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73-87 Chevy _ GMC Trucks => Electrical => Topic started by: Enginerd on February 05, 2013, 09:17:05 pm
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Hi guys. I need some help from the electrically knowledgable members here, please. This is kinda long, so please bear with me. ;D
I have a 78 C-10 (also a Big 10) that I have owned for about a year. The headlight high beams have never worked since I've owned it. I've fixed many other issues with the truck, and now I am working on getting the high beams working. The truck had both original 1978 power beam headlights when I got it (7 inch round lights). Low beams worked fine, but when I used the floor mounted dimmer switch to go to high beams, both lights went out! The blue high beam indicator lights in the speeedometer would light up, but there is absolutely no light from either headlight when high beams are selected. Then, when I used the floor mounted dimmer switch to go back to low beams, they come back on and work just fine. :o
I installed new Sylvania 6024 Silverstar headlights thinking that it was possible that both original headlights had lost the high beam filament (yes, it was a slim possibility.....LOL). I also installed the auxillary headlight harness kit with relays. I now have absolutely awesomely bright low beams. But still no high beams. :-\
I then replaced the floor mounted dimmer switch. Still no high beams. :-\
I then replaced the headlight switch. Still no high beams. :-\
I must have a wiring issue in the high beam wiring? Is there another relay or something that could need replacement? A fusible link maybe? Let me just add that this truck has had ALOT of half-done wiring attempts made by previous owners. I am really amazed that this is the only electrical problem the truck has.
Can someone please suggest where I should look, or what I should try next?
Thanks,
Gary
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Anyone have any suggestions at all?
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Your description seems straight forward enough. Use a test light to probe for power on the green wire at either headlamp. If it's dead at the headlamps, then verify power on the green wire at the dimmer switch. If green is dead at the lamps and hot at the dimmer, use the diagrams in our Technical Pages to start tracing the roughly six feet of green wire between the dimmer switch and the lamps. If you get different results from probing the circuit or something doesn't make sense, post it up. Let us know how it turns out for you.
From our Technical Pages:
headlamp switch and dimmer switch wiring (http://www.73-87chevytrucks.com/techinfo/wiring_diagrams/77-80_cab_inter.jpg)
bulkhead wiring (http://www.73-87chevytrucks.com/techinfo/wiring_diagrams/77-80_SBC_eng.jpg)
headlamp wiring (http://www.73-87chevytrucks.com/techinfo/wiring_diagrams/77-80_250_I6_eng_frt_light.jpg)
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Gary, I will add, there are no fusible links or relays in that section of circuit "from the factory." If it's a cobbled harness, anything is possible....
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Your description seems straight forward enough. Use a test light to probe for power on the green wire at either headlamp. If it's dead at the headlamps, then verify power on the green wire at the dimmer switch. If green is dead at the lamps and hot at the dimmer, use the diagrams in our Technical Pages to start tracing the roughly six feet of green wire between the dimmer switch and the lamps. If you get different results from probing the circuit or something doesn't make sense, post it up. Let us know how it turns out for you.
From our Technical Pages:
headlamp switch and dimmer switch wiring (http://www.73-87chevytrucks.com/techinfo/wiring_diagrams/77-80_cab_inter.jpg)
bulkhead wiring (http://www.73-87chevytrucks.com/techinfo/wiring_diagrams/77-80_SBC_eng.jpg)
headlamp wiring (http://www.73-87chevytrucks.com/techinfo/wiring_diagrams/77-80_250_I6_eng_frt_light.jpg)
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Gary, I will add, there are no fusible links or relays in that section of circuit "from the factory." If it's a cobbled harness, anything is possible....
Thanks for the help Rich! I have a direction to go on now.