I have fought this many, many times and the fixes haven't always been as straightforward as the bigger conductor to the key switch - did that early on. Once I reluctantly went with the relay- only thing I could get to work. Worked but then in a few years I started having the issue even with the high current bulkhead starter solenoid relay, big key switch conductor and all hot terminals shined to bare metal.
Starter tested fine at the starter / alt. shop. Finally thought to upsize my negative wire to the block and get those connections clean, that did it. Eventually that starter too gave up the ghost and wouldn't consistently pull in and spin. My latest one is fine with no relay for two years. I doubt this issue will ever be fully put to rest, and my belief is the starters Yanmar has built simply aren't very rugged, well designed or both. This happens much less frequently on the millions and millions of automobiles that use onboard solenoid starters and they last many many more starts, most going from new and perfect to dead or retired over the course of a decade or more with absolutely no attention paid or problems resulting from bad connections or undersized wire. Bring on the comments that say it's all the wire sizing and connection cleanliness, to each his own. These are some finicky ass starters.
