I have a 2000 150hp johnson sea pro. The problem I'm having is, I keep getting a low oil warning light and horn on my system check gauge. The oil tank is 3/4 full and I've checked the wiring to the motor. My question is, How do I check the sending unit? When I put an ohmmeter across the two terminals where the black wire and the tan w/blk go I don't read anything in ohms.