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PostPosted: Fri Oct 20, 2017 10:18 am 
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Hmm. I don't know.

That Diode should only have current flowing through it when the starter is engaged, and the current would be whatever the fuel pump requires. When the engine is running, it just has to block current from going back to the starter solenoid...an easy task for a diode. Diodes are usually quite reliable. Since you had it on the outside of the wire harness, any chance it's been overexposed to the environment (corrosion)?

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PostPosted: Tue Oct 24, 2017 12:58 pm 
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I went down this road for a while trying to find the diodes and a broken wire.....then on a whim tried jumping directly to the fuel pump and it spun right up.....But still wouldn't pull fuel from the tank. Swapped it with brand new pump and fuel came out at an alarming rate...moral of my story, I couldn't tell aurally if the pump was running at all, once confirmed it was running it turned out that the pump had worn out and wasn't working anyway....start simple (or easy)

I guess its just one more thing to think about, those diodes DO fail but before digging too much go simple.

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Last edited by jontmiller on Wed Oct 25, 2017 7:04 am, edited 1 time in total.

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PostPosted: Tue Oct 24, 2017 4:20 pm 
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Goldfish

Joined: Sat May 10, 2014 3:07 pm
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Location: Knoxville, Tn
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 . .

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PostPosted: Fri Oct 27, 2017 7:32 am 
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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?

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tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings


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PostPosted: Sun Jan 14, 2018 7:32 pm 
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Sting Ray

Joined: Mon Sep 03, 2012 9:05 am
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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.

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PostPosted: Thu Jul 12, 2018 12:41 pm 
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Sting Ray

Joined: Mon Sep 03, 2012 9:05 am
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So after numerous calls to local boat garages, I cannot find a source for a replacement diode. I'm thinking of just wiring a toggle switch to run power to the fp relay while cranking the engine, and then just turn it off once the engine fires and the alternator starts sending power to the fp. Anyone see an issue with this? I realize there's potential to forget to turn off the toggle switch, is there any potential for damaging the alternator if power feeds back through this circuit?

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PostPosted: Thu Jul 12, 2018 12:46 pm 
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Sting Ray

Joined: Mon Sep 03, 2012 9:05 am
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Or, alternatively... why couldn't I just connect a wire to the 'on' position of the ignition switch to supply power to the fp relay? I'd disconnect the line from the starter and the alternator, so as long as the key is 'on' power would flow to the fp relay. ??? There must be a reason the manufacturer didn't make it this simple?

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PostPosted: Fri Jul 13, 2018 11:22 am 
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Location: San Antonio, TX
Or maybe use a "momentary" push-button type switch that has to be held in the "On" position while cranking, but will release to the "Off" position after start-up?
Kind of a make-shift anti-theft situation also.
Something along the lines of this: https://www.amazon.com/SPST-BUTTON-SWITCH-MOMENTARY-NORMALLY/dp/B006M4ZGOI

That way, you don't have to worry about forgetting the switch in the on position. Food for thought...

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PostPosted: Mon Jul 16, 2018 1:10 pm 
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Sting Ray

Joined: Mon Sep 03, 2012 9:05 am
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Hi kmack, yeah I thought about the momentary switch as well... but then remembered that I'd need to have my hand on the throttle, key, and the momentary switch all at the same time... might get frustrating.

That said, I believe I may have found the source of the problem that has been plaguing me for several years now, masking itself as different issues. The contact from the fuel pump circuit breaker was corroded and loose... just enough to cause an intermittent connection, and therefore intermittent power to the fuel pump. In the past I've always included that contact in my diagnostics, but must have been just checking contact on the terminal screw rather than poking through the wire with a test light.

I took the circuit breaker off and cleaned the contacts on it and the wire ring terminals and reassembled, and she starts right up. Just goes to show that starting with the simplest things is always best... I'm sure many of you have said the same in the past. I sincerely appreciate everyone's input on diagnosing this gremlin, and I hope this is the last time I have to write about this condition!

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