
Originally Posted by
woohoo
The reason? See this:
The positive cable when unhooked from a battery almost always causes a small spark which is harmful to electronic circuit boards present on almost all cars nowadays. Unhooking the negative cable first prevents static sparking and "leaking" voltage from escaping the circuit.
For the same reason, when using jumper cables, connect the negative of the dead battery last.
Nothing happens to the vehicle if you connect the negative side first. But is is safer to connect the positive side first.
Reason: If the negative side is already connected and you put the positive side on you take a chance of touching a part of the body or radiator with your wrench while tightening it, causing the positive side to ground out.
If your negative side is not connected yet and you touch the wrench to the body while tightening the positive side nothing will happen.
So installing the positive side first reduces the chance of shorting the battery out which could cause the battery to explode.