I have a 1995 fullsize bronco with the 5.8, recently I have replaced the battery and the alternator. My problem is that the engine starts and runs fine...until you turn on the light switch and the engine dies, turn the lights off and it starts right up. Also the engine will not start with the switch on, but starts fine with it off. My local parts shop has been very helpful, and doesn't think it is the switch?

I don't know, and live up in the mountains with a 30 mile drive to the parts house or any test equipment. Anyone else ever have this problem and if so...what did you do to fix it? I don't want to just keep trading out parts.

Thanks for your help...

 

I'd say you have a short in a major wire and when you turn on the lights your completing the short and if it's bad enough it'll over power the ignition and or ecm circuits killing the engine. Any sparks or blown fuses? Does everything electrical in the vehicle die when you flip the switch, and if not what other circuits are affected?

 

I was going to say that we just had a car in that had such a bad battery in it when you hit the brakes it would kill the car because of the brake lights, but with a new battery and alternator, thats a tricky one.

The headlight switch is not very complicated, or expensive (15 bucks i think), have you ever modified any wires under your dashboard or have you accidentally stuck your foot up there or anything. It sounds like a short to me, but with that kind of load it would fry any kind of wiring used for a standard healdight circut instantly. Any aftermarket lights/alamrs/remote starters installed or anything like that?

 

All of the wiring is stock, with no additions...pretty much a stock Bronco at least in the wiring arena. I plan to pick up a headlight switch tomorrow and see if that helps. All the other electrical works fine. thanks for the help

 

electricity will take the path of least resistance. So when you turn your headlights on all of the power that is going to your ignition system is going into your headlight circut. I just dont know if there are any wires that go from your ignition circut to your headlight circut.

 

Cannot say why or just what it fixed, but replacing the headlight switch fixed it. Engine starts, runs, with the headlights on or off, no matter what I do, it seems to be fixed. the only difference I notice is the engine changes rpm slightly when you turn the lights on now, and before it just died~!

Thanks Bronco lovers Bronco forum - 1 Parts Store - 0!

LC

 

You might also want to try checking the ground on the inner fender near the battery! My dad had a 94 f150 and when you would turn the lights on it would kill the truck. It ended up being a bad ground. You may want to check just to make sure you didn't just get lucky.

 

Will do...thanks...

 

Let us know what you find...that's a real bummer!

Of course, you've probably already thought of the ovious...like don't drive at night...

 

headlights switch kills motor what do I do

 

Did you read the posts above suggesting a new headlight switch and checking the ground connection? Have you tried both of those and still have the issue? Have you replaced the battery and confirmed the alternator is working properly? What else have you checked? Do you have access to a wiring diagram for the Bronco and at least have some basic tools for troubleshooting electrical circuits?

-Rod