Re: 1987 mercruiser 165hp 3.7
OK it is not that hard: Just have to go step by step. First this is how it works: When the engine is turning over with the starter, +12V goes to the + side of the coil from the slave solenoid out through the - through through the closed points to ground. That current builds up a magnetic field in the primary windings of the coil. As the engine turns one of the lobes in the distributor causes the points to open stopping the current flow and the magnetic field collapses causing a voltage buildup in the secondary windings of the coil (20,000 volts or so). That high voltage travels from the center wire of the coil to the center of the cap, down into the rotor, which if timed correctly is pointed at one of the posts on the distributer. The spark jumps the gap and travels up into the plug wire, on to the spark plug where it jumps the gap to ground. So you got no spark. So no voltage no spark, as the engine is cranking, make sure battery voltage is being applied to the + side of the coil. Next if the points do not open and close properly no spark. Clean them up with a fine file (not sandpaper). With the key on, you should have 7 volts or so at the + side of the coil (reduced by the resistance wire). One way to check for spark, take off the distributor cap, take the center wire and place it about 1/4 inch from the block. Turn the key on. If the points are closed, open them with an insulated tool, every time they open you should get a spark. If open, short across them, and every time you remove the jumper you should get a spark. There is not much more that can go wrong. Sorry to be so long winded. Good luck.
