It's always a good idea to disconnect the battery whenever you work under a vehicle. I remember about 10 yrs ago during Bullrun (coast to coast timed race), a participant put his Lamborghini Diablo on front jackstands, he was working underneath with the key on, jumped the starter circut with a tool, car jumped foward off the stand, crushed/ran over him and proceeded across the garage area into a Ferarri F40 and Gumpert Apollo. UGLY SITUATION!!!! A battery switch could have avoided the carnage.

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'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!