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My second alternator - retrofitted to deal with my five leisure batteries, has always needed an extra few revs when started from cold, just  to get 'excited' and kill the buzzer.  Not a problem, but after pushing the engine hard on a river, knocking back the speed to tick over causes the buzzer to come on again - any ideas please? 

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Us it just a buzzer, or is there a light as well?

The usual cure with a light is either to use a more powerful light, wire a second one in parallel with the first, or put a suitable resistance across the light to increase the current taken.

I imagine the same applies with a buzzer.

This fix is needed with some "vintage" engines that rev slowly, and hence don't spin the alternator fast enough.  If you have the issue with a modern engine, then I'm wondering if the pulley ratio is high enough.  Typically the engine pulley should be around 3 times the diameter of the alternator one, to spin the alternator 3 times as fast as the engine.  Spinning it slower works of course, (provided it energises), but you may be loosing some of your charging capacity at normal cruising revs as a result.

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Tighten the belt .... change the pulley size .... increase the resistance of the buzzer/bulb ..... get into techno music so it sounds like music to your ears ....... jam knitting needles down your tabs so you don't hear it ...... strip all the electrics out and rely on candles.

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2 minutes ago, homer2911 said:

Thanks for your quick reply. It's the light as well as the buzzer. 

Extra resistance in parallel then, but in a modern installation with a modern engine I would have though it should not be required.  To me it says either wrong pulley ratio, or that the alternator is not performing quite as it should.  Do you know what the pulley ratio is?

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8 minutes ago, BWM said:

Is the increase in resistance just a matter of installing a higher wattage bulb?

I have not found that to be enough on the various HA2 & HA3 Listers we have had fitted in in either of our boats, (either A127 or ACR5 alternators), or at least not using the type of small bulbs normally used on a panel - a headlight bulb would no doubt be enough!  However these 1960s marine engines have a maximum speed of just 1,800 RPM.  On a Lister LPWS with the right pulley ratios I wouldn't expect a lot of extra "encouragement" would be needed, because both its normal operating RPM, and even its tickover, will be so much higher.

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15 minutes ago, homer2911 said:

Far from a modern engine! Original 1992 Lister 40hp LPWS4. Alternator fitted about 12 yrs ago. As I said, only annoying after hard river work 

Modern engine by my standards - ours are all over 50 years old.

Seems to be specced to be capable of up to about 3,000 RPM?

I assume as soon as you are at more than tick-over the buzzer goes off again?  Extra resistor in parallel to buzzer and bulb should cure, but I still think there has to be a reason why it is happening.  Assuming your other alternator doesn't there is clearly something different in the set up of this one - or is this the only one with a buzzer?

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from memory 47ohm 10watt resistor in parrallel with the lamp. It will get very hot if the ignition is on and engine not running........

Thinking about it your problem is the other way round, the alternator is not producing enough volts/current to shut the buzzer up. May be a fault in the alternator (diode gone?)

Edited by Loddon
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4 minutes ago, homer2911 said:

The original 70amp alternator, now entrusted solely with charging the starter, doesn't have a buzzer, and yes, increasing the revs a little does stop the buzzing and kill the light 

Does it have a warning light, or does it energise with neither?  If "neither" how would you know if it stopped charging the starter battery?

Are both alternators driven by the same fan belt, with similar sized alternator pulleys, or are each driven by a different belt?  If the latter are the pulleys at either end of similar dimensions, or is it possible there is a bigger "step up" on the original alternator than the new one?

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2 minutes ago, Loddon said:

from memory 47ohm 10watt resistor in parrallel with the lamp. It will get very hot if the ignition is on and engine not running.......

On my Beta when the batteries are fully charged and the engine is ticking over the buzzer chirps, have never got to the cause I just never use tickover its easier.

I always put that down to a slightly soggy oil pressure switch

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6 minutes ago, Loddon said:

from memory 47ohm 10watt resistor in parrallel with the lamp. It will get very hot if the ignition is on and engine not running.......

I think that;s about what I use, (from memory also......)

However 47 ohms across 12 volts is only about .26 Amps, so not much over 3 watts.  I don't think a decent 10 watt rated resistor will get excessively hot in that case, (mine don't).

None the less I make sure they are in free air, and none of the surrounding wiring can touch them, just in case.

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10 minutes ago, homer2911 said:

Each alternator has its own pulley and belt, the starter battery one with its original poly-v, and the other with twin vee- belts. Not sure about ratios, and it's dark now. Thanks fellas for your input 

Interesting, the additional pulley is often a poly v that is actualy smaller in diameter than the one its attached to. I wonder if your twin vee is from the "fire pump" spec engine.

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Once an alternator has energised they normally stay energised from the supply via the field diodes. I have never experimented but I suspect the field diode output would have to drop to well under 10V for it to de-energise. Once the belts have been ruled out my suspicion is that one or more diodes or a poor solder joint have failed.

However some sort of resistance between the alternator output terminal and warning lamp bulb (back through the batteries and ignition switch) could also cause this when the alternator tries to supply the engine electrics by back-feeding through the warning lamp and buzzer. Check all the terminals and connections, including the multi-plug on the main engine wiring harness if you have one. Remove, clean to bright metal, dress with Vaseline and replace all the battery terminals just in case.

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10 minutes ago, Tony Brooks said:

Once an alternator has energised they normally stay energised from the supply via the field diodes. I have never experimented but I suspect the field diode output would have to drop to well under 10V for it to de-energise. Once the belts have been ruled out my suspicion is that one or more diodes or a poor solder joint have failed.

However some sort of resistance between the alternator output terminal and warning lamp bulb (back through the batteries and ignition switch) could also cause this when the alternator tries to supply the engine electrics by back-feeding through the warning lamp and buzzer. Check all the terminals and connections, including the multi-plug on the main engine wiring harness if you have one. Remove, clean to bright metal, dress with Vaseline and replace all the battery terminals just in case.

This ^ a bigger bulb will not help, the bulb current gives the alternator something to start it off, if it is not self sustaining after having cut in then it's either faulty or not spinning fast enough.

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1 hour ago, Tony Brooks said:

A quick test. Put a voltmeter set to 20 volts DC between the main alternator output terminal (B+) and the warning lamp terminal (D+). Start and rev engine. If the voltmeter reads a volt or more you have faulty diodes

Acknowledgements to Sir N.

Many thanks Tony, and others who have been helpful. We've just locked down onto the Severn, headed north, so I will try your test later, or when we're back on our Shroppie mooring in a few days time. 

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