Headlights sending SOS

[lightfoot]

After driving for about 10 minutes the headlights on my 68 F100 240ci six start turning off and on like I’m trying to send someone SOS signals. I stopped and checked the brake lights and they’re not affected, turn signals and running lights are all ok but the headlights keep flashing off and on. The next day I had the same problem, run 10 minutes or so, and they start flickering on and off again. Any idea what’s causing this?

[Calfdemon]

Could be a bad ground and after driving for a bit, it starts vibrating loose enough to lose good contact. I know the grounds on each side by the front parking lights can cause the headlights to flicker and/or go out if they are not making good contact. I would check them first...

[lightfoot]

Could be a bad ground and after driving for a bit, it starts vibrating loose enough to lose good contact. I know the grounds on each side by the front parking lights can cause the headlights to flicker and/or go out if they are not making good contact. I would check them first...

Parking light grounds sounds like a good place to start but I don’t think anything is vibrating loose. I started the truck the next morning after this happened and turned the lights on and left the truck running in the driveway. Everything was fine for about 10 minutes but when I ame back 5 minutes later I found them flickering off and on again. Could this be a circuit somewhere that’s fails when it gets hot?

I'll check those parking light gounds tonight, thanks for the tip.

[HIO Silver]

The problem is likely the headlight switch being overheated by the headlight current. The best way to remedy the issue is to install a headlight relay harness which, instead of the current going through the switch, will directly source power from the battery. The headlight switch's voltage will just be used to trigger the relays. A big benefit is that the headlights will receive cleaner current without all the parasitic power losses through connectors and the headlight switch - that means brighter headlights.

About $30 from LMC or build your own for as little as $10.

[lightfoot]

The problem is likely the headlight switch being overheated by the headlight current. The best way to remedy the issue is to install a headlight relay harness which, instead of the current going through the switch, will directly source power from the battery. The headlight switch's voltage will just be used to trigger the relays. A big benefit is that the headlights will receive cleaner current without all the parasitic power losses through connectors and the headlight switch - that means brighter headlights.

About $30 from LMC or build your own for as little as $10.

How hard is it to install a headlight relay harness? If it’s not too much trouble can you walk me through the installation?

[Calfdemon]

If you get a pre-made one (LMC sells them), they are super easy to install.

They pretty much consist of hooking a wire to a hot lead (Starter relay), a ground, and then plugging one of your current headlight plugs into the harness. The harness itself plugs into the 2 actual headlights. The way it works is it uses the signal it gets from the stock headlight plug to open the relay and power the new harness directly from the hot source. The headlights then get their juice through this new harness instead of all of the juice travelling through the headlight switch.

Very easy upgrade and will take you about 15 minutes to install. I believe you can find the harness here for $29.95: http://www.lmctruck.com/icatalog/fb/full.aspx?Page=63

[lightfoot]

If you get a pre-made one (LMC sells them), they are super easy to install.

They pretty much consist of hooking a wire to a hot lead (Starter relay), a ground, and then plugging one of your current headlight plugs into the harness. The harness itself plugs into the 2 actual headlights. The way it works is it uses the signal it gets from the stock headlight plug to open the relay and power the new harness directly from the hot source. The headlights then get their juice through this new harness instead of all of the juice travelling through the headlight switch.

Very easy upgrade and will take you about 15 minutes to install. I believe you can find the harness here for $29.95: http://www.lmctruck.com/icatalog/fb/full.aspx?Page=63

Awesome, thanks for the link!!!

[lightfoot]

If you get a pre-made one (LMC sells them), they are super easy to install.

They pretty much consist of hooking a wire to a hot lead (Starter relay), a ground, and then plugging one of your current headlight plugs into the harness. The harness itself plugs into the 2 actual headlights. The way it works is it uses the signal it gets from the stock headlight plug to open the relay and power the new harness directly from the hot source. The headlights then get their juice through this new harness instead of all of the juice travelling through the headlight switch.

Very easy upgrade and will take you about 15 minutes to install. I believe you can find the harness here for $29.95: http://www.lmctruck.com/icatalog/fb/full.aspx?Page=63

Problem resolved thanks to your tip Calfdemon. THANK YOU!!!

[HIO Silver]

Gee, thanks.

[lightfoot]

Gee, thanks.

Doh!!! I see it was you that first made that suggestion, sorry about that HIO Silver.
Much appreciated, and a relatively simple job. Thanks, and I have to say this forum is a great help to amateur mechanics like me.

[Calfdemon]

Yup! I just piggybacked his suggestion as it slipped my mind when I first responded. All I did was provide you the link. Glad that solved your problem!