What happens if you accidentally reverse polarity?

MoffatdMoffatd Member Posts: 5
edited February 2022 in Hyundai
My 2007 Hyundai Accent 1.6L 4 door began to turn over more slowly than normal so I suspected a tired battery, however, the 'battery' trouble light had never illuminated while driving. When replacing the battery I managed to reverse the polarity for a brief moment (big spark), however, the car started and the lights worked so I thought there had been no damage. I didn't check the radio or heating fan (blower) at that time.
The starts were normal at first, then gradually got slower and reving the engine did not brighten the headlights so I suspected the alternator and replaced it, whereupon the 'battery' trouble light appeared and stayed on after starting. All the lights still work, however, the radio and blower won't turn on.
The battery fusible links under the hood and the fuses for radio and blower under the dash all have good continuity and the blower relay closes when energized.
Any suggestions for where to look/test next would be appreciated.


  • MoffatdMoffatd Member Posts: 5
    I bought a rebuilt alternator, so perhaps it is defective and not charging at all and the low voltage is preventing the blower relay from closing. My 'drive clean' diagnostic tool indicates a system voltage of 10 volts.
    I called the auto vendor and they have agreed to trade me up to a 'better' model.
  • MoffatdMoffatd Member Posts: 5
    Installed the 'new' alternator with no change; the battery trouble light came on, blower and audio won't power up, no charge happening.
  • MoffatdMoffatd Member Posts: 5
    I guess the only thing left to do is check all the fuses and relays...
  • thecardoc3thecardoc3 Member Posts: 5,603
    That is where you need to start. The 125 amp main fuse connects the alternator output and the blower 40 amp fuse. The radio does not share that same circuit. You will probably need to troubleshoot each system that has suffered a failure one at a time.
  • MoffatdMoffatd Member Posts: 5
    You are spot on!
    I can't believe I pulled 125 amps from that reversed battery, so perhaps the old fuse was derated a lot. I made up a 70 amp fuse from 1mm of solid copper until I buy the replacement and bingo, the blower returned and some more searching found the melted audio fuse.
    Oh happy days.
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