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Old 07-23-2016, 06:48 PM   #1
Al
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What happen with my tachometer!!!

Greetings to the forum

Interesting development.
Recently the tachometers(Motorola) on the bridge and cabin have been fluctuating a couple of hundred RPM periodically. As they would eventually come back it slipped my mind as being serious.
On this trip about into it a couple of hours, the tachs began in earnest to drop RPMs in an increased amount till around 500 RPM or so. As I was at sea, I called a local Ketchikan alternator/generator shop (White Marine).
The tachometers are run through the alternator so the call seemed correct, The technician gave me a question. ‘Did I have anything in line that called for amp usage? Well, yes, I have an inverter that runs our 110 fridge underway and if called for, a coffee pot, toaster, and micro oven, outside of the fridge, not run together.’ ‘What amperage is your alternator?’ ‘60 amp, I responded. ‘Well, the shop said, I am going to guess that you have pretty much fried your alternator as it is too small for coffee pots and such. When your tachometer dies, Johnson, your alternator is as well!’
When you return home bring your alternator in for review to confirm and by the way, plan on a larger alternator.

Okay, that is the first chapter of this adventure

By the time I arrived at the destination, Wrangell, AK. the tachometers had in fact ‘Died’. I removed the alternator, went to NAPA and purchased the same make, Delco, in 90 amp capacity and installed.
Now this is a simple four wire system, Ground connection Positive connection, Diodes two prong connector and rectifier (Tachometer connection). I re-installed all wires and connections, double checked and fired up the engine. NO TACHOMETER! What the heck. thought about it, double triple checked connections and out of whim, turned on the inverter system and activated the toaster. VOILÀ!! The tachometer came on and held RPM. What happened?

Call into my heavy duty mecanic son who at once went into a complicated explanation regarding AC/DC science which is all Greek to me. I understand 12 volt and 110 but in separate capacity, not when the integrating conversion of it comes into play. I had also text my young marine mechanic who responded with confirmation of what had been told to me and the end result is: The new alternator will have to be opened up and something with the rectifier has to be reversed or something.

The point of all this is to lay out a situation that may not be common yet common enough that a seed of what happens in our case will prove helpful.

The new alternator is providing charge to the system. the system currently is at full capacity, reflected in the tachometer not registering. How ever, were our batteries low and we start the engine, the tachometer will register dropping to zero again as the batteries become fully charged.

It is all magic that is what it is, magic!! I do not pretend to understand it.

Al-Ketchikan Marben 27’ Pocket CRUISER
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Old 07-23-2016, 10:23 PM   #2
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Al
I am with you on the part about it being all Greek.
For years I have been synchronizing my engines on the temp guage, as the tach on the port engine has been misbehaving. Your description re the batteries being charged or not, is exactly what I have observed. I know my batteries are being well served by this 120 amp alternator and its Ample Power "Next Step" regulator, as the most recent change of batteries was after 10 years of good service.
Occasionally, in tune with the batteries needing a lot of recharge, I get a good, solid, accurate tach reading. The rest of the time, the needle rests solidly on 0.0 rpm. The Stb tach is fed by a 50 or 60 amp alternator with its own internal, "stupid" regulator. Incapable of charging much, but faithful to the rpm being used.
If you find the Rosetta stone, please post your results.
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Old 07-28-2016, 07:32 PM   #3
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Do you have solar panels operating? If so, your alternator/tach combination may hit "tilt" about the same time that the house bank becomes fully charged. The fix is to turn the solar panels off when you are motoring. There's probably a better work around but I haven't fully researched it yet.


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Old 07-28-2016, 10:48 PM   #4
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If you purchased a Delco 10 SI alternator, the 2 small terminals should be marked "F" and "R". This stands for "Field" and "Relay". The Field terminal is how you turn the alternator on and off. Usually this is accomplished on a diesel with an oil pressure switch on the engine. When the engine starts and you build oil pressure, the switch closes and the field is energized. You make juice! The "Relay" terminal is the one intended for the idiot light on the dash. It puts out pulsed voltage and can also be used for a tach feed, based on all I've read online. Sounds to me like you are connected to the rectifier bridge somewhere. Since the regulator turns off the alternator when the batteries are charged, turning on the inverter drops the battery voltage and forces the alternator to charge. You then get your tach pulse. Have a good automotive electrician check your wiring.
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Old 07-28-2016, 11:16 PM   #5
Al
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Thanks Trawler Dave,

Quote:If you purchased a Delco 10 SI alternator, the 2 small terminals should be marked "F" and "R". This stands for "Field" and "Relay".
/
The alternator is a 'single wire' Delco with the alternator wire pressed on to a single post marked 'R' for resister? As I said, it is magic. and to prove that it is, when after efforts with no progress in having the tach come alive, I accepted that I would travel back home without a tachometer. No big deal, so what was I to think when I started the engine for the trip and as indicated, house battery having been drawn down a bit, the tachometer did come alive as the amp meter showed positive amount of charge. But , but, as the amp gauge came down as the charge was coming to the top, the tachometer did NOT drop, in fact for the whole trip it never quivered from the throttle setting.
I am assuming that the new alternator required a bit of use or activation,(read Magic) as it seems to be operating per instruction/claims.

Thanks for the concerns and response. another bit of 'Boating' mystery to be filed away.
Al-Ketchikan 27' Marben pocket CRUISER
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Old 07-29-2016, 06:04 AM   #6
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You are absolutely right! The single-wire Delco is called a "self-exciting" alternator. No need for an energized field. These require the the field to have some residual magnetism. As they spin, the residual magnetism starts the charge and as the charge increases, it fully "excites" the field and goes to full charging ability. They are usually packed with instructions on using a jumper wire to start this initial charge. After the field has the residual magnetism, they work fine on their own from then on. The "R" terminal stands for "rotor", I believe. Good luck!
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