1978 F250 Ranger (Lariat)

Here is a link to the wiring diagrams. http://bbbind.com/free_tsb.html

First I would suspect the dimmer switch on the steering column or wiring in the column. Second I would suspect a bad ground wire or ground connection. Here is link to a forum where others are having lighting problems with daylight running lights. The whole DRL system is rather a complicated maze of relays and connections, any of which could cause your problem. The tip on the forum to get a wiring diagram form Autozone looks like a good deal.

Headlight Wiring Diagram 02 F250 DRL Ford Truck Enthusiasts Forums

All the details are shown in these 1978 Ford truck wiring diagrams https://www.fordshopmanual.com/ford-truck-wiring-diagrams/1978-ford-truck-wiring-diagrams-pdf-download/

The basic 4-flat trailer wiring colors are as follows.
Yellow-Left turn/Stop, Green- Right turn/Stop, Brown-Running Lights and White-Ground
7-way trailer wiring colors are as follows.
Yellow-Left turn/Stop, Green- Right turn/Stop, Brown-Running Lights, White-Ground, Red or Black-auxiliary power, Blue-Trailer Brakes and Purple-Reverse lights

first thing i would check is the wiring were the plow wiring is spliced in at this is most likly the culprate. or you may have a bad multi function switch in the steering column

why do you need a diagram?, if you have a new one, just route and install it just like the old one.

yes i do but i will have to get back to you keep in touch i have a 460 but i will have to look at it lol and get back to you


Submission Mike

This shop manual will help you https://www.fordshopmanual.com/ford-truck-shop-manuals/1978-ford-truck-shop-manual-pdf-download/

It is not downloadable, but the library will probably have a Mitchell repair manual with wiring diagrams.