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  Yamaha Digital Tachometer Reading Wrong

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Author Topic:   Yamaha Digital Tachometer Reading Wrong
skred posted 07-19-2009 10:33 AM ET (US)   Profile for skred   Send Email to skred  
My 2004 Yamaha 40 two-cycle with the digital combination gauge seems to display the oil, fuel, trim, and hours just fine, but the RPM seems really low. At full throttle and about 30-MPH, the tach reads about 2,600 or so. Should be about double that, I would think.
jimh posted 07-19-2009 11:30 AM ET (US)     Profile for jimh  Send Email to jimh     
Tachometers are really frequency counters. They typically count the pulses of the alternating current from the battery charging circuit from the permanent magnet alternator via a half-wave rectifier tap in the charging circuit. On some tachometers there is a calibration adjustment to match the tachometer readings to the number of pulses per revolution of the flywheel. In different engines there are often a different number of poles and magnets in the alternator, and some adjustment has to be made to calibrate the tachometer.

It sounds like your tachometer is reading about one-half the true crankcase revolution speed. There could be several explanations:

--the tachometer is designed for a different number of pulses per revolution than your engine provides, thus it is always going to be off calibration;

--there is an adjustment on the tachometer that needs to be set to match to your engine;

--there is a malfunction in the engine that is causing the tachometer signal to become prone to being misread by the tachometer; this could be a problem in the charging circuit such as a bad rectifier.

number9 posted 07-19-2009 12:17 PM ET (US)     Profile for number9  Send Email to number9     
New boat or problem just started?

There is a small round cover on the back of the tach. Three switches for setting up for 2/4 stroke. LH is for trim sensor.
Center for tach, 2-stroke should be in the down position. RH if for oil warning and should also be down for 2-stroke. That's straight out of the 2004 Rigging Guide.

skred posted 07-19-2009 12:41 PM ET (US)     Profile for skred  Send Email to skred     
The tach is an OEM Yamaha digital multi-function gauge. Since the gauge and engine were installed at the same time in 2004 when new (before I acquired the boat), I suspect perhaps a rectifier diode, based on your responses, although I would think that if one or two of the diodes were bad, I wouldn't get a reading at all...
I know there's an on-engine test for the rectifier, and will try that first.
If that tests okay, I guess I'll check the switch settings on the gauge, to be sure someone didn't putz with the settings.
jimh posted 07-19-2009 01:16 PM ET (US)     Profile for jimh  Send Email to jimh     
Now that we have information that there is a calibration switch on the tachometer, I recommend you check the calibration switch first. Even if it is in the correct setting, the switch contact may have become corroded. Move the switch back and forth between its various positions several times. This will help to wipe away corrosion from the switch contacts. Return the switch to the proper setting. I have seen this fix problems with tachometers several times.
skred posted 07-19-2009 05:48 PM ET (US)     Profile for skred  Send Email to skred     
Boat is 4 years old, but new to me - second owner. 48 hours on the whole rig.

Thanks to all for the contributions. One is gonna work, I'm sure..

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