Turning on headlights cuts engine. Sometimes won’t start at all even with lights off. All dash lights go out as well. I.e. when auxiliary is on only radio works, turning on headlights shuts off radio. Recently got a used alternator. Could not find any issues pertaining to the alternator. New battery as well.
You probably have a bad connection coming from the battery or a bad ground connection between the battery,frame or engine.
I’ll second that RemcoW. And raise with the suggestion that if the battery cables are cleaned and snugged and the problem remains pull the carpet back and look for the grounds for the ECM and dash lights.
Agreed…I would grab yourself a voltmeter and do a test on the battery…What is the voltage at rest and while running? Sounds like you have just enough battery juice to start the engine and run it…but when you add the headlights into the mix…the engine dies because the battery is not being successfully recharged by the alternator…Due to either bad, loose or dirty connections (Pos or Neg), or a faulty alternator. You have some homework to do…Let us know