So let me start with: I have a 2003 Ford F-350 Super Duty with the 7.3L Diesel engine and regular cab, XLT trim. I have a plow and a custom bed (wooden flatbed). Full disclosure, this truck is in tough shape; the body is in tough shape and there's a fair amount of corrosion on the underside.

I THINK I know the answer here, but looking for smarter people than me to either confirm or deny.

I have a Fisher plow, and it uses a Douglas Dynamics 4-port Isolation module. Plow lights work as they should. Without the plow, the passenger headlight (both high beam and low beam; it worked before, but I did replace the bulb anyway for brightness continuity between the sides) and all running lights/turn signals work as they should. Driver's side (figures, since it's the harder one to access!) running lights/turn signals work as they should. Even after replacing the bulb, low beam is VERY dim, almost unnoticeable in daylight, and high beam does not work at all. This weekend I plan to meter the short harness between the isolation module and the light.

I'm guessing, and please correct me if I'm wrong, but since passenger headlight works as advertised, and I am getting SOME power to the driver headlight, that this isn't a fuse or relay issue. I don't really know - yet - how to test the isolation module, but they don't typically go bad, and I think I'm stuck with a bad wire between the IM and the headlight itself. Does this track? Would a ground issue cause these symptoms?

Thanks in advance for any help/insight.

 

Check the grounds, run a jumper from the battery to the HL ground and see if it brightens.