dash lights, interior lights, tail lights and park lights won't work

[bryson.feiring]

I'm new to this forum first off so nice to meet yall.  OK so I need help figuring out a problem and I can't seem to find what exactly I'm looking for. My dash lights and interior lights quit working on me one day and then about a week later I lost my tail lights and park light, side markers as well. The thing is my headlights (low and high) work as well as turn signals and brake lights so it's not the bulbs. I've tried using different headlight switches from the local scrap yard but still the problem persists. I'm lost on what to do and the Haynes manual I have isn't helping much. Any help would be much appreciated!

[bake74]

     First off welcome to the site.  But for future reference, you need to relay all pertinent information.  We do not know your truck, so telling us the year, model, make (I assume 73-87 Chevy, but assume is key word here).
     In the tech section here we have the electrical wiring diagrams, also we have the service manuals for our trucks.
     You can use the links in green to search the wiring diagrams and manuals.  Also, did you check to make sure you have good grounds on your truck.  Grounds play an important part of lighting on our trucks.
     You will need to start tracing the power supply for the systems and then tell us what you find, or do not find so we can help.

[CipherLogic]

Sounds like a bad/loose ground wire. Everything needs to be grounded to everything else umpteen different times and 1 bad ground can ruin your day. 

[bryson.feiring]

I guess year and model would be a good start lol. It's a 76 k20 single cab long bed. I don't feel like it's the ground because the lights are still getting power when activated through other sources such as the brake lights and turn signal. My best guess was getting a new headlight switch but all that proved was the original was just fine. I will be following the harness with a multimeter and circuit tester after work to see what I can find. Thanks for the help!

[bd]

I'm new to this forum first off so nice to meet yall.  OK so I need help figuring out a problem and I can't seem to find what exactly I'm looking for. My dash lights and interior lights quit working on me one day and then about a week later I lost my tail lights and park light, side markers as well. The thing is my headlights (low and high) work as well as turn signals and brake lights so it's not the bulbs. I've tried using different headlight switches from the local scrap yard but still the problem persists. I'm lost on what to do and the Haynes manual I have isn't helping much. Any help would be much appreciated!

Hello Bryson.  Welcome to the Forum!

Referencing the 1976 Wiring Manual, the dash, tail, park, side marker and under dash courtesy lamps are powered through the shared 20-amp, "TAIL LPS - CTSY" fuse.  The dash lamps are redundantly protected by the ~4-amp "PANEL LIGHTS" fuse connected in series with the tail lamp fuse.  The tail lamp fuse is hot at all times; the panel lamp fuse is hot only with the headlamp switch on and the instrument lamp dimmer rotated away from "DIM."  The voltage measured at the panel lamp fuse will vary from 12 volts (bright) to <<12 volts (dim).  The overhead dome lamp is powered through the 20-amp "ACC FUSED" fuse.

Check both fuses and check the fuse sockets for corrosion.  If the fuse(s) are blown, you may need to start tracing wire.  There are common trouble spots on our trucks for shorted wiring - at the back of the bed, over the rear axle where the harness runs close to the exhaust, and under the dash in the vicinity of the fuse block.

Now, as to the grounds...
If you want to maintain your sanity, never assume ground connections are in good shape.  And, don't try to rationalize a ground problem.  Instead, make it your standard routine to verify grounds by testing and/or servicing.  Although a simple 'open' ground (a totally incomplete circuit that grounds independently of other circuits) will prevent circuit operation, 'floating' grounds (several circuits 'grounding' through a common point that isn't actually connected to ground) will cause mysterious back feeds and senseless symptoms as circuits seemingly turn on and off with no pattern and for no apparent reason.  When a floating ground occurs, current can find alternate paths back to battery that were never intended. 

You've undoubtedly seen a taillight on one side of a vehicle flash alternately with the turn signal on the opposite side - or something similar.  This occurs when a poor ground on one lamp causes that lamp to ground through the common wiring and opposite side lamp.  Instead of a tail filament grounding through its base because of a poorly grounded socket, it may 'ground' through its mated brake filament and the brake filament of the opposite lamp, assuming the opposite lamp is grounded.  The possibilities and potential symptoms can be mind numbing.  You get the idea?  When you encounter a puzzling electrical issue, always verify your grounds early in your diagnosis.

I hope this helps you get started.

[pholliday1]

I'm giving the standard check the factory wires that go to your radio advice. Routinely when an aftermarket radio is installed the "hot" wire coming over from the head light switch grounds out and pops the fuse. Welcome to the site!!!

[bryson.feiring]

First off thanks for the replies! Secondly is a little update.  I finally got around to testing wires.  I started with the light switch itself and immediately there is a problem.  I tested first the power wire and that is good however once the switch is in full on position only the headlight wire (light blue) is getting power. I've tested every wire in the switch harness and nothing except headlight.  I've also tried this with three different switches and I'm not getting any different results! I also confirmed this with a multimeter.  12v to the headlight and nothing anywhere else. Does anyone happen to know if the light switch itself is supposed to have a ground? Just a thought.  If anyone has suggestions on what course of action I should take feel free to throw them at me! At this point I'm going to search around in my dash a little more and see what I can find!

[bryson.feiring]

Oh yes I forgot to mention the fuses are good!

[bd]

You posted that you have power to the headlights.  Do you have power to the tail/courtesy fuse?