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Alternator voltage drops almost entirely when batteries are earthed


JugaarLife

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15 hours ago, Tony Brooks said:

 

I think I have said all I can without a full engine and charging wiring diagram complete with terminal markings on the equipment. Go with what Nick says he is very hot on electronics.

You've both been awesome. Going with that, I'll report back once I've got my hands on a good leisure battery. 

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On 25/09/2022 at 09:16, Tracy D'arth said:

OK, but is there an electrical connection bus bar to hull?

Apologies, was away from the boat for a few days. No, the negative bus bar is connected to the leisure battery negative terminal.

Just now, JugaarLife said:

Apologies, was away from the boat for a few days. No, the negative bus bar is connected to the leisure battery negative terminal.

Which is not connected to the hull...

 

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25 minutes ago, Tracy D'arth said:

Where does the starter battery negative connect to?

Got terrible signal, thought I replied to this but can't see it. Apologies, my bad - both the starter negative and leisure negative connect to the bolt holding the alternator on, which is in turn connected to the hull. I'm sorry - my brain's getting fried!

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On 24/09/2022 at 12:18, Tony Brooks said:

Whatever battery is used to power the dynostart probably needs its negative connection to both the dynostatr body and the control box so you may have broken the circuit to the control box negative.

Sooo...lamp now coming on, but not going off. It is a very low revving engine, though - I'm limited in how much I can crank it up. No guage but it's low.  I'm guessing this particular model of alternator worked at some point with this engine, though. Although that is a guess. Another oddity....there is no negative cable on the Dynastart. It's bolted to the hull, but there isn't a negative back to the starter battery, which is where it gets it's power from. 

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13 minutes ago, JugaarLife said:

Sooo...lamp now coming on, but not going off. It is a very low revving engine, though - I'm limited in how much I can crank it up. No guage but it's low.  I'm guessing this particular model of alternator worked at some point with this engine, though. Although that is a guess. Another oddity....there is no negative cable on the Dynastart. It's bolted to the hull, but there isn't a negative back to the starter battery, which is where it gets it's power from. 

 

The lack of a negative on the dynostart may not be a problem, and if it were, I doubt it would start the engine. Much electrical equipment used on boats apparently has a negative terminal but when inspected that terminal is not insulated from the case. That means the negative connection to the engine block works just as well as the negative terminal. It seems your battery negative is connected to the block by the alternator bolt.

 

It is alternator speed that is important, not engine speed and that is determined by the pulley ratio. As it is a slow revving engine I would expect the alternator pulley to be around 2" in diameter and the engine pulley in excess of six inches, it could be ten inches.

 

I would not be so sure the system worked at some time. That Sterling A to B strongly suggests to me that the owner had been trying to solve charging problems.

 

Have you tried it with the charging lead from the alternator connected directly to whichever output terminal the is connected to the battery that feeds the warning lamp? This will probably be the engine battery, but that is by no means certain. That will bypass the A to B so if it still does not charge it suggest the alternator is faulty.

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1 minute ago, Tony Brooks said:

That means the negative connection to the engine block works just as well as the negative terminal. It seems your battery negative is connected to the block by the alternator bolt.

 

 

Yes, it is. Also, yes, you're right about the pulleys - they're massive, I'll abandon that train of thought. 

4 minutes ago, Tony Brooks said:

 

Have you tried it with the charging lead from the alternator connected directly to whichever output terminal the is connected to the battery that feeds the warning lamp? This will probably be the engine battery, but that is by no means certain. That will bypass the A to B so if it still does not charge it suggest the alternator is faulty.

Doing so now...

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13 minutes ago, JugaarLife said:

Yes, it is. Also, yes, you're right about the pulleys - they're massive, I'll abandon that train of thought. 

Doing so now...

 

Both pulleys driving the alternator should not and must not both be massive, only the one on the engine should be large, the alternator one should be small. That way the engine revs gets increased at the alternator. The larger the size difference the faster the alternator will run compared with engine speed.

Edited by Tony Brooks
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21 minutes ago, Tony Brooks said:

 

Both pulleys driving the alternator should not and must not both be massive, only the one on the engine should be large, the alternator one should be small. That way the engine revs gets increased at the alternator. The larger the size difference the faster the alternator will run compared with engine speed.

They are as you described in your first comment about them. The alternator one is a bit larger than 2", the engine one 10-12". I'm tired, I got lazy in my description. I realise that doesn't help, I will be more specific. 

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9 minutes ago, JugaarLife said:

They are as you described in your first comment about them. The alternator one is a bit larger than 2", the engine one 10-12". I'm tired, I got lazy in my description. I realise that doesn't help, I will be more specific. 

 

That should be fine for now.

 

Incidently I don't think that you have told us what engine it is. That always helps, as do photos.

