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On my old boat, it was voltage drop due to the size of the wire used from the key switch to the solenoid. As it aged, its ability to support the necessary voltage degraded. This apparently is a common issue with Yanmar engines to the point I saw a 'repair" columnist in Sailing magazine discuss it. His solution which was also discussed on Sailnet previously was to install a second solenoid. I'm sure if you search sailnet, you can find the discussion and the details.
 

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Firstly, Mainsail is pointing in the right direction. It is most likely not the batteries, but somewhere along the line the resistance is to high. Note that the Voltage drop over the resistance is R*I, and I being in the 200-300 Amps, then even a small resistance will result in rather considerable voltage drop which in its turn will show up as as a buzz in the solenoid, but no more.
My guess, there is not much info to go on so it has to be guesses, is that your cable is too thin. Maybe just intended to take some charging (which usually is in the range of 5-20 Amps, most often i the lower range), then small cross area is not a problem. There are other things to say about this, depending on how your charging relays are working etc.

Also the switch is probably not built for these currents of 200-300 A. Simply, you are not intended to start your engine in this way. For the occasional start using the support batteries use jumper cables. Some few $, a good investment.


Secondly, yes Yanmars may be difficult to start. Symptom: engine doesn't start when strating bottom is pressed, one has to do this 3-4-xx times and the it starts. That is usually another story:
For some reason Yanmar is taking the complete solenoid current through the starter key, ie from starting battery up to the key (often in the ****-pit) and then back again to the solenoid. Typical current is some few amps, but the wires are rather thin. There are also usually one or two connectors on the way, and then the key in itself.
If this happens then:
a) open key lock and clean. Easy to do. Bottom is a solid plate of cupper, clean, even sand it. (It is obvious that this becomes corruded after some years in a salty environment, so why Yanmar has this solution is unclear).
b) open up the connector(s) and clean.
If this doesn't helt then you have to invest some $5 in a rely that you place on the soleniod, and control by the the key switch and wire (some easy chaning the wiring). This will usually fix all these cases.

/J
 

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I am no electrician so I always start troubleshooting with the most simplest, hoping to solve my problem without a headache. First thought would be, are the house bank batteries fully charged when you try to start the engine? I know when I first started boating I kept forgetting to turn the selector switch from the #1 starting bank to #2 or all position in order to get the house batteries charged up before shutting off the engine. Second gremlin I had to deal with was a short somewhere in the house bank which caused the batteries to become drawn way down during long periods of non-use. Not being an electrician my solution was to disconnect the cables from the house bank batteries when I was leaving the boat for a few weeks or more.
 

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After a long head scratch over similar problems I focused on the either/neither/both switch. Turned out the internals had gummed up over the years and a good cleaning solved the issue.
 
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