Can jump starting a car with a battery charger burn the ECU?

Kind of a rookie mistake but today I left my lights on, on my 91 Mazda Miata went to jump start it with a battery charger but accidentally attached the positive and negative cables on the wrong terminals(both cables are black give me a tiny bit of slack) only for a couple seconds once I realized I switched them back, car still wouldn’t crank or start and I’ve heard jump starting it wrong can fry the ECU. Some places say only on newish veichles but I don’t know, the lights still go up and down when hooked up to the battery charger. I haven’t had a chance to further test the battery. If the ECU was fried would the lights still go up and down? How likely is it my ECU is gone?

Yes, the EC can be fried by reverse polarity. But there should be a fuse protecting it.

The lights have nothing to do with the ECU.

Are you using a battery charger or a jump pack?

Trial and error, try the cheapest things first. New battery try one?

Hm I just figured if the EC didn’t work nothing electrical on the car would work as well, also it was a jump pack

The problem with connecting any jumper backwards is, you run the amps thru the circuits before they encounter the fuses.

So an electrical component may fry first before the fuse.

So, yes.


Current cannot flow through a component until there is a path from one polarity to the other. So whether the electrons (or pick your favorite description of electrical flow) encounter the fuse first or last is of no consequence. What does matter is reversing polarity on a junction that is not designed to function that way. Then the component can burn up before the fuse can react.

Electrons flow from negative to positive.

OP could have a dead battery with an internal fault, or bad connections. Either could keep a jump or a battery charger from working. The reversed hookup might have harmed nothing.


So the Main fuse 80Amp blew nothing else, Replaced it and it was back up and running. Thank you guys for the help!

That’s great news. You dodged a bullet.

1 Like

Yeah. Usually the $500 computer will blow to protect the 50 cent fuse.

I would check main battery or alternator fuse under .

I don’t disagree with anything in that article. But unknown how you got from there (or anywhere else for that matter) that the position of the fuse in the circuit matters. Because it doesn’t and would defy basic electronics theory.

I don’t know about anyone else, but every wiring diagram I’ve looked at for a vehicle shows the fuses on the + side of the circuit, before the circuit. And not on the - side of the circuit, after the circuit?


I have repaired a number of vehicles that have the jumper cables connected backwards and in most cases the only damage was blown fuses. Fuses will blow no matter which side of the circuit they are on.

Electrons, and therefore current, run from negative to positive. I imagine fuse are put on the positive leg of the circuit because many devices use their metal cases attached to metal on the body for their negative leg. While this is less true in modern cars, the fuses remain there for convention.