Yanmar 3GM 30F stopped while motoring, won’t re start.

[Cheechako]

Originally Posted by skipperpete

Here’s an update on the 3GM. The engine just did an 8 hour run after the new OEM drive plate was installed with seemingly no dramatic problems or seizures. It’s now possible to rotate the engine by getting a good hand grip on the front pulley with the decomp levers engaged.
Now there’s a new problem...The engine barely idles and cuts out when returning to idle in gear ( which made docking a bit problematic). I found out that the alternator is part of the problem because at startup the panel light stayed on while the engine idled happily in the normal way but as soon as the revs were increased in neutral to make the light go out , a slight belt squeal could be heard and when returned to idle the revs dropped to an unhappy 500rpm. In gear there was a lot of dead lever travel till the revs started to pick up and once past 1500 all seemed ok, with no belt squeal. The journey proceeded uneventfully from there on at 2,500 rpm using both engines but with noticeably different lever positions.
After docking I took the belt off the alternator then ran the engine and ALL the symptoms totally disappeared so it’s definitely related to the alternator and here’s where I’m a bit puzzled........ is it totally an alternator problem or is it a combination of low engine power and the alternator drag?
Has anyone else on the forum had these problems with a 3GM30?
Pete J

I have had that problem. The belt squealed and smoked. It's because when you idle up faster the regulator tells the alternator to kick in. If you have a higher output alternator than stock it can be too much for a single belt system. Mine was a 100 amp alternator.

If yours is stock and doing this, you either have the wrong belt or the alternator bearings are maybe shot? Does it spin easily by hand with the belt off?

There's also a situation where the alternator will run like a motor from the battery, maybe that could do it against the diesel engine? That means a blown diode in the alternator IIRC.

[Wotname]

What Cheechako said plus -

Assuming both alternators are charging all batteries while underway, there should be very little alternator load after 8 hours of running i.e all batteries should be fully charged.

Thus the alternator drag should be almost nothing and certainly not dragging down the idle rpm down to 500.

Again assuming both alternators are paralleled, run up the known good side so that alternator is supplying all the load required and then retry the problem side as the alternator drag on the problem side should be minimal while the good side is providing the load as required.

Or - swap alternators and see if the fault is transferred or not.

[skipperpete]

The port alternator was replaced in Cairns and has only done the hours for the trip to Southport but I don’t know the amp output..... not a heavyweight like Balmar though and is internally regulated. It got warm but not too hot to touch and the belt lost a bit of bark over the recent 8 hour trip. Interestingly, I slackened off the belt to the point where I could turn the pulley by hand and it still pulled the revs down to 500 so I took the belt off completely and she ran nicely ( for about 5 minutes). The batteries are both new, located in the saloon, on independent systems ( no paralleling link) and stayed cool for the 8 hours. I didn’t have a multimeter so no clue as to the alternator voltage or output but the effect on the engine is immediate when the charge light goes off.
I really like the “Swap em over” idea Wottie but I was keen to get off the boat yesterday..... the traffic on the inland route to Bris was incredible and now I really really hate jet skis. I might get down there during the week and do as you suggest but the owner wants to get that alternator to an auto electrician for a health check first.
We were just happy that the crankshaft thrust bearing never seized again, but this new problem is an annoyance. Incidentally, if that alternator light and the little belt squeal hadn’t tipped me off, I’d be on the verge of blaming the injector pump or governor.
Pete J