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Author Topic:   Odd Tachometer Problem
kaferhaus posted 01-17-2010 09:57 AM ET (US)   Profile for kaferhaus   Send Email to kaferhaus  
With a new Teleflex analog tachometer wired to a 1998 Yamaha 50-HP two-cycle outboard engine, and with pulse setting correct, the tachometer works normally until the engine exceeds 2,000-RPM; then [the dial indicator on the tachometer] jumps up, flickers, and goes back to 2,000, until the engine speed is brought below 2,000-RPM. What could be causing this? The engine was previiously without a tachometer, so I have nothing to reference too.
jimh posted 01-17-2010 11:28 AM ET (US)     Profile for jimh  Send Email to jimh     
In am not familiar with the electrical system of a 1998 Yamaha 50-HP outboard, but I suspect it is quite similar to the general configuration on a two-cycle engine with regard to the derivation of the tachometer pulses. Usually the tachometer pulses are derived from the output of the battery charging circuit. There is a coil winding in the under-flywheel alternator that drives the battery charging circuit via a full-wave rectifier. The output of the battery charging circuit is a pulsating direct current. The tachometer signal is usually derived by connecting to the rectifier at a point where a half-wave rectified output is obtained. This results in a pulsating direct current of half the frequency. The tachometer is actually a frequency measuring device that measures frequency of these pulses. The frequency of the pulses varies in direct proportion to the engine speed.

The rectifier has the effect of limiting the voltage of the pulses to the tachometer to a range from very close to zero volts to very close to the battery positive terminal voltage. In a normal situation this means the tachometer pulses are ranging from roughly -0.6-volts to +13-volts. If there is a problem with the rectifier, the signal to the tachometer may be affected.

The output voltage from the alternator coil tends to increase as the engine speed increases. Because the onset of the problem with the tachometer occurs as engine speed increases, it is reasonable to link the two. It seems like the tachometer begins to malfunction when the voltage output from the alternator increases. This tends to point to the rectifier as perhaps being the cause of the problem. Something odd may be going on with the alternator output and the rectifier.

Because the tachometer signal is a string of pulses, it is hard to assess precisely what is occurring with a simple voltmeter. The voltmeter will read a voltage which is more of an average voltage present. The best way to diagnose problems with alternating waveforms is to use an oscilloscope, although this is probably never done in repair of an outboard motor charging circuit. Instead, a measurement of the tachometer signal is typically made with a peak reading voltmeter. You can make an adaptor for a average reading meter to convert to peak reading by simply putting a small electrolytic capacitor across the input terminal shunted by a resistor of perhaps 1-megaOhm. In outboard service this is often called a DVA (perhaps for digital voltmeter adapter), and is sold for $65. By using a peak reading meter you could investigate the tachometer signal to see if there was an abrupt change in peak voltage when the engine speed was 2,000-RPM or more.

jimh posted 01-17-2010 12:26 PM ET (US)     Profile for jimh  Send Email to jimh     
Another source of a problem whose onset would vary with the voltage output of the alternator is the coil winding itself. As the voltage output increases, the coil could be shorting to the engine block or to other windings in the coil due to loss of insulation. The wiring of a coil typically uses an enamel-like insulation material applied directly to the copper wire. If the temperature of the wire becomes too high, the insulation melts away. This allows the copper windings to become short circuited. This behavior may be dependent on the voltage being produced.

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