Ok guy I'm new to the forum and still trying to get the hang of using the website. I figured the easiest way to go this was to post so y'all could redirect me or help me if y'all haven't heard of anything like this before.

my battery voltage at the battery is 12.21 volts but my cargo light will not come on. I can unplug the bulb from the socket and dome light works just fine on the cargo light switch, but when the cargo bulb is in and turn the switch on both the dome light and the cargo light do not turn on. Been very frustrating for the past couple days and finally decided to try and get help for someone.

 

Have you cleaned the connections make sure everything is grounded properly. may clean the ground?

 

Welcome to FTE...lots of good people here, and often willing to help! If you get a chance, post up some details of the truck such as year and model. Pictures help too-we love pictures!

Anyway, problems with the dome light could be the door switches. Mine are finicky, and usually only work if I flick them with my finger.

The dome light and cargo light are grounded through the cab body. Check the grounds, and make sure the cab is grounded, too. The cab usually has a strap to the cylinder head.

And check the fuse...

 

Welcome to FTE. Does the dome light come on with the headlight switch turned fully counter clock wise and the doors close? Might be a bad cargo light switch?

Dome light 101, maybe this will help with the cargo light issue too?

For the dome light wire, hot-at-all-times power comes goes to the headlight switch as a GREEN with YELLOW stripe wire. Dome light power comes back out as a BLACK with BLUE stripe wire. The same color scheme is used at the door jamb switches. BLACK with BLUE stripe runs out to the actual dome light. BLUE with a RED stripe is instrument panel illumination. It comes out of the headlight switch to the fuse box, leaves the fuse box again as the same color and is distributed throughout the dash.

There is no ground connection to the dome light switches or the headlight switches. These switches switch power, not ground.

1) Make sure you have 12 volts on both sides of the courtesy lamp fuse. This circuit is hot at all times, so it doesn't make a difference what position the key is in. If you have power on only one side, replace the fuse. If you don't have power on either side, then there is other work to do first.

2) Pull the dome light switches out and remove them. Make sure the terminal for the GREEN with YELLOW stripe wire in the connector has power.

3) Check the dome light switches for continuity. With the plunger all the way out (switch CLOSED), the switch should be shorted. With the plunger pushed all the way in (switch OPEN), the switch should be open. If not, replace the switch.

4) With power to the switch proven to be good, and the switches proven to be good, the dome light should light up. If the dome light does not light up, remove the cover and check for power at the dome light (not the side that's permanently grounded to sheetmetal) with the door OPEN. If you have power here, yet the bulb does not light up, replace the bulb.

This will prove out the door-side; if you get to the point that the doors turn the light on but the headlight switch will not, then we can dig a little deeper.

 

The switch has voltage going to it. For some reason I think I'm getting a small voltage drop. I plugged in LED bulbs to both the dome and cargo and both come on, but are somewhat dim. When I plug in a regular halogen bulb to the cargo plug both the LED in the dome light and the halogen bulb in the cargo both turn off. I will double check my cab ground and see what I get from there. Thanks everyone one for helping out. I will give you an update ASAP.

 

Yeah, that kinda sounds like a grounding issue, maybe causing the two bulbs to be somewhat in powered in series or something? And if that's the case, with LEDs being Diodes which are essentially check-valves for current, one could be denying current to the other somehow. What does the system do with two incandescents installed?

I'd like to see what you find out.

 

ok guys sorry i didn't exactly explain what truck i had for those who are wanting to know. it is a jade green 78 f150 custom step side 4x4 400 with a little engine work done to it, not really for sure what all is done to it. my dad bought it in 82 from a friend and his friend had done a couple things to it and thats all i know. told me if i would fix it up and get it running good i could have it.

i did a little research today with a multimeter but didn't have enough time to really dig into detail to find something out. i noticed that when the dome light is on just from the door jamb button switch it is not as bright as when i actually have the cargo light switch on. when i turn the cargo light switch on the dome light is bright when there is no bulb in the cargo light. with the cargo bulb out and dome light in i am only getting around 9 volts at the cargo socket. that is as far as i could dig into it today, hoping to try something with grounds tomorrow and see if that helps.

 

I was thinking about this thread today as I was looking over my new cab which is equipped with the cargo light. It fortunately has I believe all the wiring and the cargo light switch. From what I've read, the cargo light only comes on with the dome light.
Your wiring is probably fine. What you said about the cargo switch making the dome light brighter makes me think that perhaps the switch contacts are dirty or/and corroded. Perhaps the dome lamp gets its current through that switch. I'll bet that simply spraying some contact cleaner into it will make an improvement.
And I just went out and looked, and saw that the dome lamp gets its ground from a wire which is screwed to the support beam above the headliner. I forgot to look, but I think the cargo lamp connector has two wires - so it might also get its ground from that wire. I would definitely drop the headliner to check that lamp ground, but you have to be careful because the headliner is probably brittle.

I can tell you that the wires come up the driver side A pillar from I believe the light switch, and run over the door and probably to that switch. From there wires would go to the dome light and cargo lamp 'fixtures.'

 

Ok guy I'm sorry it took me so long to reply I've been out of town, but I finally figured out what was causing the problem. It was where the harness plugs in behind the dash, one of the hot wires was loose causing it not to have a good connection. Thanks for helping out guys I'm sure there will be questions in the future I have working on this 40 year old truck.