Tacho only reading half revs?

[surfingminniwinni]

New boat on the way. When we were testing and inspecting I noticed the tacho reading quite low, existing owner said it was only reading half the revs? Is this a common problem and is there a simple solution?

[atoll]

could be a slipping fanbelt,wiring to the alternator or most likely alternator sensor on its way out.

[daddle]

Some tachometers have an adjustment on the back to compensate for the pully diameters. The engine speed is derived from the sinusoid (AC) voltage of one of the three alternator windings. Someone nearby probably has an optical tachometer you might borrow for calibration.

[cburger]

One sure way to find out is to get your hand on a digital laser tach that will allow you to read the rpms of the flywheel as a baseline to determine where the problem lies, engine, alternator, tach, belt tension, etc.

[S/V Antares]

Hey Surf... There are various ways tachos get their signal.

Alternator driven, where an "AC" tap is available. Most alts have 12 pole but there are variations and pully ratios. Common for some perkins, Kabota, Mitsubishi

Inductive, where a magnetic pickup typically counts flywheel or other gear teeth. Common in Yanmar, Cummins, Volvo

Generator sender, where a small signal generator is installed typically on a cam gear. Older Cat, Detroit,

Most tachs have some way of adjusting for the various inputs. Some OEM types are fixed (Ususlly indictive)

What is you engine? This will give us a hint.

[SailFastTri]

Originally Posted by daddle

Some tachometers have an adjustment on the back to compensate for the pully diameters. The engine speed is derived from the sinusoid (AC) voltage of one of the three alternator windings. Someone nearby probably has an optical tachometer you might borrow for calibration.

I think daddle has your answer. It's a push-button setting. Get hold of the tach documentation or let your dealer take care of it (new boat, you said?)

[osirissail]

Originally Posted by S/V Antares

. . . Alternator driven, where an "AC" tap is available. Most alts have 12 pole but there are variations and pully ratios. Common for some perkins, Kabota, Mitsubishi . . .

Check the back of the tachometer and you may see the below example of "pole" selector setting.

[S/V Antares]

Many OEM panels use VDO tachs, The ones with the digital window / Hr meter in the front have a push button or small DIP switches that allows programing. You will need the directions!

[surfingminniwinni]

Originally Posted by SailFastTri

I think daddle has your answer. It's a push-button setting. Get hold of the tach documentation or let your dealer take care of it (new boat, you said?)

I guess I should of said another boat, we have sold Crazy Lady! Our new boat(1987 model) is a Oceanic 46 designed by Jack Savage, just passed survey so it's a done deal. The surveyor seems to think it will be a button setting at the rear of the tacho as well.
Thanks for all the helpful answers. New boat pictures soon.