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Old 05-05-2006, 15:06   #1
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REV counter pickup

OK, so can anyone explain to me exactly how the REV counter picks up a pulse from teh alternator. I am about to delve into mine to track down a problem and have never played with one before. I also always find, the best way of becoming an expert, is to have the job go wrong, becomes bigger than you expect and end up replacing everything till it works. Well I am sick of becoming an expert in things. I just want a job to be simple and straight forward for once.
The fault is, my rev counter would not register at all till the engine reached 1000RPM. Then it would kick in and run fine. Now it has decided to just vaguely drive the indicater needle all over the place and then sometimes it's dead, no matter where the engine revs are. I am hoping a simple matter of a bad connection, but it's called a boat, so I doubt it will be that simple.
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Old 05-05-2006, 16:40   #2
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Alan, the alternator produces A/C power before it is rectified by the internal diodes. The tachometer samples the A/C voltage. Your analog meter will use a "discriminator" (? may have wrong term here) to produce a dc voltage which will drive your meter. Correct RPM reading also depends on correct pulley sizes.

If your meter movement is not damaged then you are probably correct in assuming a bad connection. The whole thing is fairly simple. Somewhere in the circuit should be selector switches to adjust the value for different pulley ratios.

hth
Phil
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Old 05-05-2006, 17:06   #3
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Alan, forgot to mention that if you have an external regulator, you may have to drive the alternator from the regulator, rather than the alternator. If your regulator has the "soft start" feature the alternator will not put out a big enough pulse. This may also happen on "float" as well.


Phil

ps; I think the tern I was looking for in previous post is "integrator". things are a little foggy back there <g>
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Old 05-05-2006, 17:12   #4
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Wheels, Phil is correct in that externally regulated alternators using 3-step regulation just will not work for you if you need a good tach indication. When transiting from acceptance voltage to float voltage the tach will completely drop out until the regulator "sees" the first slight need to increase the float voltage.

Hopefully you can install a digital tach which reads the teeth on the flywheel.
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Old 05-05-2006, 22:43   #5
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Just a note about flywheel transducer/tachs

The new tachs come with a three position switch which indicate a range of number of teeth. So you'll need to know the number of teeth on the flywheel for calibration.

Personally, I prefer the flywheel type over altenator. Most marine diesels came with them.
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Old 06-05-2006, 01:54   #6
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Deepfrz, are you thinking a Rectifier??

Thanks guy's. OK, so now I know a little more, I can explain a little more.

Firstly, the engine is a Perkins 6.354. The rev counter is VDO. There is no multi-stage regulator on the Alternator, it is just plain simple straight off the Alternator charging with an 80A alternator. Must increase that one day, but....
I only use the main engine to give a form of bulk charging and then go to the genset and 3 stage AC charger for the rest.
Calabration is done with a variable pot in the back of the Rev counter dial. It has been calibrated using a laser calibrator about 6months back. Up untill just recently, everything had been working fine.
OK, so I will look for faulty connections then.
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Old 06-05-2006, 16:14   #7
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Faulty connection

There are two symptoms of worn or contaminated field brushes with alternators. First, the alternator driven tach does not register any more at low rpms. Second, the alternator output current becomes erratic at low to medium rpms and, later becomes downright intermittent and then zilch!
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Old 06-05-2006, 16:28   #8
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Alan, I like Rick's idea (worn brushes) but would still look for bad connections before opening up the alternator. Brushes are generally cheap & simple to replace and having a spare set on board isn't a bad idea anyway.

But don't give up on your 80A alternator. If you can't get any numbers...I'd guesstimate the 80A is a peak rating with 60A sustained output (with good cooling air) likely. That could give you 20A to run ships' lights, radio, etc., and still provide 40A to a battery bank during charging.

If you were using wet cells that can only recharge at 1/5thC...five hours of charging would give you 200A, and if you're only running the battery bank down to 50% cycles as recommended, that alternator would be perfectly matched for a 400AH battery. Or 320AH AGM battery, which can accept a charge faster.

