Tachometer error at high speed

Electrical and electronic topics for small boats
Oldslowandugly
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Tachometer error at high speed

Postby Oldslowandugly » Mon Jun 07, 2021 1:44 pm

When my 1985 Evinrude 30-HP engine with electric start and charging accelerates to 5,200-RPM, the tachometer begins to read lower going down to 4,3000 to 4,500-RPM, while the engine speed is still increasing.

Years ago I saw this on this engine, and then I added new charge coils [apparently part of the alternator stator]. The engine then would charge the battery fine, but the tach would get to 3,000-RPM and then drop down to 1,500-RPM and stay there.

I then swapped an entire stator and coil assembly from a 1980 Johnson 35, and the tach then worked fine for many years.

Now after ten years of storage I I have put this engine back in service, and the tachometer problem is occuring.

I had consulted electrical engineers at OMC and Teleflex, as well as my cousin who is a radar engineer at Lockheed-Martin. They all say the same thing: the waveform is collapsing.

But no one can explain what causes this and how to fix it.

I understand how a tach works: the [permanent magnet] alternator produces alternating current. The rectifier changes the alternating current to [pulsating] direct current. This charges the battery and powers accessories.

Amyellow wire with [gray] stripe from the alternator provides the "the tachometer signal."

As engine speed increases, the AC voltage increases, and the tach interprets this as RPM.

[AT THIS POINT THE EXPLANATION OF HOW THE TACHOMETER WORKS IS WRONG--jimh]

I tested the voltage increase and it was what the FSM specified.

If the alternator is charging properly, why would the tachometer signal collapse?

jimh
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Re: Tachometer error at high speed

Postby jimh » Tue Jun 08, 2021 11:34 am

I have no idea what "collapse" means with regard to an electrical signal. You need to go back to your consultants and have them explain what "collapse" means.

Also, you do not understand how a tachometer works. The tachometer is not a Voltmeter. It does not indicate engine speed as a function of an applied voltage.

The general method for a tachometer to work on an older two-cycle-power-stroke engine is for the engine to provide a HALF-WAVE RECTIFIER tap into the FULL-WAVE RECTIFIER. This produces a distinct pulse of DC current. The FREQUENCY of the pulses changes in proportion to the engine speed.

The Tachometer is a device that measures FREQUENCY of the pulses of an electrical signal applied to the SENSOR input to the tachometer.

Now the actual meter that indicates the engine speed is responding to a flow of electrical current through the meter--all electrical meters are actually measuring a current flow through them that creates the power to move the dial pointer. But the meter responds to engine speed because the circuitry in the meter produces current in proportion to the frequency of the pulse input.

Your prior experience shows that the general solution to the problem of poor tachometer accuracy is to replace the coil set in the alternator. I have had similar experiences, particularly with older two-stroke-power-cycle engines. As long as the rectifier was not damaged, the stator coil assembly was the problem.

The behavior of the coils in a permanent magnet alternator is as you described, the voltage induced into the coils increases with the speed of rotation, But this behavior does not go on forever. At some point the coils become saturated, and the output voltage stops increasing or might even decrease slightly from the coil saturation.

You should make a very careful, very close visual inspection of the coil assembly, looking for signs of damage from too much heat. Typically the insulation on the wires can melt with high temperature, and the coil winding short out to each other.

If there are sorted turns in the coils in the stator, the waveform being created may still work for battery charging, but the half-wave tap output waveform may be affected in some way that is causing the tachometer circuitry to operate incorrectly in its method of frequency-counting and conversion to an electrical current for the tachometer.

Oldslowandugly
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Re: Tachometer error at high speed

Postby Oldslowandugly » Tue Jun 08, 2021 2:36 pm

Jim, THANK YOU, for correcting me!!! No one has explained it that way before. The first thing that was suggested by the Dealer was to check the rectifier. Several brand new ones were tested, installed, and no change resulted. That leaves the stator charge coils. Initially, the Dealer installed brand new coils. The 1985 30hp motor was a rope start tiller without charging capability. On the 1985 model the charge coils were separate from the other ignition coil and trigger and easily installed. I had removed the flywheel and inspected the new coils and they still looked factory fresh with no discoloration or damage. I concluded that they may have been defective from the factory. They charged the battery fine, with just the tach problem. I then decided to swap the complete 1980 stator assembly on to the 1985 30hp motor. In the 1980 model year the electric start model stator included all the coils needed to power the ignition and charge the battery in one combined assembly. It too looked to be in perfect condition. Once installed, the tach worked as it should. I can't check the 1980 stator for damage without removing the flywheel and since the boat is in the water now, that will have to wait. At least now I understand what may be causing this. In your experience have you ever seen the tach read a steady 1000 rpm above what you know it should be? I see that ocassionaly on my 48SPL motor. Usually when the battery has been charging for a long time and the volt meter reads 13 to14 volts.

jimh
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Re: Tachometer error at high speed

Postby jimh » Tue Jun 08, 2021 5:07 pm

You have given me an idea: the problem might be related to the battery voltage.

If a diode is bad in the rectifier it could be allowing the battery voltage rise to creep into the tachometer signal. The difference in voltage between the pulse-on state and the no-pulse state in the tachometer signal should be about 12-Volts. Maybe there is a leaky diode and the off-state voltage gets too high, then the tachometer cannot count the pulses correctly.

Changing the stator coils is a pain—the flywheel usually has to come off. An impact driver and a special tool to hold the flywheel by the teeth in the outer ring are needed.

Last time I worked on this type problem I used an oscilloscope to monitor the tachometer signal. That isn’t the usual c.1970 boat mechanic method.

jimh
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Re: Tachometer error at high speed

Postby jimh » Tue Jun 08, 2021 5:10 pm

The tachometer signal usually is taken from the battery charging coils, not from the spark coil primary coils.

These old outboard engine two-stroke tachometers are an analog measurement, and I don’t know the exact circuitry used—but it must be rather simple.

Oldslowandugly
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Re: Tachometer error at high speed

Postby Oldslowandugly » Tue Jun 08, 2021 6:17 pm

...voltage gets too high, then the tachometer cannot count the pulses correctly.

Could the reason be that this is an unregulated charging system?

At high rpm could the charging voltage disrupt the tach signal?

ASIDE:Funny you should mention the oscilloscope- my cousin told me to use one too. Back when he was in school he actually had a portable scope that took a zillion batteries to power it. He regrets getting rid of it.

The tachometer signal usually is taken from the battery charging coils, not from the spark coil primary coil

That was the secondary reason I had the charge coils installed. The primary reason was battery charging.

Oldslowandugly
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Re: Tachometer error at high speed

Postby Oldslowandugly » Tue Jun 08, 2021 6:27 pm

I was just browsing the parts places and the 1985 separate stator charge coils that started this whole thing are now obsolete. And they had only cost $50. However the one-piece stator that the 1980 used has been replaced by a more modern assembly that IS available—for $400.

goldstem
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Re: Tachometer error at high speed

Postby goldstem » Wed Jun 09, 2021 10:28 am

I believe you can just disconnect the entire charging rectifier and regulator, leaving the tach connected. If that solves [the problem with the tachometer error] you might just need to replace [the rectifier and regulator], much easier and usually cheaper.

Oldslowandugly
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Re: Tachometer error at high speed

Postby Oldslowandugly » Thu Jun 10, 2021 9:33 pm

The rectifier was replaced three times. And swapped with know good ones. Nothing changed. I think Jim nailed it- the charge coils are the problem.