 

I think that your engine is probably bolted down onto the steel engine beds without any rubber mounts. If so then the hull is connected to negative by the alternator mounting bolt, engine block and the mounting bolts. If the engine sits on wood make sure the bolts make good contact with the underside of the steel engine beds.

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15 minutes ago, Tony Brooks said:

 

That should be fine for now.

 

Incidently I don't think that you have told us what engine it is. That always helps, as do photos.

 

I think that your engine is probably bolted down onto the steel engine beds without any rubber mounts. If so then the hull is connected to negative by the alternator mounting bolt, engine block and the mounting bolts. If the engine sits on wood make sure the bolts make good contact with the underside of the steel engine beds.

 

Or have a cable connecting the engine to the common negative bonding point.

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5 minutes ago, StephenA said:

 

Or have a cable connecting the engine to the common negative bonding point.

 

That is for another day when it is time for best practice. For now, we are trying to solve a perplexing fault so as long as both battery bank negatives are connected to the hull and electrical equipment, as it seems to be, it will do.

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27 minutes ago, Tony Brooks said:

Incidently I don't think that you have told us what engine it is. That always helps, as do photos.

 

I think the OP mentioned earlier it is a JP1, so tickover I would guess is about 200rpm and a max speed of 1,000rpm, if it ever gets up that fast. Being s single pot it might seem a bit scary running it the fast.

 

And the alternator pulley at 2" seems unusually small. I'd guess more likely to be 2.5" so perhaps a 5:1 pulley ratio, so the alt is spinning at 1,000rpm at tickover and 5,000 rpm at full chat. 

 

I's suggest moving the alt and wrapping the drive belt directly around the whole flywheel given I think its exposed on the JP. Works well for me on my vintage lump! 

Edited by MtB
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6 minutes ago, MtB said:

 

I think the OP mentioned earlier it is a JP1, so tickover I would guess is about 200rpm and a max speed of 1,000rpm, if it ever gets up that fast. Being s single pot it might seem a bit scary running it the fast.

 

And the alternator pulley at 2" seems unusually small. I'd guess more likely to be 2.5" so perhaps a 5:1 pulley ratio, so the alt is spinning at 1,000rpm at tickover and 5,000 rpm at full chat. 

 

I's suggest moving the alt and wrapping the drive belt directly around the whole flywheel given I think its exposed on the JP. Works well for me on my vintage lump! 

Yes, it is a JP1, and you're about right with the revs. Also with the alternator pulley - I haven't actually measured it (I will), but probably closer to two and a half, I can't really run the engine now so I'm going back to this in the morning. 

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3 minutes ago, JugaarLife said:

Yes, it is a JP1, and you're about right with the revs. Also with the alternator pulley - I haven't actually measured it (I will), but probably closer to two and a half, I can't really run the engine now so I'm going back to this in the morning. 

 

Ok thanks, I thought you'd mentioned it earlier! I'm never sure about measuring the effective diameter of a small alt pulley, when the depth of the vee is significant in relation to the overall diameter. I think the true effective diameter is about one third down into the depth of the pulley vee. Or higher if it's a toothed vee belt in use. 

 

Wrapping a non-toothed vee belt around a flywheel like on a JP works fine in my experience for getting the alt speed up. Yours will still be spinning at perhaps 2-3000rpm when you raise the engine revs which should be enough to get a passable output at all, so this chat about pulley ratios is a bit of a poisson rouge for now. Your main problem lies elsewhere, I think. 

 

 

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10 minutes ago, Tony Brooks said:

Agree with what Mike said. Seems it might just be low alternator revs

 

Perhaps combined with even that oldest of classics that catches most of us out early in our tinkering careers, a loose vee belt.

 

Hands up who here has never in their life bought a new battery for the car then found out a week later it was just a loose fan belt! 

 

 

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6 minutes ago, MtB said:

 

Perhaps combined with even that oldest of classics that catches most of us out early in our tinkering careers, a loose vee belt.

 

Hands up who here has never in their life bought a new battery for the car then found out a week later it was just a loose fan belt! 

 

 

🙋 Hand Up Emoji Meaning with Pictures: from A to Z

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The rpm required to put the light out and get it self-sustaining is quite a bit more than the rpm required for it to keep the light out and generate power. Depending on the bulb wattage.

 

So I would run the engine reasonably fast and momentarily short out the bulb, putting 12v into the D+ terminal. Hopefully the light will remain out.

 

If that doesn’t work I’d suspect a faulty alternator - diodes blown or the like.

 

Bottom line is that with sufficient rpm and the warning light illuminated, it should work. If it doesn’t, the alternator is faulty.

 

If shorting the bulb does work, the situation might be helped by using a bigger wattage bulb or adding a resistor in parallel to the bulb, so the there is more initial current into D+

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