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Old 05-05-2009, 08:59   #1
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Yanmar Start Buzzer Question

I have a small cat (Tobago 35) with two 2GM20s. When acquired from the charter company, the field sensors from the two alternators were connected to the diode block, on the upstream (current-wise) side of the diodes. Of course, this means the alternators do not sense the voltage of the batteries, and therefore tend to put out fewer amps at a lower voltage (measured at the batteries).

I found that when either sensor was switched to a battery, either singly or at the same time, the buzzer at the starter(s) would sound. Since the buzzer does not sound after the engines are running, I rigged a switch to route the wire around the diode block, so that the alternators' built-in regulators would see the true voltage at the battery and cause the alternator to send out a higher voltage to compensate for the diodes. I have heard this causes the alternators some extra stress. I often use a jumper with alligator clips to bypass the diode block from time to time to better charge a particular bank.

As I rely more on solar and wind for most of my house bank, this is more of a curiosity than a problem, but in those uncommon moments when eveything else is fixed, I often turn my mind to it.

How are they wired on a monohull? How would they be wired when there were not two battery banks, like on a shore vehicle or machine? Wouldn't they buzz all the time?

So, what is so unique about my situation that they buzz whenever they sense a 12-volt battery?
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Old 02-12-2009, 10:12   #2
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By Jove, I Think I've Got It, Partly

Hi folks:

I'm sitting in Prickly Bay, with a good internet connection, and was researching Hitachi alternators, when I found the following post on another site, responding to another sailor with a similar problem:

From: Yanmar Alternator- oooops - Yachting and Boating World Forums
I may be wrong but I would bet good odds that:

The live input for the alternator has been wired direct to the output coil of the alternator. In a car or any instillation without a splitter diode the output coil would normally be live as it is connected direct to the live terminal of the battery. However standard marine practice was to introduce a splitting diode between alternator and battery banks. The output coil and hence input to the alternator is now disconnected from both batteries. You rely on residual magnetism in the core to provide some output to power up the alternator and the faster the rotor spins the greater the voltage produced in the output coil. Once the output voltage of the alternator reaches the level needed to "turn on" the alternator control circuit, the activating coil is energised and the alternator now generates its own electricity and the revs can drop and you still have an output.

Any comments from the gang at Cruisersforum?
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Old 02-02-2010, 08:54   #3
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Me again. I found the truth. Previous owner, charter company, spliced into wires that run from the alternators to the charging lamps, and ran the wires to the diode block. This closes the circuit whenever the key is on, and the alarm goes off whenever the key is on and the alternator is not being run. Evidently, this was to make the key switch alarm whenever the charterer left the key on when the motor was off. With two engines, and potentially dull charterers, it might make sense to do this.
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Old 04-02-2010, 04:47   #4
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Alarms normally sound on high temperature or low oil pressure, manufacturers normally do not consider an alarm for the charging ciruit that is normally has a charge staus lamp. When the engine is not running and the key is on the alarm will normally sound due to the electrical ciruit being on and zero oil pressure. It makes no sense to me that the charge ciruit would be modified to provide "the key switch alarm whenever the charterer left the key on when the motor was off."

It was probably done to make sure the alternators started charging at idle as with the diode partly blocking the battery voltage the alternator would not sense the voltage needed to 'excite' them and put out a charge voltage.
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Old 04-02-2010, 08:10   #5
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R.Furborough - thanks for clearing that up. I thought the alarm always sounded, when 1st starting my Yanmar, because the engine was not yet revving enought to produce a charge from the alternator - not because it had not reached it's normal oil pressure.

Either way, the alarm stops after revving the engine up a bit, but this knowledge may be useful in future troubleshooting.
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