Car wont start after it rains!
 

Please help! My 03 R53 has thrice now not started after it rained in the last 2 weeks. The first time the battery was just completely dead and I jumped it and it got going. The second time I got the same thing. The third time now the alarm wouldnt stop going off even after I disconnected the battery and I didnt have any means by which to jump it so I got a ride home. Anyone have any ideas? Battery and alternator are both new less than 6 months ago. Havent had to jump it any time other than these 3 since the new battery. Any ideas? Thanks!

The alarm was going off after you disconnected the battery?

It must have a backup power source and is monitoring the car's 12-volt power supply. Sounds like it's going off because the car's 12-volt power is being interrupted ( like disconnecting the battery).

Have you checked to see if water is leaking into the battery compartment in the boot? Sometimes the rear hatch will leak and water will wreak havock with the electronics...

Also, check all your ground straps making sure the connections are clean and tight especially in the battery box. Whoever replaced the battery may not have gotten everything torqued down and/or didn't clean the connectors. Same for all around the alternator.

+1 , that's were I would start also. I would check that main ground strap: Loose or corroded. 12429805422

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