Dome Light Circuit

[BlackTomC]

Quick question for y'all, my dome light power feed works just fine, door jamb switches have been replaced and function accordingly, however, in order to get the bulb to light off, I have to ground the bulb to the chassis, the white ground wire doesn't do anything.... Any quick tips for getting that diagnosed and fixed?

TL:DR - Orange power wire works, white ground wire doesn't do anything, have to ground bulb to chassis to get it to light up. Both door jamb switches replaced.

1978 GMC Sierra K25 Suburban

[zieg85]

Does your horn work?  I was having a similar problem and my horn fuse was blown on my 85, the horn relay is somehow tied to that circuit and was bad.

[bd]

So, jumping the white wire directly to a clean ground at the door jamb switch doesn't illuminate the dome lamp?  Have you checked continuity on the white wire between the door jamb switches and the dome lamp?  Do the dome and courtesy lamps illuminate with the headlamp switch rotated all the way CCW?  Is the dome lamp bulb making good connection with its socket? 

There's a two-wire inline connector between the dome lamp(s) and the left side interior lamp harness, IIRC, under the driver side sill plate or in the left B pillar.  The wiring and connector are susceptible to corrosion below the sill plate due to water from the floor in wet weather and blowing past the door weatherstrip.  This becomes especially true if the wires have been pinched by the sill or penetrated by the sill screws.  Check there for wiring damage and loss of continuity.

[BlackTomC]

@Zieg85 - The horn does work, very well Took a while to get that up and running too...

@bd - I will start tugging on wires/checking continuity tonight. If this is any help, the interior lights stopped working after rain water got in through the wing window during a fairly gnarly rain storm.. To be completely honest, I've got the entire burb pretty much gutted, the only orange/white wires (dome light wires in this case) I see, run straight over to one of the pillars from the dome light, down under the sill plate, then straight up to the switch, I don't see any inline two wire connector anywhere, just wires running straight from the switch

I will double check everything later on tonight

[bd]

I guessed on the inline connector since the pickups use them to connect the bed load light option.  I assumed the harness would be the same up to that point, but it's quite possible the burbs use a continuous run of wire.  IF there is a connector, it maybe in the B pillar.  Harness damage caused by water intrusion under the sill plate is still a good possibility.  Bear in mind that the wiring could be severely corroded inside its jacket and not visually apparent.  Check the wiring closely.

[BlackTomC]

@bd - You were right! The inline connector was located in the door "pillar"? The wire ran from the fuse block/grounds, through the sill, up into the pillar, then out to the roof, and those grounds connections were completely green and corroded. Ended up chopping all the terminals off, as much as I enjoy keeping old school things old school, I might just slap some weatherproof connectors on here, then see if the dome lights work.

Random question, so if I trace the power wire up into the fuse block, it connects to the block however there's another wire branching off of that that ends with a male fuse block connector, not connected to anything. Any idea what that's for?

I'll check back in later on tonight after I get done soldering and heat shrinking all these wires.

[BlackTomC]

You guys know of any place I can purchase 3 dome light housings with the clear lens covers? Possibly even the rear plastic housings for suburban rear AC?

[bd]

...Random question, so if I trace the power wire up into the fuse block, it connects to the block however there's another wire branching off of that that ends with a male fuse block connector, not connected to anything.  Any idea what that's for?

It's the power connection for the clock option.  The clock harness, when present, plugs into that connector.

I recommend that you survey for how/where water entered the cab to result in severe corrosion of the wiring to help avert similar repairs down the road.  Just make sure there isn't an active leak or previously unrecognized rust-through.  Electrical connections (especially those that are hidden behind panels and difficult to access or exposed to weather) should be coated with antioxidant paste available from the electrical departments of most hardware retailers.

[BlackTomC]

Problem solved!

Stupid me missed a spot on the wire that had an incredibly small area that had rubbed away over time under the sill plate, totally corroding the wire within, chopped her out, stripped, soldering and heatshrinked. The applied a nice vinyl edging on the door sill sheet metal to prevent it from happening again. The balance has been restored in the force! Lesson learned, get a second set of eyes and try not to work at night!

[bd]

Good job!  For even better long-term service, near permanent protection can be afforded by shrouding the wires below the sill plate in asphalt impregnated fabric loom.