Still puzzled: what is pin “D” for on the alternator, or is it “W”?

[Krumelur]

Thanks to the forum I was able to confirm that the 1202 diode splitter on my boat is broken and would lead to overcharging my starter battery.
I’m soon going to replace it but I still want to understand, why disconnecting the wire going from the alternator’s “D” pin to the housing bank dropped the voltage level of the starter battery.
What is the “D” connection exactly about?
Or is the connection I’m referring to “W”? See highlighted pin in the image.it’s not quite clear to me what letter is meant to be assigned to which connection.

In any case: D or W…the wire connected to it leads to the housing’s +12V connector. Why did it stop the over voltage alarm and decrease the charging voltage the moment I disconnected it?

[sy_Anniina]

Google for explanation on how a standardi car alternator works.

My brief summary:
D is connected through alarm light to +12v, giving initial current to rotor (rotor becoming a magnet)
Rotating rotor generates AC current on stator coils
Rectifier (within alternator) transforms this AC to DC
DC is fed back to stator through controller, adjusting voltage to relevant level...

W would give AC signal to tacho.

Disconnecting D coins affect on alt output

[Krumelur]

I’m usually good at finding things on Google but I fail to find the article “Alternators for Dummies” :-)
Your explanations are the best so far, still I have questions:

“Disconnecting D coins affect on alt output” - sorry, what does that mean?

Also, you are writing: “adjusting voltage to relevant level”. What role does D play here?

[SYJetzt]

The marked connector is neither D nor W. It is the voltage sensing input for the alternator regulator. If disconnected, your alternator charges with full power, until the voltage reaches 14,4V (measured on B Connector of alternator). If you connect this terminal to your house bank, which has lower voltage than 14,4V , the regulator will force the alternator to generate more current/power to bring your battery up to 14,4V and reduce power when the house bank reaches 14,4v. This circuitry serves as compensation for the voltage losses on cables, diodesplitters in the charging path. In your case (blown diode splitter) the sensing input remained on 13,something Volts, forcing the alternator to produce maximum power, which only could go to the starter battery (the path in diodesplitter to housebank was blown) , causing the Over voltage alarm and probably cooked your starter battery.

[Krumelur]

This makes perfect sense and is what I thought was happening. But it did not make sense given the definitions of D and W. It drove me crazy 😜
Does this connector not have a letter or numeric value assigned?

[SYJetzt]

Yes it has a „S“, but unfortunately not stamped on the alternator case, but only in the drawings. I strongly suggest loading down the VP service manuals from this website, which are a great ressource for any troubleshooting on your engine.

[Krumelur]

If I replace the diode splitter with a Victron Cirix-ct to separate my batteries if needed, the sensing cable should no longer be required, is this correct?  There should not be major voltage drops occurring, according to Victron’s data sheets.

[SYJetzt]

No, that‘s not correct. If you don‘t connect the „S“ Terminal, your batteries get only 14,4Volts less the voltage drops on cabling and the cyrix. It’s right, that the cyrix has lower losses (and i would suggest the cyrix), but you should not forget the losses on the cabling. All lead acid batteries need the full charging voltage to do an absorption and equalisation cycle. Every tenth of a volt missing at the charging voltage (on the battery terminals) will cost lifetime of your battery.

[sy_Anniina]

Not related to regulation, but answering to Krumelur:

Disconnecting D would affect output - "coins" was my lousy phone spellchecker

If D is totally disconnected, I would assume the alternator will not even strart generating power - but just would spin freely without any voltage or current output.

Tommi
s/y Anniina

[Krumelur]

Thanks for clarifying this!