So my gf has a 2000 Monte Carlo SS and I really need helh with this one. At first the battery would go dead afteranot starting a couple of days, put in a different battery and get same results so its not that. i decided it had to be a drain after the sudden large spark when hooking up the cable. like when something is on. so took the multimeter out and found 0.4 amps through the BCM fuse. I notice a click and the instrument cluster goes on for a second as if I put the key on. also the flood interior light gives me 2.5v with fuse out and 10.5v with fuse in. that is suspicious.

any help? what is my electrical problem and what can I do?

When you do the drain test, are you taking an immediate reading, or are you giving the modules, 5-10 minutes to go to sleep?

What I usually do is disconnect the neg cable.....then I attach a jumper between the neg terminal of the battery and the negative cable.....I make sure everything is off and there is no key in the ignition, all doors are closed, and if there is an underhood light, I disconnect it......

After 10 minutes, I then attach my ampmeter in parallel with the jumper, and then I disconnect the jumper to get my amp reading.....

The reason you do this, is because when you disconnect the battery and then attach your ampmeter immediately, all the modules are awake......and this will cause an initially high reading......as time goes by, modules one by one go to sleep and the reading will gradually drop.....after 10 minutes, they should all be asleep and you will get a true reading....

On cars with many modules like PA'* and Cadillacs, it can sometimes take upwards of 20 minutes......I chased a draw, on a PA that never was, long ago....