I have a 1996 Z28 M6. The ABS INOP light has been coming on intermittently since I had the engine rebuilt(AIR pump removed also). In March I installed a set of GMPP polished valve covers and had to grind a little of the back of the alternator case so it would fit(I read here that this has been done before). Recently the ABS light is coming on more often and I seem to have charging problems. right after I start the car, the voltage seems ok but after I drive the car for a while it will start to drop and fluctuate, sometimes to almost 12 volts on the gauge. Then today I tried to start the car and the battery was completely dead! When I jumpstarted it, there was a big spark off the negative cable were it was clamped to the frame.
Any ideas on what could cause this and were to start looking?
Matt

 

I think you already nailed it. Your alternator is not charging properly.

 

What would cause the battery to completely drain overnight?
Matt

 

The car is what's draining the battery. With the alternator not charging, the car has to run strictly off the battery.

 

I had the alternator tested today and it was fine. This is a shop that only rebuilds starters and alternators. The owner(who I know personally) said it was probably the battery. I'm taking the car to the same shop tomorrow to have it checked out.
Matt

 

If the battery is not accepting the charge from the alt, then same result-dead. Maybe you have a bad cell.