rpengr wrote:
VolDad wrote:
A solenoid is a large inductor. When the flow of current to an inductor is shut off, it will generate a voltage spike. The diode is most likely there to catch the spike so it will not damage other electronic components. The diode has a reverse break-down voltage, you should make sure this diode is rated to at least 80V. (Higher would not hurt) But higher voltage usually are slower, and the reason for the diode is to catch a rather quickly rising pulse. So higher, faster . .
All true, but...
A diode used to shunt a voltage spike would be connected between the two leads of the coil. In this case, the diode is in the wiring harness leading to the alternator and fuel pump. Seems to me that this diode is being used as a "one way valve" to allow current to the fuel pump while the starter is cranking, but to prevent alternator charging current from going back the opposite direction on that same wire. Thoughts?
Sorry for the late response guys, thank you for your input.
rpengr is correct, in this case the diode is used as a one-way valve to activate the fuel pump while cranking... once the alternator is generating enough electricity there's another wire from the alternator that runs the pump, and the diode prevents the current from backfeeding to the starter. The first time the diode failed, I experienced a situation just before total failure where the starter was engaging while the engine was running....(ting ting ting ting goes the flex plate!) shortly after that the diode failed completely and the boat wouldn't start. Replacing the diode with one I had from another electrical harness resolved the issue.
Since then we've had a couple of instances where the boat died, which had me convinced that the boat was running out of fuel and my fuel guage was just bad. After some experimenting (ie. I carried some gas with me and ran the boat WAYYYY longer than I had evern dared before without filling up) I determined that the guage is indeed working, so it's not running our of fuel... there was just something else going on. This time the boat just died and wouldn't start... testing the fp relay connections with a test light shows that there's no current going to the relay when cranking it over, so I'm fairly confident it's the same thing again. I'll maybe get the alternator tested, but I guess I could also just rewire the point where the diode is in the harness so it's easier to get at, and just carry some extra diodes with me to get me by in the mean time.
Thanks,
C.