you need to start with basics 1st...............did this problem just occur? or has it been doing it? test voltage of each battery and load test each one.....take SG readings of each cell with a hydrometer, keep in mind it only takes one battery to mess things up...........also check and clean/tighten all battery cables, if your unsure about battery readings post #,s and don,t just assume they are good test them......it could also be a solenoid going bad even though its rare they go bad in that way on occasion they do but start with batteries/cables....................................