hi.........on the 250sl you have side covers on the main carriage x 2, held by 2 x philip screws at bottom on each........depending on your s/l being l/h or r/h on stairs ,you will find behind one cover 2 x 12v batteries held by a zinc plate which also needs to be taken out to remove batteries/test..[must turn the carriage isolator switch on front of carriage off before removing batteries] behind the other carriage cover is the main pcb which i would leave alone until you have tested the batteries as these do need replacing....hoped this has helped.........regards
IAN
March 2011