License plate light reading voltage but not lighting up

[jmcm]

As the title says, the license plate light is reading about 10 V on the multimeter but the bulb is not lighting up. I tried lighting the bulb across the battery terminals and it worked. Made sure the multimeter reads 10 V even when I push in the spring on the socket and it does.

Could a frayed wire cause this? I plan to check over the Thanksgiving weekend but if there's anything else I should like for I'm all ears.

 

[Michael B]

Make you are getting a good ground.

 

[MrMoMo]

Yeah, when you test 10V are you using the same ground as the bulb uses? (Or the frame). If you have no ground, obviously you technically have no voltage.

 

[jmcm]

Thanks guys for weighing in.

Yeah I'm using the same ground the bulb uses, which is just the metal socket.

Any good way to check ground besides running a wire from the battery, around the rig, and to the socket?

 

[Solace in Solitude]

The socket is grounded to the metal on which its mounted... See if you can clean the metal between the ground.

Or, run a ground wire, from the socket to the frame rail, below the back of your truck... Make sure it's a clean ground point.

 

[4x4veteran]

Clean the socket and clean the metal on the bulb. Slightly tweak metal on the socket enough that it is a tight fit when putting the bulb back in. Also check that the pos end of the bulb is not mashed so flat that it does not touch pos wire in the socket. It would be best if you used a new bulb. You can check the ground with a continuity tester. Your muti-meter should have a setting for that. Hook one wire to the socket and the other wire to a clean spot on the frame.

 

[lc69hunter]

Just had that trouble with my new 4 Plus bumper. I had to scrape away the powder coating on the license plate bolt hole to get a good ground for the light.

 

[Living in the Past]

Running a separate ground is not a bad idea. Currently the ground is made thru the hinges on either the barn doors or tailgate. That is if you only have one license plate light and it's in the stock location.

 

[jmcm]

License plate light is stock 1972, one light.

Just ran a wire from the pos terminal on the battery to the bulb, with another wire taking it to the socket to test whether the socket was grounded. Bulb lit up.

This leads me back to my frayed wire hypothesis. Could the positive signal coming from the wiring be strong enough to show voltage but too weak to light up a bulb? My electrical knowledge is pretty weak.

 

[73FJ40]

License plate light is stock 1972, one light.

Just ran a wire from the pos terminal on the battery to the bulb, with another wire taking it to the socket to test whether the socket was grounded. Bulb lit up.

This leads me back to my frayed wire hypothesis. Could the positive signal coming from the wiring be strong enough to show voltage but too weak to light up a bulb? My electrical knowledge is pretty weak.

The test you indicate above replaces both the positive feed the bulb and the negative ground. Keep the ground in place and see if the bulb lights with the normal supply from the wire. This will help determine whether the problem is a bad positive wire or negative ground.

 

[jmcm]

Thanks 73FJ40

Used the same ground as the bulb uses, only replaced the positive. Works off of the battery, but not when I use a wire to connect the positive feed from the socket to the bulb.

 

[Living in the Past]

Your positive feed is spliced off the connection for your left tail light and left clearance light. If those work the issue is from the connection in the left side and the light.

 

[mtquivr]

License plate light is stock 1972, one light.

This leads me back to my frayed wire hypothesis. Could the positive signal coming from the wiring be strong enough to show voltage but too weak to light up a bulb? My electrical knowledge is pretty weak.

Yes it is possible. The wire could be broken with just enough conductor to read voltage but not enough to carry the amperage requirement of the lamp.

Had this happen on a fuel pump once. Your lamp is drawing considerable less current, but it is a possibility.

 

[73FJ40]

@jmcm,

The outer socket for the bulb can only be grounded IF it has a path for electricity to flow from the socket back to the negative terminal on the battery. If corrosion has affected the frame of the the license plate holder, or the door it's mounted to, or the hinges that attach it to the body, or the body to the frame, or the frame to the battery, the circuit cannot be completed in order to light the bulb.

 

[GoPats]

After I had painted the license holder and remounted, I didn't have any more license lights. For me, I had to scrape off some of the new paint so the bolts that attached holder back to the door had bare metal to tighten against.

 

[Living in the Past]

While I never did it running a ground wire would solve any problems. I know on my 68 with the engine running the vibration was enough to see the license plate light to flicker. Between oil and dust buildup in the hinges was enough to create a poor ground. For the rest of the doors use weatherstripping to prevent any metal on metal wear. Only other place on barn doors for a ground is the pin the locks the one door.

 

[emac]

A little hard to see in this picture, but I have a ground on the door running to a body ground on the right side where the marker light grounds. This of course assumes the license plate holder and bulb socket is well grounded to the door.