cargo lamp wiring

[markus]

today i finally got to solve some electrical issues, mostly dome lamp, cigarette lighter and cargo lamp. dome lamp and cigarette lighter were easy, turned out to be a blown fuse.
but i don't get the cargo lamp system. the bulb is double, 8W and 16W. one of them is connected to the ground line of the dome lamp (white, #156) and the other to always hot (orange, #40). the orange part makes sense to me, but the white one doesn't. could anybody explain how that is supposed to work? i already had a closer look on the diagrams, but it just doesn't make click in my head.

in the end i didn't have voltage at the orange wire, for whatever reason... so i bridged the orange from the dome lamp to the switch of the cargo lamp, works great (well, 16W only).

i don't think i'm gonna change the actual wiring, but i'd be seriously interested in how the original configuration is meant to work...

[1979C20]

The dome light has an always hot orange wire, and the white wire is a switched ground. The ground comes from the door pins and the headlight switch to complete the circuit and turn the light on. Im not surr about the cargo light as I dont have one.

[markus]

thanks for your quick answer!

right, the white one is the switched ground. now the thing with the cargo lamp is that this white cable goes to the socket of the bulb (+, not ground), as if it would deliver power to the bulb (whichever, 8W or 16W). that's what seems strange to me, this looks to me as a parallel ground through the bulb...

[bd]

So, you're saying the cargo lamp has two filaments?  Generally, the cargo lamp uses a bulb with one filament, two contacts.  One bulb contact is wired to fused & switched battery (orange wire), the other bulb contact is wired to switched ground (white wire).  The cargo lamp harness plugs into the dome lamp wiring along the vertical run at the left rear corner of the cab.  As I recall, the dome lamp must be turned on at the headlamp switch (grounding the white wire) and the cargo lamp switch must be turned on (connecting the orange wire to dome lamp feed) for the cargo lamp to light.  Sounds like someone substituted the wrong bulb and maybe unplugged the orange wire in the cab corner.

[markus]

right, the bulb in there comes with two filaments. and reading your reply it gets clear what's going on...

there's just the wrong bulb at the right place or the other way around... after a bit of research i found out that it's supposed to be a 1076 bulb (23W btw), one filament with two pins, so it gets ground by switching on the dome lamp and + by switching on the cargo lamp.

now that makes more sense! anyways, i'll stick with the configuration i have right now, cargo lamp to be switched on whenever i want, without needing to switch on the dome lamp, more comfortable i guess... and with 16W instead of 23W i also get to save the planet 

still no exact clue why there wasn't voltage present at the orange wire, but that doesn't matter, if anything goes wrong with my wiring repair... so what are fuses there for?

thanks a lot, problem solved!