i have a 1997 f350 with a 7.3 powerstroke and it is doing something that is just driving me crazy because I can't find the problem, the problem I am having is with the speedometer, at times it works just fine but other times it just jumps around all over the place or doesn't work at. now here is the troubling part lets say I'm going down the road and its working well when I turn my headlights on the speedometer will stop and the abs light will come on, I can turn my lights off and the speedometer will start working again I haven't started changing parts yet I was hoping someone on here could give me some advice on where to start.

 

You may not have any defective parts, but what you could have is a poor ground in the system where it is using the other systems to get a path to ground.
Starting right at the battery , check the ground connections to all the sheet metal especially those up near the radiator and headlight area.

 

I checked the only one that I could find by the battery, it had about 3 smaller wires on it and one larger one that comes from the battery, do you know the location of any others I need to check?