Old as Dirt!
Join Date: Nov 2008
Location: Tampa Bay Area
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Re: bouncing tach
How could that work - that would require a feed back circuit and none exists.
The voltage regulator is what does or doesn't send a charge to the batteries based on voltage/state of charge (depending on how sophisticated it is).
The tach is simply reading and counting RPM's either from the alt (which is still turning exactly the same whether or not it's making amps) or from the flywheel (ditto, still making amps).
I'll admit I've had a similar problem with RPM's bouncing around once I get past 2400 RPM, but that's a matter of calibrating the gauge, not the alternator (flywheel in my Yanmar's case).
As the problem was explained to me (as I am not an electrics person), both the internal regulator on the original Motorola stock alternator and most external regulators (ours is an AR5) on larger after market alternators (ours is a 120 amp) include a sense circuit that determines the static voltage in one's batteries. If they are fully charged, or nearly so, close enough to the tripping point at which the alternator is activated/deactivated, when the alternator is activated, with the resulting extra voltage the sense circuit "senses" that the batteries are fully charged and so, deactivates the alternator. The tachometer on the stock Perkins engine panel relies on an activated alternator. Hence if the alternator is cycling on/off/on, the tach bounces. We were instructed to simply activate some power demands to suck up the "extra" voltage--lights, fridge radar or whatever--and, by doing so, the "problem" was solved. Actually, at this point we've used our engine for so long I/We can pretty much tell our RPM's by the sound of the engine but I do like to be able to look down at see that she's steady at 2500 to 2700 RPM's from time-to-time, our usual cruising speed range when powering or motor sailing.
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."
Last edited by svHyLyte; 09-09-2013 at 08:08 PM.
Reason: Correct typo