cargo light and courtesy light go dim...

[tpl77]

... when I pull the switch to turn on the cargo light, it comes on but real dim, and if the door is open and the courtesy light is on, I notice it goes dim also. I checked the ground above the gas tank and it's good. I suck at wiring... I'm looking at the schematic but do not see any clues. Any help is appreciated. Thanks!!

[MadJoe]

First things first: You have a multi-meter, right? Ok, good. Now make sure you're getting 12V at the bulb socket. Probably not, but clean up the socket and replace the bulb anyway. These trucks are old, those connections were bunk from the factory, and the previous owner(s) most likely never heard of dialectic grease, so corrosion in the bulb socket is a real possibility. If you rule that out, the switch would be second on my list of suspects: Check that it isn't corroded, or otherwise damaged and verify 12V is present. I'm assuming this is a simple circuit, since it isn't dependent upon the headlights or even the ignition for this lamp to work, so if the switch and the socket/bulb aren't the culprit, there's a wire damaged somewhere.

Of course this is assuming you verified the ground electrically, and not just visually.

Good luck.

[tpl77]

...thanks... The light is new apparently. I removed the screws and looked it over and I suspect someone replaced it right before I bought this truck (2012). I dug out a tester, but before I tried anything, I looked closer at the ground. I messed around and found that if I disconnect the ground, the light works perfect- nice strong light coming off the bulb. The courtesy light looks fine too. So, I'm now experimenting even more. I pull on the headlights and they are fine. Everything works as it should with the cargo, courtesy, and headlamps- as long as the ground wire (above the gas tank)is disconnected. So, my last move was to test the ground wire. With the cargo switch off, there's nothing reading on the tester. When I pull the cargo lights on, then test the ground wire(still disconnected) I find 12.4 volts. I also connected the ground, turned on the headlamps, and pulled the switch to activate the cargo light. Headlamps dim down. Just like the courtesy light.

All I can think of was a headlamp switch that went bad last summer. I changed it out and everything was fine. I rarely use the cargo light, so did I change the switch with a new one that is tied to the cargo light and somehow energizing the ground wire? I'm just not experienced enough with electrical diagnostics. Maybe this cargo light is bad and when I pull the switch it's jumping the current around the bulb and that's the short that dims everything? I'm thinking back to when I last used the cargo light and I know it work fine about a year ago...

MadJoe- thanks for your input. I kept the ground wire off for now so I don't pop a fuse or melt the wires. I suspect the fuse is too large for the circuit as I should have blown it from all this tampering...agree?

[MadJoe]

The good news is that if the cargo light is working with the 'ground' removed, it's finding ground some other way which might indicate the wire you suspect is the ground, isn't. This would line up with what I always thought (but never confirmed): that the cargo lamp is grounded directly to the cab and should only have a power lead. Hopefully someone with more experience with these trucks can confirm or deny this for us, since I only have electrical experience, not much experience with these trucks (I've only had mine since December and haven't had a chance to dig into it yet).

Either way, considering that the lights work correctly with this 'ground' disconnected, my next step would be to disconnect the 'ground' and then check to see what, if anything, no longer works correctly <--I would suspect what ever does is most likely your culprit, and if nothing stops working, then I would suspect this wire as the culprit.

Just make absolutely sure that your tank is still grounded when you remove that wire (the mounting screws should provide a path to ground, but it never hurts to be sure).

[colnago]

tpl77,

Sounds like your ground wire either got connected directly to a power lug, or (less likely) the plastic coating rubbed raw over 50 years of bumps, and is rubbing against a power source. Or (again less likely), this was never a ground wire, and was used to bring power somewhere. Can you tell if it's stock? Does it go to any switches on the dash? What color is it?

MadJoe,

I think you're right about a single wire to the cargo light. I have a switch in my bucket of parts, and it only has a single in/out line. I'm sure this is for power, and the light is grounded above.

Joseph

[tpl77]

...I've been slammed at work and haven't had time to look further. I'll be back at it in a couple weeks.

PS- I changed the turn signal switch 2 years ago and it broke again over the weekend. Now I have another job!!...