Diesel stalls under alternator load - SailNet Community

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Old 11-26-2007
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Diesel stalls under alternator load

I've got a 4-cyl 25hp Westerbeke M4-30 in my 38' NorthStar that I've refitted with a beefier alternator, a Balmar 90 amp unit. After a typical day of sailing and draining the batteries with fridge and tunes, the alternator load appears to be contributing to engine stall when docking. I'm darn near positive it's related to the extra load that this higher output alternator is putting on the engine, but I don't know what the fix is. I'm sure more batteries (I have 3 AGMs, 1 for the engine and 2 for the house) would postpone the symptom, but that constraint must remain. For now, when I hear the engine running at lower rev, I idle up. Is there an engine setting that should be adjusted or is my throttle up approach ok?

Much obliged, as always.
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Old 11-26-2007
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If you turn off your ignition key you can take the alternator load off your engine. There will be a few here that will tell you that you can burn up your alternator that way but that hasn't been the case in the past three boats I've owned along with every single car I've ever driven. You could at least trouble shoot it a little further to confirm it's the additional load from your alternator this way.

On the other hand, turning off your battery switch while your engine is running will most likely burn your alternator out.
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Quote:
Originally Posted by erps View Post
If you turn off your ignition key you can take the alternator load off your engine. There will be a few here that will tell you that you can burn up your alternator that way but that hasn't been the case in the past three boats I've owned along with every single car I've ever driven. You could at least trouble shoot it a little further to confirm it's the additional load from your alternator this way.
I may be wrong, but I believe on my M4-30 the electric fuel pump is controlled by the ignition switch. [I am not 100%certain since this is a new-to-me boat ...]
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That's true, pegasus1457.

I also know that there are factory settings on idle and high-rev that I've always been told not to mess with. But could a new engine component be cause for tinkering with these?
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i doubt it's an alternator problem. i have a 100 amp alternator on my W30 charging 4 6v deep cycle (total 440 a hr in the house bank) and the starter battery, and i don't have any problems at idle. It is possible that the idle speed needs to be compensated for the extra load the bigger alternator is putting on it. throttling up a bit is probably the best response as adjusting idle for these conditions means it'll be running fast when your batteries are charged.
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Pub...

You really must keep your revs up when you begin re-charge from that state of discharge. If you try to charge from idle, you will give the drive belt a hard time... often it will get very hot as it slips. At higher revs, the belt tension will not be so great. An alternator output of 90 ampere at (say) 14.2 V is nearly 1300 Watt, and the belt will struggle at low revs. Low revs at 1300 watt will mean very high belt tensions.

Keep your revs up until the charge eases up in the ammeter. On no account begin re-charge from idle. Certainly go to idle after you have reduced the charge rate, but not before.

With revs up, you will not stall.
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In my experience, adding a high-output alternator can cause the engine to lug/stall at idle speeds. I would think the quick solution is to increase idle rpm slightly, say by 100 rpms or so. On our Yanmar, doing so is a very simple process (by simply adjusting a set screw that limits the throw of the throttle/cable assembly).

I am not recommending charging at idle speeds, just offering a suggestion to keep the engine from stalling during docking procedures.
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John...
Absolutely. Keep the revs up when charging, particularily when first charging after discharge. In theory, if you charge at (say) 1800 rpm, the belt tension is half what it is at 900 rpm.
I have a 100 A alternator from Ample Power of USA, and if I keep the revs up when charging, the belt seems to last a very long time... typically 3 seasons... perhaps 300 hrs. I spend a lot of time at idle, when going through the canal locks (my boat is on a canal), but as a rule, I will not charge initially at idle.
I can speak very highly of that alternator.
I have no formal interest in Ample Power, but it havs ben the best money I ever spent.
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