I have a 2004 ford f-350 super duty gas motor just bought it.Tried driving it last night.When you start it the headlights wont work,turn the truck off and the headlights come on.The high beam light on the dash(blue light) will still work even when truck is running and the headlights arent working.There is a remote starter installed that the guy I bought the truck from couldnt find the remote,if that helps?

 

If you can get to the bulbs easily, unplug them and with the headlight switch on, probe the plug and see if you have voltage. I suspect if you put your testlight alligator clip on a piece of metal on the truck, and then probe the plug, it will light. Then take the alligator clip off the metal ground and try to put it on the ground terminal in the plug. If the testlight won't light(make sure the truck is running) I suspect the ground is missing to the lights. When the truck is off the lights are finding a ground through a different circuit, but when the truck is running, that path is blocked so they cut off.

In my state, a vehicle new or used must pass state inspection before it's sold. If you have such a law in your state, you can take it back and complain about it, and they are obligated to fix it. It may have been wrecked in the front, and someone forgot to hook a ground back up, or the ground spot was painted and the paint is blocking the ground.

 

Thanks I'll try that and let you know how I make out.I bought it off a private owner,and I knew it had certain issues but he never mentioned the lights.