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 Author Topic:   Engine Starting Circuit jimh posted 09-16-2007 09:36 AM ET (US)         ENGINE STARTING CIRCUIT CURRENT PATHSHigh Current Path in very great detailPOSITIVE LEADBATTERY--battery Positive Terminal--terminal lug on wire--wire--terminal lug on wireSOLENOID--terminal on Solenoid--solenoid--terminal on Solenoid--terminal lug on wire--wire--terminal lug on wireSTARTER MOTOR--motor positive terminalNEGATIVE LEADSTARTER MOTOR--motor housing--mounting screwsENGINE CHASSIS--threaded boss on engine block--engine block--threaded boss on engine block--terminal studBATTERY--terminal lug on wire--wire--terminal lug on wire--negative terminal postLow Current Path in less detailMOTOR--battery positive primary distribution terminal post--fuse--battery power to ignition switch via wiring harness and connectors--ignition switch--neutral safety switch--wiring harness and connector--solenoid positive terminal--solenoid negative terminal--engine chassisA poor connection at any point in these paths will prevent the electric starter motor from operating.In the event of trouble in a battery operated electrical system, the first component to test is the battery.A good technique for locating a break in the current path is to use a voltmeter to measure the voltage drop across each connection. jimh posted 09-16-2007 12:02 PM ET (US)             A simple way to evaluate all the connections in the high-current portion of the engine starting circuit is to measure the voltage drop occurring across each connector. One way to accomplish this is described below:Connect the positive lead of an accurate voltmeter to the positive terminal of the battery. Connect the negative lead of the voltmeter to each point in the high-current path described above, and observe the voltage measured. If the connection is a proper electrical connection, the voltage drop across it will be very close to 0.0-volts and the meter will read no voltage drop. Beginning from the positive battery terminal and working toward the engine, testing each point while an assistant operates the ignition key to the START position. As soon as you find a connection which is open-circuit you will read 12-volts on the meter. You will have found the open branch of the circuit.In a normally operating starting circuit, the only branch of the circuit which should have any significant voltage drop across it is the starter motor, and then only during engine cranking. If the open branch of the circuit is indicated across the solenoid, this means the solenoid is either defective or not being energized.To assess the function of the solenoid, measure the voltage across its coil terminals. If the solenoid has 12-volts across its coil terminals but there is an open-circuit indicated across its contacts, the solenoid is defective. However, it is often seen that there are problems in the low-current or control wiring for the solenoid. These may be explored in a similar manner.On some OMC motors the routing of the low-current path through the remote shift and throttle controls in order to pass through the neutral safety switch is often accomplished by an unusual in-line knife-like connector which is encased in a rubber protective and insulating cover. To remove this cover to check the connector you can wet the rubber with alcohol to act as a lubricant to allow the rubber to slide off the wiring and connector.The cost of a replacement solenoid is modest, particularly so by marine electronic standards, and a new solenoid can be purchased even at retail and from an authorized OEM dealer for as little as \$25.I have had my own frustrating experience with intermittent electrical problems in outboard engine cranking, and the source was difficult to pinpoint. I ultimately cured the problem by replacement of the solenoid and by thoroughly and carefully cleaning and restoring every contact in the wiring path.

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