Dome light wiring

[cyates219]

Hey all, I’m new to the forum and I just got a 76 c20 std cab with some wiring issues..to say the least. I’m trying to get a new dome light wired up and when I took a look under the dash I just saw a mess of wires.

It seems one of the previous owners decided to completely cut off one of the orange wires coming off the headlight switch (I’m guessing it was the 12v fused hot wire) and the other orange wire was jerry rigged with electrical tape to the orange wire coming from the courtesy lamp AND a random red wire (guessing that was the new 12v source)

I’ve found that one of the previous owners put in door jamb switches and had those hooked up to the white wire for the dome light (that would ground the circuit when at least one door is open right?)

So I’ve found numerous diagrams and pictures and I think I have an idea how everything is supposed to be hooked up (novice at electrical work here)

From my understanding I need to have the orange wires from the dome light and courtesy light connected to the orange wire on the headlight switch AND to the T/L & Courtesy terminal on the fuse box for a hot wire. One problem, I’m not getting any voltage from that terminal on the fuse box (fuse is fine). Are there other fuses that can fail and not supply voltage to this terminal?

Also, if I want the dome light to turn on with my headlight knob do I just need to splice it into the white wire on the headlight switch?

I’ll upload pictures when I get a chance. Thank you in advance!



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[Rapid Roy]

Welcome to the site.

[Henry]

Hi cyates219:
Yeah, if you are not familiar with the cab lighting circuits it can be non-intuitive how it works. You definitely need a good hard copy wiring diagram and I would recommend you buy the GM wiring diagram document X-7630 ST-352-76 for 1976 trucks so you can study the fold out diagram in question. I recommend you draw out the specific circuits in question on a separate paper so you can understand it.

Dome light and cargo lamp: One orange wire from lamps (18 ORN-140) running from lamps all the way forward to the fuse panel. One white wire from lamps (18 WHT-156) running from lamps all the way forward to splice-156 either near the fuse panel or the courtesy lamp. Then one branch of this splice goes to passenger door jamb switch (18 WHT-156C) and the other goes to the courtesy lamp. They are all white wires.

Courtesy lamp: One orange wire from lamp (18 ORN-40B) goes from lamp to back of the headlight switch. One white wire from lamp (18 WHT-156) goes from lamp to back of headlight switch. Where this white wire plugs into the courtesy lamp with a connector, there is supposed to be another white wire. This other white wire (18 WHT-156B) runs to the drivers side door jamb switch.

Headlight switch: The main power coming into the switch is a red wire (12 RED-2D) that runs back to a splice near the fuse panel with a whole bunch of red wires in the splice. One of them (12 RED-2A) runs to the firewall connector and one of them (12 RED-2B ) goes to the fuse panel.

It is confusing to try to explain where the wires terminate at the back of the fuse panel and the headlight switch...the picture on the wiring diagrams show how and you should be able to match this up with your connectors that plug into them.

Regards,
Henry

[Henry]

Hi cyates219:
From my wiring diagram, this is how I see the power delivered to the circuits in question:

Red wire (12 RED-2A) comes from the firewall connector to the main power splice (splice-2) with a whole bunch of red 12 gauge wires in it...this should be near the fuse block. Then one red wire (12 RED-2B) goes from the splice-2 to the back of the fuse panel and supplies power to three fuses in a row: (1) ACCESSORY, (2) TAIL, STOP, CTSYLAMPS, (3) TRAFFIC HAZARD. The middle fuse, (2), is a 20A and this is what  orange wire 18 ORN-40A is connected to and runs to the back of the headlight switch. It is spliced at this location to wire 18 ORN-40B which runs to the courtesy lamp under the dash. The right hand fuse, (1), is a 20A and this is what orange wire 18 ORN-140 is connected to and runs to the dome/cargo lamp. The left hand fuse, (3), is a 15A and this is also a orange wire, confusingly identified as 18 ORN-140, and this wire should run to your brake light switch located on your brake pedal arm somewhere.

I have a spare headlight switch squirrelled away somewhere and if I can find it I can let you know where the white and orange wires from this light circuit are supposed to plug into...I should be able to take a photo of it.

Regards,
Henry
1976 C-20 Custom Deluxe

[cyates219]

Thank you for the all the responses Henry! I haven’t gotten time to get around with it yet but from what you’ve told me I feel like the random red wire that was attached to the orange courtesy lamp wire is supposed to be going to the back of the fuse box as you stated. Thank you for your time, I will provide updates whenever I can.

Respectfully,
Colin


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[Henry]

Hi Colin:
I have attached a photo of the headlight switch without the wire harness connector attached. If you have the original Delco-Remy switch, yours should look like this and the blade contacts are marked with a number. The connector that plugs into this should have 6 or 7 wires in it and I have attached a second picture of what the contacts on your headlight switch should connect to. Hope this helps.
Regards,
Henry