Ok, so after 18 years of the lights in the instrument panel not working, I decided to finally change the bulbs... to my surprise as soon as I started unscrewing it, the instrument panel literally disintegrated! The plastic was in such a bad shape that it would break just by touching it... so today paid a visit to my local Pick'n'Pull and scored a an instrument panel which to my surprise came with a new voltage regulator and new bulbs(new, in packages taped to the back)

So... got home, cleaned the terminals and installed it, gave it a try using the bulbs it came with, and after 18 years all the lights worked, but sadly my happiness lasted a few minutes after I decided to test brake, headlights, and signal lights....

NOTE: Truck still running yet, and its been about a year since I last checked the exterior lights, last time I remember they were working..

*Headlights (low/high beams) = Working

*Taillights = Not working Brake Lights stuck (probably the pedal switch, BUT worked last time , ill replace the switch)

*Left signal lights = Semi Working, rear work, but front don't...

*Right Signal Lights = Semi Working, rear work, again front don't...

*Reverse Lights = Not working....

*Hazard Lights = Not working (when they are turned on, both lights in the instrument panel stay on , but if I touch or semi move the hazard switch it acts like if it has static, by that I mean the lights flicker.. and not to mention didn't notice them working either... )

Now, I've checked the bulbs, wiring, and fuses, everything seems to be in order, even removed the Instrument panel just to compare it with my old one, and its identical.... removed all fuses since I noticed a bit of rust, will buy new ones tomorrow. The ONLY things I've done has been replacing the panel and removing an after market "Sure Stop Brake Controller" that wasn't even connected, well actually ONLY one wire was connected to the brake pedal switch, the rest were rotten out under the hood, there were even wires missing, so I decided to uninstall it..

Any ideas what could be the problem? any help will be highly appreciated, thanks!

 

Most likely bad grounds. Run a wire to the frame from the signal light housing to test.or a jumper cable to ground.

 

I did a basic test with a wire screwed into the frame, and the other end in the light housing like you described, sadly nothing, tried moving the wire to different location still nothing.... I don't understand how I ended up with bad grounds.... nothing has changed to the wiring after a year when they were working, well just disconnected that one cable to the brake switch (which was doing nothing)

Well I guess tomorrow Ill spend more time on this issue, hopefully I figure this out..

Edit ^: Actually I just remembered the cables for the Brake Proportioning Valve are disconnected since I'm working on them, AND the cables for the Dual Tank Valve are disconnected as well since I want to use only the rear tank, would this be the cause?

 

Try grounding directly to the batt negative terminal with a jumper cable. All my light problems were caused by bad grounds. They sort of work so not likely its a bad fuse. Old truck wiring harness connections get corroded and make poor connections as well. If you disconnect the instrument cluster connector, do they work? Signal lights should work without the cluster plugged in. They may not blink, but they should come on.

 

I think it's bad grounds too, you may even have to replace some of the light bulb sockets. When i got my 73 i had the same problems,some times they would work and then they would quit . Cleaned up the connections ,grounds and replaced a few sockets , so far still working.

 

Ill give it a try tomorrow without the cluster, tried the direct method with a jumper cable, nothing....

 

Look for the squar-ish 4 way plug on each side fender apron, c-801 on wiring schematic for my 78.

The colouring might be different but hope this helps:

Park (Brown), Left Turn (Green/White) or Right Turn (White/Blue), and High beam Low Beam wires (Green/Black Red/Black). Test from these plugs to determine which way to continue diagnoses, either towards the cab, or the front end wiring.

Also, I really don't like the old lamp sockets and will be upgrading mine to the newer sockets which happen to fit the older trucks.