[Unber-j]

I have a 2001 748s. My high beam light went out and i replaced it with a new lamp and It still wont work. When the key is turned on the only way low beam will work is if the high beam switch is on otherwise it doesnt work either. Is this a bad switch or bad wiring harness

 

[bhmax16]

You're going to have to do some probing with a power probe (a must in my opinion for electrical diagnostics) or at the bare minimum be somewhat competent with a multimeter. I think it's unlikely it's your switch, and you more than likely have a wiring issue (ground or power wire bad on high beam...shorted to low beam, so when you flick the switch on high beam you get power to low beam). See where I'm going here? Check all power and ground connections for continuity from the switch to the end of the headlight harnesses. If they check out THEN you know the switch is bad.