Interior Lighting

[BlackTomC]

The interior lighting on my 76 GMC worked a few weeks ago, now, doesn't work at all.

Can someone please explain basically how the wiring for the door jamb switch/main light switch work and how it's setup and ran? I know that sounds like a fairly involved request, I really just need a simple breakdown so I can better understand where the wires are going so I can fix my lighting issue.

I have two new door jamb switches I am installing (previous ones rusted out) and I've installed brand new bulbs in both dome lights.... for some reason, even with the door jamb switches grounded and the bulbs installed, when I twist the "Lights" knob, the interior lights don't turn on. Thanks in advance.

[BBM3]

Do the interior lights work when a door is open?
If so and they don't work with the doors closed using the headlamp switch check your headlamp switch.
Good chance the rheostat wound wire is broken. Very common.

[BlackTomC]

They don't at all, nor when the door is closed and I twist the "Lights" knob all the way to the left/right. The switch does turn the dash lights on and off as well as control the headlights perfectly.

Rheostat... I just learned a new word LOL

[a.graham52]

Check for a blown fuse

[BlackTomC]

I visually inspected the fuse box, none of the fuses appear to be blown...

Just for testing purposes, my friend had my run a wire from the power side? or one of the dome lights, to a screw on the other side of the dome light (grounding the light/circuit out) and what's interesting is the interior lights magically come alive and work, however they are unresponsive to the switch and never shut off at that point.. until I undo my little hackjob wiring.

[bd]

The dome lamps receive constant battery power (via orange wires), but illuminate only when the ground is completed through either one of the door switches or the headlamp switch (via white wires).  So check the 'white wire' for being unplugged near the base of the left B pillar, or perhaps being severed as it passes under the left door sill plate (a common scenario), etc.

[BlackTomC]

bd,

Thanks for the informative response. I'm going to replace both door jamb switches tonight, can you dive a little more in depth, I think I have an idea what and where the "b pillar" and "door sill plate" are but want to be absolutely sure I'm checking the right places.

[bd]

The "pillars" support the cab roof at the corners of the cab (and in some cases the center of the cab along the left and right sides).  The A pillars are in front of the driver on either side of the windshield and the B pillars are directly behind the driver on either side of the cab.  In cases where there exist center roof supports, such as many four-door vehicles, the B pillars are the center supports directly behind the driver and the rear supports on either side of the rear glass are C pillars.  So on your two-door truck, the left B pillar is the vertical roof support directly behind and left of the driver's head.

The aluminum sill plates run along the floor of the cab from the rear of the door openings to the front.  Typically, they retain the edges of the floor covering (mat or carpet) while providing a finished appearance. 

On two-door trucks, the Siamesed wire for the dome lamp (and optional cargo lamp) runs beneath the left side sill plate to protect the wiring from scuffing and to prevent it from snagging your feet as you enter/exit the cab; I'm less certain of wire routing on Crews & Burbs.  It's not uncommon for the dome light harness to be nicked by a screw or crushed by the plate during sill plate installation and then wick up moisture and subsequently corrode inside the insulation, severing the wire.  On two-door models, there is an in-line connector in the harness within about a foot above the floor as the harness feeds up the left B pillar.  That connection can pull apart or become compromised by corrosion.  From your description, the problem is in the ground (white) side of the circuit.

An incorrect bulb inserted into the dome lamp socket can also cause problems.  Dome lamp bulbs have one filament and two contacts.  One contact is B+ (via the orange wire), the other is switched ground (via the white wire).  On single-filament, dual-contact, bayonet-base bulbs the metal case of the bulb makes no electrical connection with the filament.  Make sure someone did not install a single-contact bulb in a dual-contact socket.

[BlackTomC]

Looks like I've got some digging todo. I've got a 76 Suburban, both dome lights are wired in correctly.

Maybe this will lend some insight as to what the issue may be..

I had a broken wing window (drivers side) for the longest time, lots and lots of rain got down in the truck , once I finally got around to fixing it I noticed that the dome lights/courtesy lights didn't work anymore, before the wing window was broke and left for water to wreck everything, the interior lights worked fine.

In the meantime, I'll start tracing wires to see what I can find.