Mainship Starting Interlocks

I have an intermittent problem starting the 370 HP Yanmar engine on my 2006 Mainship 34 Trawler. This is the 4th year we've owned the boat, and this year is the first year this has occurred.



1. I switch the "Eng 1" switch on (is a single engine boat).

2. I press the button on the electronic engine control - orange indicator is on.
3. I turn on the engine key switch - immediately the "low oil pressure" alarm sounds - as usual.
4. And I press the start switch - nothing. No mechanical noise, no starter grinding, nothing.


My question is what signals would interrupt engine starting. For example, the fire suppression alarm would I believe shut off the engine. Are there other alarm conditions that would prevent starting the engine. By the way, once started the engine has run fine.


At first I thought it might be a "low voltage" interlock preventing starting the engine - since it seemed to be an issue if I disconnected the AC power from the dock prior to turning on the electronics and then starting up the engine.



This last time, after refueling at the fuel dock, the engine was off for 15 to 30 minutes (all electronics on) and wouldn't start. I finally got it to start after charging the batteries with the generator for 30 minutes. But the battery voltage was not low - maybe 12.5V on the small panel meter.


Also, the electronic engine controller appeared to be working - I could hear it shift the transmission for example. So I don't think that was blocking startup.



The yard load tested the batteries - all tested ok. The mechanic couldn't reproduce the problem - engine started fine.



Yesterday I went down to boat figuring I could reproduce the problem. Turned off the battery charger, put a DC load on the batteries for a while, tried turning the engine on and it started up. Kept load on the batteries for a while still started up every time.


Only difference was the last couple of days the outside temp drop down 20 degrees.


And one final piece of evidence - the "low oil pressure" alarm is normally silenced with the "off" position of the engine start switch at the helm. But when this problem occurs and the engine won't start - the alarm buzzer cannot be silenced.


Any ideas?

You Should have a neutral safety switch. This switch might need to be adjusted.

tiltrider1 said:

You Should have a neutral safety switch. This switch might need to be adjusted.

Click to expand...

My first thought as well... jiggle the shifter fwdm rev, neutral several times and try again.

I don't think his electronic controls have a neutral safety switch. I think the controls won't activate unless they are in neutral. (He says "I press the button on the electronic control-orange indicator light is on." This is the same starting sequence as our boat, so I understand what he is saying.


To me, this sounds like the same problem I ran into about a year ago albeit on a different Yanmar motor, 4LHA-STP rather than a 370 hp Yanmar; a failed start relay. It can be an intermittent failure.


I posted a thread about this sometime back. I will see if I can dig it up and post a link in another post.

Hey Doug, you make me want to get a spare start relay. Do you still have the part number handy to pass along? 4LHA-STP. Thanks.

What's up Gary?


Here is the relay, I bought a spare too.


129100-77910 Starter Relay


For some odd reason, the relay is on the opposite side of the motor from the starter, and my stb one went bad, so it was on the outside of the motor and not super easy to reach. But it is doable. About an hour and a half job, as I had to replace a couple of terminals as well. Diagnosing it was the hard part, I got a lot help from the TF.

After re-reading the OP's post, I'd be willing to bet a very small sum of money that it is his start relay. The guys at the Yanmar dealership where I get my parts said it is a very common failure. It's easy for me when diagnosing this kind of thing to get led astray by what seems like contributing evidence, but is really just coincidence. I think the OP is facing some of that.


Then again, I could be wrong. I'm often wrong.


I looked closely at the port side for reference when I replaced the stb one as the port side is easy to reach and see. One of the underappreciated benefits of twins, lol.

Thanks Doug, just ordered one as a spare. I don't have any issues now but it's a good spare to have. Is that the relay behind the corrugated cable cover in this picture?

No, I don’t think so. It’s closer to the on engine fuel filter. When you get the apare in you will be able to compare and recognize it.