We are having a issue in our 91 f350 with the headlights. They are going on and off kinda like a strobe light. The first time it did it my husband had been driving it up a really rough road. He had been driving with lights on for about a hour. Since then its been doing it on smooth roads as well. We could be driving with the lights on for a few hours before they start "strobe lighting" . We can fix most things bt electrical and wiring is something we just dont understand. So what would be causing this issue?

 

Are the lights completely going on and off or are they just dimming and getting brighter. if they are just dimming and getting brighter I would start with checking the alternator and voltage regulator

 

They are completly going off.

 

how quickly do they flash? Is your truck diesel or gas? and are any other lights on the truck having the same problem?

 

It's the headlight switch,, the thermal breaker is opening and closing because the contacts are worn and heating up...

Also check the headlight switch plug for any burnt places or signs of overheating/ melting of the plug material..

 

They will go off for a few seconds and back on then off again. Its gas. all other lights are working.

 

Here is another thread with the same issues....
https://www.ford-trucks.com/forums/1...ght-issue.html

 

All of the other lights are on fuses, the headlight switch has a THERMAL BREAKER.. It gets hot and opens the circuit then cools then resets itself, Perfectly normal...

 

well you beat me too it white. I was leaning towards the switch but wasnt too sure since I havent really seen these switches go out all to often.

 

I've seen it hundreds of times in the last 40 or so years... These trucks and about every automobile on the road!!!

 

Actually this is the third or fourth case of flashing headlights in the last month right here in this forum...

 

Ive only been a certified mechanic for 2 years now...

 

Ok thanks White 97 xlt. this may sound really silly but where do i find the thermal breaker? I try to do most of the work on my truck and I dont know where a lot of things are. thanks guys

 

You don't!!! It's inside the headlight switch... The cure is to replace the headlight switch, it is not the thermal breakers causing the problem, it is saving your truck possibly burning from an OVERHEATING contacts in the switch.

 

Bingo...A simple swap, to even a donor trucks switch will immediately solve the issue...

Its a simple deal, pull the headlight switch ****, then the instrument bezel.....2 screws, and a pigtail swapped later...It's done...