Denso alternator Beta 20hp

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transom

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I have just replaced the alternator on my 20 year old Beta 20 engine with a like for like Denso 40 amp unit. The new alternator appears to be identcal to the old one and is charging at 14.5 amps through a Sterling smart charger. However, the tachometer has gone haywire and is showing 4000 revs at engine tickover which is about 800 revs. The company who I bought the alternator from say the tacho wire is correctly insatalled and cannot understand the problem. This company who I have used befoe are excelent and whose judgement I completely respect. I have made extensive internet searches and spoken to many people but as yet have received no explanation. It seems inconceivable that the tacho went wrong at exactly the same time as I replaced the alternator so I am reluctant to replace the tacho at £125 before I have exhausted all avenues of enquiry. Any help will be much appreciated.
 
It may not help at all, but on my slightly older Beta 622 I had tachometer problems which I managed to resolve by checking and adjusting (I think I must have fiddled with it :() a small recessed ‘knob’ with a screwdriver slot, on the back of the tacho. I’m sorry I cannot remember the details as the engine manual in which I recorded them is on the boat, but ISTR an A and a B and two numbers, for different engines / numbers of cylinders or somesuch. Beta were helpful as usual, so you might give them a call.
 
transom said:
the tachometer has gone haywire and is showing 4000 revs at engine tickover which is about 800 revs.
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Could perhaps stem from bad contact in the engine harness?
I had the same problem intermittently for some time with my Beta engine (younger, not the same alternator), although it was usually sorted by just shutting down the engine and starting again.
The problem has largely disappeared since I checked the multiplug/socket in the harness and treated with some contact spray.
 
Thank you for your suggestions. I did clean all the contacts for the new alternator in particular the earths but have not checked the multiplug on the harness which I will now do. I am reluctant to start altering the settings on the alternator just yet as I would first like to find the root cause of the problem. As with most electrical faults the solution is usually simple its just finding it that's the problem. The tacho is obviously getting the right sort of electrical signal but this signal appears to be corrupted in some way
 
Sounds a bit like your old alternator had a non insulated return and the new one is insulated. It can cause weird issues with the engine panel. Running an extra earth off the negative alternator terminal will rectify if it is indeed the issue.
 
Thank you for your reply. The Denso alternator is earthed directly through the engine block and does not have an independant earth terminal. In addition there is a diect earth wire connected to the main support bolt. I too thought the earth was probably the source of the problem but I cleaned both earthing points thoroughly and checked them with a meter.
 
transom said:
The new alternator appears to be identcal to the old one and is charging at 14.5 amps through a Sterling smart charger. However, the tachometer has gone haywire and is showing 4000 revs at engine tickover which is about 800 revs.
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Did you have the Sterling device on the old alternator?
 
transom said:
… I am reluctant to start altering the settings on the alternator just yet as I would first like to find the root cause of the problem. ...
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Sure, I well understand that reluctance - but if all else fails I would advise checking with Beta what the setting should be. Trying to remember exactly what happened, ISTR that mine went 'haywire' and having checked the connections to no avail, I may have fiddled with the setting and got it wrong - but the selector did not seem to have a very positive 'click' on the positions, so it just might have vibrated off the correct one. Anyway, after moving it positively to (and to make sure, slightly back and forth at) the correct setting as advised by Beta it did again give consistently sensible readings - though it has never zeroed fully with the engine switched off.
 
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pvb said:
Did you have the Sterling device on the old alternator?
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I was wondering if there is any confusion between the wire that has to be added to a standard alternator for a Sterling advanced alternator regulator ( I assume that is what the OP means by Sterling smart charger ?????) and the W terminal which provides the signal for the tacho.

Disconnecting the Sterling regulator will eliminate that as the cause .
 
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VicS said:
I was wondering if there is any confusion between the wire that has to be added to a standard alternator for a Sterling advanced alternator regulator ( I assume that is what the OP means by Sterling smart charger ?????) and the W terminal which provides the signal for the tacho.

Disconnecting the Sterling regulator will eliminate that as the cause .
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If you mean that the Sterling wire to the regulator and the W wire for the tacho might have been transposed, I don't think this is the case, as otherwise it wouldn't be charging at 14.5v.
 
pvb said:
If you mean that the Sterling wire to the regulator and the W wire for the tacho might have been transposed, I don't think this is the case, as otherwise it wouldn't be charging at 14.5v.
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If it is a modern alternator it might charge at 14.5 anyway.

I was wondering how the tacho would respond if it is connected to the field control .
 
The firm that supplied me with the alternator have assured me that they would not have connected the Sterling connection to the tacho as they had the original alternator in bits and merly transferrred the wires accrosss to the new alternator. If they had done so I do think the tacho would not have worked at all since it operates off ac current pulse where as the Sterling connection to a brush is a direct current. The mystery of this problem is that the tacho connection is to the stator coils and so theoretically the tacho should be working properly as there does not appear to be any way you can connect the tacho wire incorrectly. It may well be that the tacho malfunctioned at exactly the same time as I changed the alternator; stranger things have happened on boats.
 
Assuming the tacho. is OK, I wonder if the Sterling unit is putting some AC component voltage to the field, making the alternator generate a higher frequency wave-form than it should, which would appear on the W terminal.
This is a bit of a long-shot I realise.
Can the Sterling can be temporarily disconnected, thereby putting the alternator's own regulator into control, assuming it's still fitted? This might show if the tacho. reads properly.
 
Yes I can easily disconnect the brush connection to the Sterling smart charger as it is a bullet and will try this next time I am at the boat. I cannot see the two are connected but 12 volt electrics does sometimes do strange things. In the abscence of any difinitive solution I think my first action will be to remove the alternator and check the connections; even the experts can get things wrong. I then propose to put the alternator on the bench and connect it to a different tacho to see if that works. I was hoping to avoid this action as getting the alternator off and on is a pain as it is a very tight fit. I wish to thank all the contributers to this thread as it has given me some options to try and has also concentrated my mind as to where I should start. I live in hope that someone out there has had exactly the same problem and has a simple fix.
 
transom said:
The firm that supplied me with the alternator have assured me that they would not have connected the Sterling connection to the tacho as they had the original alternator in bits and merly transferrred the wires accrosss to the new alternator. If they had done so I do think the tacho would not have worked at all since it operates off ac current pulse where as the Sterling connection to a brush is a direct current. The mystery of this problem is that the tacho connection is to the stator coils and so theoretically the tacho should be working properly as there does not appear to be any way you can connect the tacho wire incorrectly. It may well be that the tacho malfunctioned at exactly the same time as I changed the alternator; stranger things have happened on boats.
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Fair enough. I do not know enough about the way in which an alternator regulator works, beyond knowing what it does, to pursue my line of thought any further. I
 
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