Blew all fuses after installing battery backwards

Tiny
MONICA BIRT KAKOW
  • MEMBER
  • 1998 BMW 740
  • 0.8L
  • V8
  • 2WD
  • MANUAL
  • 240,000 MILES
Put battery on backwards cannot roll up windows and car will not start.
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Tuesday, February 13th, 2018 AT 10:59 AM

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Tiny
CARADIODOC
  • EXPERT
There are dozens of computers on your car, and they cannot tolerate reverse voltage. To prevent damage, they will have "diodes" between their 12-volt supply terminals and their ground terminals. Diodes are one-way valves for electrical current flow. They are installed backward so under normal conditions, it is like they are not even there. When the battery is connected backward, the diodes become "forward biased", and they act like a piece of wire. They become a bad short. That is what causes the fuses to blow before those computers can be damaged. Sometimes a fuse will not burn open fast enough and a computer can still be damaged. If this involves more than just fuses, the inoperative systems will need to be diagnosed, but knowing the battery was connected backward is a valuable clue in knowing what to look for.

There will be multiple fuse boxes so be sure to check all the fuses. At a minimum, there will be one under the hood and one inside the car. The owner's manual will show if there are others.
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Tuesday, February 13th, 2018 AT 4:57 PM

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