More important is to use a proper external marine regulator, so your alternator is giving the batteries the right amount of power, up to and including full output. With thermal sensing to protect the alternator, and adjust the charge to the batteries.

Sometimes, you can also get much better power form the alternator by changing the pulley size, but you need output ratings for the alternator (from its maker) and to know your engine RPM before you can even think about that. A $100 custom-made pulley can literally double your charging efficiency if the stock one was chosen "because it was round."

You also won't see good tach numbers unless the alternator is putting out, and if the sense lead is not making good contact, or you've got a "1-wire" alternator (no sense lead, or tied right back to the positive post) and your *alternator* is running under ~~1500RPM, it won't turn on.
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Old 07-05-2006, 02:23   #9
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Yeah I was suspecting the brushes to begin with. Hence why the ask of the "how does it work" question. It has been wet and the brushes are quite possibly locked in their holders, which means they won't be make good contact as they wear. I do a lot of extreme 4WDing and I have to strip my 4WD alternator regularly for the same thing. The brush holders get gunked up with crap and water and I loose charge.
Yeehaa, another job on the to do list.
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Old 07-05-2006, 11:56   #10
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I could understand a 4WD running the alternator under water and getting gunk in there...But on the boat?! You shouldn't be able to get enough debris into the area of the brushes or brushes holders to jam them, they're usually a fairly tight fit into the holders and, since they are self-lubricating, that works out well. If they are stuck, I'd look at replacing the springs for them as well. I've seen specs on some alternators that "required" replacing brushes when a, ah, dare we say cheap?<G> user might think "They're still good enough". But at that point they are too short for the springs to push out properly on them. Just something to doubel-check on.
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Old 07-05-2006, 13:17   #11
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OK, a little more to the stroy then. I've had one of those months, a couple back. I had a cooling hose blow, spraying the entir engine room in saltwater. So I gave everything a good wash down. Then I had a little problem with the engines oil breather hose (my fault but we wont go there) but needless to say, it was blocked off. So the engine decided to blow oil all around the engine room. I was lucky enough to find the problem just and I mean JUST before the engine ws totaly out of oil. So everything ahd a good was once again. So yes, the alternator has been wet. Not intentionaly inside, but who's to say what got in their. It ain't a sealed up unit.
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Old 07-05-2006, 13:18   #12
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Dang, this is like being in a Police interview. Eventually all the beans get spilled. (not that I have ever been in a police interview)
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Old 11-06-2006, 18:10   #13
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Wow ... great info guys --- now, unfortunately, I have an alternator to fix/refurbish... although... I have been eyeing those Balmar units ... and for 1/2 a boat buck, that would include a good regulator. :::sigh:::
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Old 12-06-2006, 06:57   #14
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Hi Guys,On the same topic I've got a problem you could possibly help me with. When I shut down the engine by turing the key anti-clockwise for a couple of seconds the rev counter stays at about 700 rpm even with the engine stopped.If I leave it like this it will drain the battery overnight.To prevent this I have to turn the ignition key one stop clockwise and then return it to the vertical (normally off position). The rev counter then drops out and returns to zerro.It is then safe to leave without drainng the battery.I've had electricians look at this but no one can offer a solution other than have the alternator overhauled.Any suggestions as to the cause?
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Old 12-06-2006, 09:54   #15
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"When I shut down the engine by turing the key" Leighton, this is a gasoline engine then?

It sounds like the normally off position is the real off position, and the 2-stops-counterclockwise (as we call it in the States<G>) position is what used to be called the "accessory" position on common ignition switches. It could be supplying power to the alternator or to an accessory relay circuit, which is still powering that meter.

Without a schematic or some times with a voltmeter, you can only guess. It could be there is a relay sticking someplace, powering the tachometer. Or, a bad diode in the alternator, which is causing the drain. (That would cause other problems and eventual failure, so you might want to check the alternator diodes in any case.)

There's got to be a difference in those two key positions, but you'll need to explore to find out what it is.
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