Small Outboard Tachometer

[gm280]

kestrel452, back in my post (post #6) I stated most any Marine tach will work. Not any tach... When an alternator generates electricity, it is AC, meaning Alternating Current. And that means it produces both a positive voltage and a negative voltage output. However, you can't send that AC voltage to your battery. And that is because your battery is only DC. DC meaning Direct Current. Which also means only a positive voltage. That is how you will charge your battery. So to make the AC voltage into DC voltage you have to allow only the positive voltage of to the battery and block the negative voltage going to the battery. And to do that you have to use a electronic device called a rectifier. I won't go into how that accomplishes changing the AC to DC for this explanation, but it does that for you. So you have to use a rectifier and then the DC (actually a pulsating DC voltage) goes to the battery and charges it. How all that happens is available on the net if you wish to read about it. I don't want to get into more confusion here to hopefully help you understand. So I don't know what your engine is supplied with presently. It could have the magnets installed and not the coils. or it could have everything and not the rectifier. IDK. But if you can get the "guts" installed, then you can use that output voltage to charge your battery while the engine is running... Hope that helps just a little...

 

[kestrel452]

It does help. I called my local Mercury authorized service center and asked them a few questions too. He too said to not operate the engine without a battery completing the circuit, and to never let the charging leads touch.

Also asked him about the tach too. He said it has to be a gauge that will work with 12-pole outboard alternator systems. And like you said, any outboard compatible tacho will work. His instructions were to attach the gray tach signal lead to either of the two yellow wires on the AC side of the rectifier, and attach the other wires to the battery. Does that sound correct to you?

 

[gm280]

Yes that does sound correct. The AC side of the rectifier will have the pulses to work the tach coming from the coils under the flywheel. The DC side will be the voltage to charge the battery after it changes the AC to a DC source...

 

[achris]

Rectifier. Changes AC to DC.
Regulator. Stops the DC output of the rectifier going above 14.7 volts (in the case of most outboard systems, and the old Mercruiser 470s, just dumps the excess voltage off as heat)... In a car, the regulator controls the current going to the inside of the alternator and controls the output that way. But the outboard uses magnets, not coils, so can't be controlled like that....

Now, for your problem, if it was me. I would buy the charging coil from Merc and install it (usually involves pulling the flywheel). Then I would pop down to the local electronics shop and buy a simple 'bridge rectifier' package. Anything with a high voltage rating (about 400v is ample) and as the maximum current from those small charging coils isn't much, a current rating of 35A would be plenty. Something like this -> http://www.jaycar.com.au/productView.asp?ID=ZR1324&w=bridge+rectifier&form=KEYWORD

That package has 4 legs, 2 are the AC input (marked '~') which you would connect to the yellow leads from the charging coil you just installed (and use one of those for the tacho signal feed), one leg is negative ('-') which you would run a black wire to the engine block (and battery negative) and the last leg is the positive ('+') which you would run to the battery positive.

Now, with that setup you could happily run without the battery connected (but why you would I don't know). Just insulate the wires when you do.... If you short the battery wires in all likelihood the rectifier would survive, but I wouldn't try it.

Chris.......

 

[kestrel452]

I'll just run it with a battery, it isn't worth trying to jerry rig something, the last thing I want to do is burn up such a nice motor.

What size gas tank should I be running that thing with for a whole day or half day out? Any advice on Moeller vs Attwood tanks/lines? Do I need something like the "fuel demand valve" that attwood sells? I don't want to be scrambling to find a fuel dock on a 12' inflatable. Though of course I would carry a spare quart of premium plus TC-W3 just in case...