No power to headlights

[84lngbed]

I have no power going to the connector for the headlight switch at all. Is there a fuseable link in the bulkhead block or something? The truck is a 84 with daul headlights.

[gafiredog]

had the same problem with my 86 about a year ago. I had no lights anywhere. no headlights, taillights, dash or or anything. after tracing all the wires I could for 2 days, and not finding anything broken, or any fuseable links, I finally found a hot terminal in the fuse box and ran a wire from there to the pink wire in the plug that plugs into the headlight switch, and problem sloved, still working great after a year or longer.

[MikeB]

Headlight switch power typically comes right off the 12-volt battery buss in the fuse panel, which in turn is fed straight off the battery terminal or starter via a fusible link.

However I have seen some aftermarket fuse panels where headlight switch power comes off a thermal breaker that looks similar to a plug-in fuse.

If the engine starts, that means there is power on the buss, so you'd really have a strange problem if there's no 12 volt power reaching the switch. Did this just happen out of the blue, or after some electrical work had been done?

Is it possible the wire that runs from fuse panel to headlight switch broke loose from the fuse panel?

Another thing: Some headlight switches have a separate fused 12 volt input for tail lights and front parking lights. Do those lights work?

[gmachinz]

Power does not come from battery per se, but from the firewall common junction behind and above your distributor. This junction receives full voltage from the alternator output post and distributes power from there-including all the BAT and fused IGN buss bars within your fuse panel. Probe both ends of each fusible link with a test light @ your common junction and see if you have a break in any of them. Are you sure your headlight grounds are good too? Did you probe your tan and light green leads with a test light to verify they are indeed dead coming out of your switch?

[gmachinz]

On some schematics GM also used the starter as a secondary common +12V junction (which I think is an outdated circuit design and I don't use in my harnesses) so there may be a bad fusible link there as well.

[MikeB]

Originally Posted by gmachinz

Power does not come from battery per se, but from the firewall common junction behind and above your distributor.

On my 82 C10, terminal 2J on the firewall connector block is fed DIRECTLY from the starter BAT terminal. However, terminal 2E is fed via the firewall junction block, which is also where the ALT output terminal is tied to. Really weird, but the schematic backs it up. However, I'm not sure if those both feed a common buss on the fuse panel.

Not sure why PictureTrail or this site is shrinking the pic below, but you can use your browser controls to zoom in. I've circled the starter, junction block, alternator, and firewall connector block. Quite a bit going on compared to my 69 pickup!

[gmachinz]

There's 2 reason why GM did that-one is that by using the starter lug as a battery junction you can have your charge lead routed from there to the junction. And, GM decided hey this is a good spot to also provide 12V batt power from too! Now, why GM would do that instead of just pulling everything off the common junction is beyond me. They certainly don't do it that way anymore so I opt to update my harnesses as such.

[84lngbed]

Originally Posted by gmachinz

There's 2 reason why GM did that-one is that by using the starter lug as a battery junction you can have your charge lead routed from there to the junction. And, GM decided hey this is a good spot to also provide 12V batt power from too! Now, why GM would do that instead of just pulling everything off the common junction is beyond me. They certainly don't do it that way anymore so I opt to update my harnesses as such.

On your harness. You changed so the alternator is the 12v right? Not power to the fuse block for where the headlights, park lamps, and horns fuses and plug in are located.

[gmachinz]

GM used both the starter post and junction for +12V sources. I prefer to use the common junction as the sole source for power-this way everything basically pulls 14V+ from the alternator. The battery needs only a charge lead from the junction. So yes my power pulls from the junction.

[84lngbed]

If I put 12 volts to the red wire on the head light switch all running lights and headlights come on take it away nothing. On my harnes I had alt end plug changed to the CS style I was using. The S wire (which is the sensor wire if I am correct?) on the Cs alt plug the sensor wire is normally ran to the alt charging stud or to a power junction point which would make me think the sensor wire doesn't supply power but reads it, right? But on my harnes the sensor wire on alt plug is ran to firewall bulk head in spot 2. the bulk head spot 2 is supposedly the spot for head light power which the wire is ran to the fire wall junction block on the factory to wiring diagram from one posted above and one I have.
Let me know your thoughts.

My brother looked over everything and says we need to cut the sensor wire so it doesn't go to the bulk head but instead it needs to go to the firewall junction block and run a new wire from the bulk head spot 2 to the firewall junction block so headlights get power.

[gmachinz]

That would work. As long as the sense wire is at a common point be it the junction or starter lug it should be ok. Ideally the common junction would be the way to go in your case....run the sense wire from alt to junction, then from junction to the bulkhead connector.