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I tried the test in this video. https://youtu.be/dhf6IdBbE3Y The alternator is putting out 14.45 volts, the diode test passes but it seems to be putting out a lot of AC according to my meter 30 volts. I wonder if I am measuring the right thing. The excitor wire has 12.2 volts at the alternator end when running. 

 

I am on the boat until I get back home from the challenge now so can quickly try anything people suggest.

 

Cheers Graham

 

 

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16 minutes ago, Graham and Jo said:

I tried the test in this video. https://youtu.be/dhf6IdBbE3Y

Why?

 

Why didn’t you do the test described by Tony B in this post?

 

Depending on the multimeter, many of them will give a spurious reading on an AC range when connected to a D.C. voltage - it means nothing. 

 

If you’ve measured 14.45V on B+ and 12.2V at the same time on D then you have a blown diode as Snib described. 

 

If your readings were accurate that would have shown 2.25V between B+ and D

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Problem solved, I don't have much time to fix this so just got a new alternator from Calcutt. The alarm is silent when running.

 

Thanks for all the advice, I really appreciate it.

 

Cheers Graham

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4 minutes ago, Graham and Jo said:

Problem solved, I don't have much time to fix this so just got a new alternator from Calcutt. The alarm is silent when running.

 

Thanks for all the advice, I really appreciate it.

 

Cheers Graham

And the charge light no longer glimmers?

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24 minutes ago, WotEver said:

And the charge light no longer glimmers?

Sorry for the slight delay in my normal prompt service but I had to put the tonneau cover back on to make it dark enough to be sure. The charge light is dark! Yes! 

 

Don't worry I have checked that the light comes on when the engine is stopped and that the alarm does function. So they are both working properly for the first time in years. 

 

Cheers Graham

Edited by Graham and Jo
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There was a little bit more to tell about this issue but I got sidetracked with Birmingham. 

 

When I removed the old alternator it had a wire connected to the w pin. I think this is intended to feed a rev counter but Dotterel hasn't got one. As I removed the wire the split charge replay vibrated sending a nice bit of back emf up the wire enough to make me jump. 

 

I was a bit confused by this but decided to tape up the wire and fit the new alternator. 

 

When I started the boat the alternator worked, loads of volts, no alarm, No warning light and at that point no domestic battery charge. 

 

Confused I was. Anyway after quick chat with the boatyard I removed the split charge relay and replaced it with one of the newer voltage sensing devices. This doesn't have any wires vanishing into the loom so I didn't need to find out how the split charge relay was wired or how it should have been wired. 

 

Now everything worked.

 

Two interesting side effects.

 

1) I had noticed in the past that when the batteries were isolated the florescent lights would still glow slightly if you switched one on. Now they don't.

 

2) As Dotterel has an Aldi boiler with a separate pump you can use the pump to heat the radiators from the engine using the hot water tank as a heat exchanger! The radiators would get warm but not hot, now they get hot presumably as soon result of more volts on the pump.

 

Cheers Graham

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What happened when you removed the the W wire is that it was being used to energise the split charge relay. When you disconnected it the magnetic field around the relay coil collapsed, producing a fairly voltage that gave you the shock. You would portably have heard the relay cilick out.

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1 hour ago, Tony Brooks said:

What happened when you removed the the W wire is that it was being used to energise the split charge relay. When you disconnected it the magnetic field around the relay coil collapsed, producing a fairly voltage that gave you the shock. You would portably have heard the relay cilick out.

Colloquially known as ‘back EMF’

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2 hours ago, Tony Brooks said:

What happened when you removed the the W wire is that it was being used to energise the split charge relay. When you disconnected it the magnetic field around the relay coil collapsed, producing a fairly voltage that gave you the shock. You would portably have heard the relay cilick out.

Yes, as I mentioned in my post it was the back emf that caused the shock from the oscillating relay but why was the W line connected to the relay?

 

Cheers Graham

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16 minutes ago, Graham and Jo said:

why was the W line connected to the relay?

Convenient place to feed the relay from. At tickover the voltage might well be too low to operate the relay but as soon as the revs increase so does the voltage and the relay drops in. 

