I'm having trouble with the head lights on my 1978 F700. It's driving me nuts. I know I can wire past the whole switch, but the lights work every now and then, so it should not be a huge deal. I'm thinking there must be a relay or something, but can't find it. I have power all the way to the hi/lo switch, but nothing to the lights them selves. Running lights work fine. When the lights do work, the hi beam indicator on the dash still works as well if that helps.

 

Hi, if I understand you. the head lights are intermitant. The probable cause is the dimmer switch, or the wires at the dimmer switch. I have had the same problem, switch would get hot on high beam and quit. Switch is cheep, check it out, replace if needed.Good luck. OBCB

 

Thanks for the reply. I did replace the dimmer switch as well as the main headlight switch. Still no headlights. The head light switch has a good ground as well as the headlights themselves. This is why I think there must be a relay or solenoid in the wiring somewhere.

 

According to the diagram I have there are no relays.The wire go from dimmer to headlights

 

Thanks for the reply. I'll try to run new power wires from the dimmer then.

 

I finally got it fingered out. I had traced the wires under the dash several times, but I obviously missed one several times. I decided to trace them once more and found a wire that led to an unmarked dash mounted switch. The switch is very close to the PTO/dump *****. The previous owner had told me the switch did nothing. Guess he was wrong. Intermittently I guess I bumped the switch when dumping. This is why the lights would work sometimes and not sometimes. I believe the p.o. used to plow with the truck. The switch must have been for fog lights or something. My guess is running them this way, you could not leave them on by mistake. Thanks for all the help.

 

Prevpous owners! Go figure. Glad you got a handie on it.Much more fun to drive um.OBCB

 

Hey rbaker6336. I know this is over a decade now, but I’m hitting desperation as no one I’ve contacted to purchase a service manual or wiring diagrams has been able to help out, so out of curiosity, you wouldn’t by chance have the wiring schematic for the starting/ ignition circuit for that 78 F700 available that you might be willing to share? I’m actually working on an 81 F700 bit of I can get an idea of what’s supposed to be going on here that would be immensely helpful as the former owner did a crazy hack job on all the wiring after severely burning up the starter. (So much so, a witness said the starter was glowing red, and the 2g ground cable from battery to engine ground split through insulation over a 10” section). I don’t know whether this truck is does you have 1 or 2 fusible links or the guage they are supposed to be bc the PO just “got rid of a couple wires that looked burnt that he didn’t know where they went”

Any help would be greatly appreciated! Thank you so much!

 

These folks might have what you need:

https://www.faxonautoliterature.com/Product/Index/24699