Expert: tyler boehner - 2/24/2008

Question
Hi Tyler...I've got a problem with an 08 50HP Mercury 4 stroke. When I added some lights to my boat they worked for a while and recently quit. I've discovered that my engine to boat ground is no longer effedtive. I will now run these grounded to the battery but I need to locate the defective ground wire on the engine. The engine runs OK and of course is grounded by the neg cable to the starter but because the engine is isolated by rubber mounts there should be a ground wire from the engine back to boat. Any clues where I might look for this connection?

Answer
Your motor has its own ground wire that runs from under the starter to the battery.  The boat itself has it's own ground wire that runs from the battery to the negative side of the fuse block.  That is what you need to be looking at.  On your battery, there will be at least two sets of wires plugged into each terminal.  The thick cables going to the battery is your motor connections and the smaller red and black wires are for the boat side.  Ignore the motor and start tracking your problem from the battery.