Cross-connecting the positive and negative terminals on a battery. Lucky it's just the engine that won't start.

Dear Tom and Ray:

I was in a hurry to reconnect my battery, and I cross-connected the positive and negative terminals. A rather large spark occurred. Now the engine won't start at all. Any suggestions?
-- Mike

RAY: Yeah. Don't be in such a hurry next time, Mike.

TOM: You probably blew a fusible link. A fusible link is sort of a "main fuse," like you have in your house. It's the first line of defense, and it keeps an electrical surge from ruining every other electronic device up the line.

RAY: In a car, fusible links are usually placed very close to the battery so that if anything goes wrong, they're the first thing to blow. Most cars have several of them -- between the battery and the ignition switch, and between the battery and the fuse box, for instance.

TOM: And, like fuses, they're basically lengths of wire designed to carry a certain number of amps. If the amperage exceeds that limit, they melt and break the circuit. They give up their lives to protect stuff further up the line from getting fried.

RAY: The reason cars have them is so that if some moron connects the battery cables backward, he won't ruin every electrical component in the car, including the computer, which costs a thousand bucks to replace.

TOM: So, my guess is that you blew one or more fusible links. They'll probably cost you 100 bucks or so to replace. You should kiss the old ones and thank them for saving you a lot more than that, Mike.
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