Yet another dome light issue

[USSkoval]

I have a problem with my dome light that seems to be different from every other (I searched a bit). It's a '79 crew cab with the center dome light and one under dash courtesy light. At first, neither would come on with the door open or the headlight knob rotated, and both front door jamb switches are seized. I replaced both bulbs, and now the courtesy light will come on with the headlight knob, but not the dome light.

Now here's where it gets weird (to me at least). With my volt meter, the dome light has a constant 12 volts on BOTH wires. So I got curios and tested the working courtesy light. It's the same way, constant power on both terminals. How is this possible, and why doesn't the overhead dome light work?

[joe231]

The dome and courtesy lights are ground side switched. The hot wire is always hot and will feed through the bulb and you will see the ground side hot until the circuit is completed by either the headlight switch or the door jamb switches. My guess is that your dome light isn't getting a ground to light it. Maybe you have a broken wire somewhere.

[USSkoval]

Hmm, I wonder if I have a short between the two wires somewhere along the way. The light was spliced in poorly by a previous owner. When I undid that mess, the volt meter still showed 12 volts on both wires... and that was with no bulb. But if there was a short, would the other light still work?

[joe231]

did you take the bulb out of the courtesy light socket?

[USSkoval]

No, it stayed in place the whole time. Should I use a test light to test for power, rather than a volt meter?

[joe231]

either will work just fine
you need to pull the courtesy light though. I believe they share a ground wire so you might be reading voltage through that bulb.
from your previous post it seems that you found a splice in the dome wiring?
can you ground the white wire and make the dome light come on?

[USSkoval]

Yeah, both wires had been cut and spliced, one of them had the remains of another wire spliced in. I'll ground the white wire at the dome and see what happens.

[joe231]

do you have a diagram?

[USSkoval]

Ok, I got it working. I had to replace the wiring going up the b-pillar and along the roof... boy what a pain that was. While I was at it, I replaced the door switches with newer ones that have blade terminals, and I added switches for the rear doors since the truck did not come with them. The LF door switch has two terminals so maybe I can add a key buzzer sometime. Thanks for your help!

Now it's on to the wipers, blower wiring and fuel gauge/dual tanks

[joe231]

no problem!

it is a never-ending battle on a neglected truck huh? I went through the same thing on my '80....nothing worked correctly.
all the riggin' about drove me nuts....some people shouldn't be allowed to own or use tools

[USSkoval]

Haha, no kidding!

It turns out it was just a bad wiper motor and a bad blower motor, so that part was easy. I started with the dome light because I THOUGHT that would be the easiest to take care of

[RuralRoute C-30]

Your day sounds so familiar to my evenings last week!. Glad to hear it went well.

I can relate to how one thing does lead to another!

Learning to find the books today!
Mark