Strip an inch of insulation from each end of a scrap piece of wire. Touch one end of the wire to the clean metal case of the speedo and the other end to a good clean ground. If the lights come on you need to provide a good clean ground for the speedo case. If the lights do not come on remove one light from the back of the speedo and touch that bare wire to the outside metal socket of the light holder. If the light doesn't come on replace the bulb with a known good bulb from the other instrument pod. Still doesn't come on? Not getting voltage to the lamp.
What Bill said I found that most of my electrical issues were related to poor grounding. With everything painted bare metal in a priority. I will brake out my grinder in a minute to get clean metal. Grounding is required.
Dash Gauge Grounding For both the lights and the fuel gauge's accuracy , there's supposed to be external star washers under each of the four nuts holding each gauge pod to the dashboard . Often it helps to wire brush the threaded studs .
