I recently replaced my dash lights with a HiPo LED set, but the cluster was still dim. So I pulled the headlight switch to clean it and noticed some melting of my wires. The blue and black stripe pair each look melted in places. Do you know what these feed? They don't appear to lead to the headlight hi/lo switch. For added context, my headlights look pretty dim too - although I only have the low-power Sylvania H6054 bulbs. I was hoping to upgrade the headlights, but not until I figure out this wire situation. Thanks!

Areas of melted wires:


Just showing where the blue/black wires plug into the harness.


'78 F100

 

Bad switch ? Short somewhere ?

 

I dont know if you can get that plug as a pig tail and replace it & the melted wires.

Now to help to keep that from happening again and make the head lights brighter you need to add a head light relay kit.
I know LMC sells them and you can buy them on Ebay, I am using LMD as I have had it for years for a different project but got my 81 F100 on the road first.
It will send a full 12 volts to the head lights and the switch is only used to open & close the relay.
It is a plug & play install: unplug the head lights and plug the trucks plug into the relay harness.
The relay harness them plugs into the head lights.
Run power to the relays, has 2 one for low the other for high beam.
Runs 2 wires for ground and that is it.

I will say the relays are an odd size and if you need to replace them I got 2 normal size relays and 2 plugs to fit them.
I removed the wires from both plugs and swapped in the trucks wires into the new plug.
Now I can use normal size relay if I ever have to.

Oh how do I know this you ask?
I had a relay go bad because when I washed under the hood the relay filled up with water and stopped working.
Because I needed to use the truck before the fix I just unplugged the kit from the bulbs and plugged the trucks harness back into the bulbs and I had lights till I could fix the relay.
Dave ----

 

that would be the dome light/cargo light circuit wiring...

 

Thanks everyone! I tracked it down in the wiring manual. Forgot I bought that! That's surprising it has melted. It seems the battery would drain with a short like that, but maybe not. I suppose the wise thing would be to trace the wires through the door switches, to the dome and LH/RH courtesy lights (which I don't have), and either replace it or wrap the questionable areas with some tape.

Thanks again!

 

FuzzFace2 - After you did the relay kit, which bulbs are you using? The ones I have are just too dim to be much help!

 

X2 on headlight switch is related to dome light, dash lights, side marker and front (park) lights and of course headlights. I have always understood that the headlight switch wiring is almost to small for the amps it is carrying, and you NEVER want to tap in it to run accessories like aux/roll bar or push bar lights.

The black elec tape right behind the plug looks to be PO installed or modified. Make sure nothing else if added or tapped into the wiring harness. Before you just black tape over it I would want to know WHY the wiring it getting hot and melting. FIRE HAZZARD.


Relay harness https://shop.broncograveyard.com/197...ctinfo/24800H/

Headlights https://shop.broncograveyard.com/LEN...artments/1085/

Awesome diagram in color. https://classiccarwiring.com/product...saAjkfEALw_wcB



Ok do I have your attention now?

 

I run your normal Halogen sealed bulbs you get in any parts store.
None of them fancy glass leans with the even more fancy bulb that goes in from the rear.
I am really happy with the light out put with the sealed bulbs and relays.
When I use the truck to / from work I leave any were from 12 mid night to 3am so it is dark.

I would like to know what the voltage is at your head lights with out the relays as I know after it will be a full 12 volts.
If you lights are only getting say 10 volts they will be dim.
Dave ----

 

77&79F250 - since the melting appears limited to the black-blue wires, I suspect the issue (if it still exists) is limited to the dome lighting circuitry and switches. If this was a short or overload caused by the headlights, wouldn't there be other visible damage to wires? There should be two black-blue wires at that harness location. It appears one was pulled out at some point - hence the electrical tape to hold it back in. But it warrants some investigation.

The truck also has 2 after market fog lights and a spot light. The spot light operates independently, but the fog lights can only be toggled on when the headlights are on. Makes me think they tapped into the headlight lines. I suppose the fog lights would need a separate relay connected to a 12V switched supply line, huh? Is that the right approach for wiring fog lights so they don't accidentally stay on when the truck is off?

 

FuzzFace2 - I'll check the voltage at the bulbs. The hi beams look nice. I just wish the lo beams were similar.

 

For fog lamps ... I use a relay taking it's main power from a dedicated fused wire to battery+, but it's switching power from a toggle switch after coming off the low beam circuit.
For a spot lamp with it's own integral switch I would run off a dedicated fused lead to the battery.

In the case of your blue/black/melted insulation, if off the door /dome lights, since they have power even when the headlights are off, could be someone tapped into one them for an always available spot light feed. That melting could be "just last week" old ... or 20 years old?

 

great thought, tbear853. I checked, and they tapped the spot light into the fuse for the Emergency Flasher / Stop Lamp. I hate that practice. I'll pull it out and run a dedicated, fused supply. It's probably time to buy a small fuse box for under the hood.

 

If 1 of the black / blue wires pulled out of the holder then it could of had a bad connection and heated up and wire and not the holder.

I am with Tbear on the wiring of the fog lights.
If you run the fog light with a relay you can still have the head lights turn them on.
Just like the relay for the head lights the switch is not passing the power of the fog lights thru it, it is only the trigger to turn them on just like for the head lights.
Dave ----

 

I installed the headlight relay kit (Bronco Graveyard) and a separate fog light relay this weekend. Easy project. Thanks again for all the advice! I also installed an aux fuse panel next to the original panel to cover the fog lights, spot light, and 2 open slots for future use. Now I just need new headlight bulbs. I'm not sure what is standard these days.

I also reconnected my battery voltage gauge - However, it is now connected to an always-on feed. I've read this won't drain the battery if it's a mechanical dial. Any thoughts on that? I could track down a spare, switched source if that was better for the battery or gauge.

 

I'm glad you got the fog and spot lamps wired.

For headlights, you have the glass body sealed beams. In years of usage, they do dim some as the filament burns / ages. One day at my part time post retirement part time job I showed up early, it was still dark, buddy walking across the parking lot commented on my lit up grille. I knew my lights were dim, but they worked. I turned my headlights back on, took a look. Probably half of the reflector material had fallen in the sealed beam and the headlights were lighting up the radiator. I bought two new "Sylvania Silver Star" sealed beams and later that day, put'em in my truck ... next morning he was there again, he said "Wow". I told him that I felt like it was setting the grass along the ditches on fire now, they were suddenly so bright. I gave him the 20-25 year old ones, he wired them with a toggle to use in his old camper as overhead lighting. I'd rather have the glass sealed beams, no desire to be dealing with moisture or bulb changes through the back. I'd look for 6054H plain glass sealed beams, be they standard or Silver Star or ExtraVision ones. None of those LED things, etc ... if I had a '78 or '79.

An always connected battery volt gauge will represent a steady always low drain on a battery. If they didn't use current, they'd read right out of the box. It might run a long long time before it pulls a battery down, but it's pulling. The way to do it is use a relay that is tripped with a feed from the ignition switch accessory terminal, so it's only reading when the key is in and "on" something.