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5 minutes ago, WotEver said:

Convenient place to feed the relay from. At tickover the voltage might well be too low to operate the relay but as soon as the revs increase so does the voltage and the relay drops in. 

I thought the W line provided the stator pulse to a rev counter. Also why would the relay respond when I removed the lead on an isolated stationary engine?

 

Cheers Graham

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5 minutes ago, Graham and Jo said:

I thought the W line provided the stator pulse to a rev counter. Also why would the relay respond when I removed the lead on an isolated stationary engine?

Yes it does (provide a stator pulse) but it would pull in a relay. It may be that the alternator objected to that load on the W terminal and that’s what killed it. As to why the relay was energised with the engine stationary, the alternator was faulty, was it not? Probably a shorted diode internally. 

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34 minutes ago, WotEver said:

Yes it does (provide a stator pulse) but it would pull in a relay. It may be that the alternator objected to that load on the W terminal and that’s what killed it. As to why the relay was energised with the engine stationary, the alternator was faulty, was it not? Probably a shorted diode internally. 

Agreed (edited comment)

 

 

That is the danger of feeding the relay from W although mine has been fine for years. My rely used to gently sing to itself because of the half wave pulse but this winter I did what the OP did and changed it for a VSR so the solar would charge both battery banks. The alternative to using W is feed it from the ignition aux output but that will connect both batteries before you starts. Alternatively it sometimes works to connect it to the alternator side of the ignition warning lamp (D+) n the alternator but sometimes the warning lamp current can still energise the alternator before the engine starts and it may also make the alternator cut in speed higher. Its all compromise.

Edited by Tony Brooks

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2 minutes ago, Tony Brooks said:

Which was Sir N's specific suggestion of a field diode while some of us just said diode.

 

 

 

36 minutes ago, WotEver said:

Yes it does (provide a stator pulse) but it would pull in a relay. It may be that the alternator objected to that load on the W terminal and that’s what killed it. As to why the relay was energised with the engine stationary, the alternator was faulty, was it not? Probably a shorted diode internally. 

Thanks for the replies guys, it all works now but I do like to try and understand what happened.  I am a lot more knowledgeable about the boat's electrics now than when I started.

 

Cheers Graham

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Note: I quickly edited my reply quoted above because it was rubbish, for the faulty diode to feed the W terminal it would have to be on or more main diodes. A blown field diode would, I think, only do it if the ignition was on.

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12 minutes ago, Tony Brooks said:

A blown field diode would, I think, only do it if the ignition was on.

We don’t actually know that the ignition wasn’t on, only that the engine wasn’t running. So, to clear this up, @Graham and Jo, was the ignition on at the time?

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6 minutes ago, WotEver said:

We don’t actually know that the ignition wasn’t on, only that the engine wasn’t running. So, to clear this up, @Graham and Jo, was the ignition on at the time?

When I disconnected the W lead, the main battery isolation switch ( in the negative side of the system) was off and the ignition switch was also off. The split charge relay was connected to the starter battery negative terminal bypassing the isolation switch ( I discovered this when I took it off). 

 

Cheers Graham

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This gets weirder unless the wire connecting the split charge relay to the battery negative was the thin negative relay coil wire.

 

The only reason to fit a SCR  in the negative is if you have a positive earth boat and that is bad news for steel boats. I have not seen a positive earth alternator for maybe 30 years so its unlikely.

 

So it looks as if the wire was the thin negative coil wire and it was a main doide that had shorted.

 

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19 minutes ago, Tony Brooks said:

This gets weirder unless the wire connecting the split charge relay to the battery negative was the thin negative relay coil wire.

 

The only reason to fit a SCR  in the negative is if you have a positive earth boat and that is bad news for steel boats. I have not seen a positive earth alternator for maybe 30 years so its unlikely.

 

So it looks as if the wire was the thin negative coil wire and it was a main doide that had shorted.

 

 

Yes it was the negative coil wire. The relay connects the positive terminals. I guess the relay was picking up positive from the W lead so was on when I disconnected it.

 

Cheers Graham

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