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Author Topic:   Tachometer Question
whaleryo posted 06-01-2001 09:43 AM ET (US)   Profile for whaleryo   Send Email to whaleryo  
My Yamaha tach isn't working. The rest of the components (ie; oil light etc) are OK but the actual tach part doesn't move. What can be measured on the wiring (purple, I believe) to see that the proper signal is present before I buy a new gauge?

bigz posted 06-01-2001 02:03 PM ET (US)     Profile for bigz    
Check first, I know your other warning lights work but check anyway, that there is power to the tach with the ignition key on, and that the safety lanyard key is installed, that wire should be the purple usually marked as "ign" or "batt" attach your red lead from multimeter and the black lead to the "gnd" terminal, should show power . I would guess this is ok.

Let's check the continuity of the ground with your multimeter, attach a lead to the "gnd" terminal to a good ground on the back of your instrument panel, if the continuity is good then the power circuit is fine.

Next check what usually is the "gray" sending wire could be called "pul" or maybe "sig", find out where it attaches to our engine and then check for continuity between there and the gauge with the ignition key off. If no continuity you have a gauge problem.

Well if all of the above checks out ok then you might have an ignition problem but this would be highly doubtful.

Hope this helped.

triblet posted 06-01-2001 05:00 PM ET (US)     Profile for triblet  Send Email to triblet     
I think bigz had a finger check there.
If no continuity on the gray wire, you do
not have guage problem, you have a grey wire

Also, check the voltage with engine running.
In some cases at least (my 90HP Evinrude)
the tach pulse is generated by the voltage
regulator, and a lack of tach reading can
indicate a bad VR.


lhg posted 06-01-2001 07:37 PM ET (US)     Profile for lhg    
I had a dead tach signal, and it proved to be the engine rectifier.
whalernut posted 06-02-2001 02:20 AM ET (US)     Profile for whalernut  Send Email to whalernut     
Larry, what actually does that little round silver rectifier do on an outboard engine? I always wondered the purpose of it`s being? Thanks-Jack Graner.
bigz posted 06-02-2001 06:14 AM ET (US)     Profile for bigz    
Thanks Chuck your correct it was a "finger" check error.

The rectifer is part of the ignition/charge system and could be the cause, then again would be experiencing problems with your charge system.

fester posted 06-03-2001 12:02 AM ET (US)     Profile for fester  Send Email to fester     
There is an old post dated 10/10/2000 titled Mercury burned rectifier. Jim H wrote a reply which very clearly discusses the purpose of a rectifier.


whalernut posted 06-03-2001 05:34 PM ET (US)     Profile for whalernut  Send Email to whalernut     
Thanks Jeff-Jack.
jimh posted 06-03-2001 09:18 PM ET (US)     Profile for jimh  Send Email to jimh     
In most older 2-stroke outboards the tachometer is connected to the pulsating output of the charging coil (Yamaha calls it "lighting coil").

The coil is located under the engine flywheel. Afixed to the flywheel are a series of permanent magnets. As the engine flywheel rotates, the magnets pass over the coil, inducing current flow in the coil.

The alternating current from the coil is applied to the rectifier. The rectifier converts this to pulsating direct current.
This supplies the battery with charging current.

The coil output--prior to the rectifier--is also usually applied to the tachometer. The tachometer is configured to integrate the pulses into a curent which drives a meter movement (analog gauge tach) or the pulse are counted by a simple counter circuit and the result displayed (digital tach). Depending on how many magnets the engine used, the calibration must be adjusted accordingly. Most outboards use 6-12 magnets. You select the appropriate setting on the tach to calibrate it to the engine based on how many magnets are under its flywheel.

Lack of tachometer pulses usually also indicates problems in the charging circuit, since the two use the same coil for their source.

The tachometer pulses also may rely on the rectifier to create a ground reference for them, so if the rectifier is bad, it also can cause the tach to stop working.

Outboard engine "mechanics" often have very limited understanding of electrical circuits. They tend to replace things that aren't the cause of the problem in many cases, but in the process of replacing a bunch of stuff they find the actual problem.

We used to call this approach "easter egging" the problem. If you replace enough stuff you are sure to fix the problem.

Of course, sometimes it makes more sense to replace a $30 part than to burn up an hour of testing at $75/hour trying to figure out if the part is bad or not. Particularly when the guy doing the testing is not an electronics whiz...

Also, if there has been a failure of some element of the circuit, the mechanic might figure it is just as well to replace everything because other parts might be "weak." That may be nebulous thinking but it is good for service dollar volume.

In newer outboards the ignition circuits are more complex, and the tachometer pulses may be derived from some other source, but in the old classic 2-stroke engines, they are generally configured as I have described above.

jimh posted 06-03-2001 09:21 PM ET (US)     Profile for jimh  Send Email to jimh     
More direct answer to your question:

An AC voltmeter should show a reading on the Tachometer lead. (I'd guess about 6 VAC RMS)

Hook an oscilloscope to it and you should see some pulses, about 12-13 volts peak.

A frequency counter should show a freqeuncy proportional to the engine RPM X No. of Magnets.


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