If you're seeing large sparks when you connect the last battery cable, suspect the diodes have become shorted in the alternator. Those are one-way valves for electrical current. Current from the alternator can flow through them to the battery and other circuits in the car, but they block flow from the battery when the engine is off. In effect they appear as a dead short when the battery is connected backwards. They can't handle the resulting huge current so they overheat and short within seconds.
Unbolt the large output wire from the back of the alternator to see if the battery still drains. If it does not, replace the alternator. There will typically be other fuses blown too. Luckily you still have one of the "good" cars like I drive that only has the one Engine Computer. It has diodes in it that only turn on and act like a short circuit when the battery is connected backwards. That is to force it to blow a fuse to protect it from further damage. The radio has similar protection diodes.
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Friday, June 3rd, 2011 AT 8:36 AM
