2003 Toyota Corolla - Electrical Problem; Potential Reversed Polarity on Battery

My battery died and I had a guard from work come out with his portable charger. I wasn’t paying close attention as he was hooking it up, but he sparked it and I’m not sure if he reversed the polarity and completely fried the electrical system. I replaced the battery but ever since the incident with the other guy I can’t even get it to “click over” with key in the ignition.

I discovered that I blew the ALT 100A fuse, the AM2 fuse, and the DOME fuse. Those are the only ones that I could visually confirm were blown. I replaced all and I’m getting further than I was before, but I still can’t get it to click over at all and start. So now I’m looking for some guidance on what to look at next?

  1. Is the starter relay blown?
  2. Is the ECU dead?
  3. Is the alternator fried?
  4. And how could I test these to figure out which is the issue?


I’m really sorry to hear what happened to your car. Many people charge batteries/ jumpstart cars the wrong way. The guard might have reversed the polarity.

I would start from the fuse boxes under the hood (there maybe more than one location). Check all fuses and relays. You should be able to test relays with a 9v battery and alligator clips and see if you can hear it work.

You have to diagnose this issue step by step. You won’t know if the critical systems are intact till you check the fuse box. Don’t forget that there are multiple fuse boxes inside the cabin too (under the dashboard on driver’s side and in some cars on passenger side too. See owners manual) . Check all.

If you cannot even hear a click from the starter, it is possible that the voltage is not reaching the starter motor. If you have a continuity tester, you can test the wires to see if they are damaged too. It is possible since you have multiple high amps fuses blown. After checking the fuses, diagnose the starter. With the ign key in start position, there should be about 10-11v on both terminals of the starter. If there is voltage on the starter but it won’t turn over, that is the first part to replace (assuming the wires are good). But, i highly doubt if your starter is affected.

Your best case scenario is that only the fuses are blown and you just have to replace them. This is the easiest and cheapest repair in your situation. Good luck.

There may be fusible links near the battery’s positive terminal. These are lengths of wire that act as fuses, and melt internally. If your car has them, check them for continuity.

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It sounds like you experienced a dead short or polarity reversal, given the 100 amp fuse is blown. After replacing the dead fuses you can find, if you aren’t hearing even a click with the key in “start”, don’t assume that’s caused by a burned out ECM. the ECM usually isn’t involved in the cranking operation. It’s involved in the catching and starting part, but not just the cranking, the rrr rr rrr sound. I expect you still got another fuse that’s shot. On my Corolla there’s no lengths of wire that act as fusible links, but there are fusible link cartridge-type fuses near the battery positive terminal. They have a clear piece of plastic on top so you can see the short piece metal that comprises the fusible link. Use your volt meter to test on both sides. If those are ok, you might have damaged your transmission’s neutral safety switch, so check that. If all else fails, you’ll have to start at the starter motor, measure the voltage on the two terminals that have wires going to them. If both measure over 10.5v with the key in “start” (terminal to starter case), and it doesn’t crank, you’re starter likely needs to be replaced.

Thanks for the guidance and responses! Looks like George was right. There was one last fuse that was still out that did the trick; the #18 15amp AM2 in the interior fuse box. Real pain to get to, but thankfully that was it and my ol’ girl is running again!


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Congrats and thanks for reporting back!

Good for you for sticking with it OP. It appears the AM2 fuse is in the path that sources battery power to the ignition switch. If it is blown turning the key to start won’t power up the small under dash starter relay, and when that isn’t powered up you won’t get power to the “s” terminal on the starter to make it turn the engine. Sound like after replacing that fuse, you are good to go.