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Tachometer not getting signial.

AllenHamel

New member
My tachometer is not receiving a signal from the engine. Where on the engine would the sending wire hook up to?

I have a
[h=1]1990 Johnson 70 hp VJ70TLESB Outboard Motor[/h]
Thanks
 
Gray wire from the rectifier. Do you have the harness that plugs into the controller? You could have a bad rectifier or tach.
 
Voltage from the gray lead at the rectifier should be 9 volts + or - 1 volt at 1000 RPM. Just above an idle since you don't have a tach. See pic for your rectifier. Reference #63 is the rectifier. Should find the gray lead there.


 

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Here is Faria troubleshooting and they state 2 volt DC or more when testing???

faria-instruments.com/site_manuals/IS0100_Tachometers.pdf
 
​I am use to texting primarily 35amp big motor systems which is why I quote the 9 volt + or - 1 volt spec. Guess I should clarify that as to what system you are running.
 
The two or three wires coming out of the stator is A/C voltage and the rectifier or bridge converts it to D/C. The tach counts the pulses per revolution same as a diesel engine uses the alternator pulses before it is rectified into direct current.
 
It runs on the pulse per revolution. If the stator puts out 6 pulses per revolution multiply that by 700 rpm equals 4,200 pulses per minute or 70 pulses per second at idle speed. Its kind of amazing to me a tach can count the pulses per second at 5000 rpm thats 500 pulses per second! At least thats my understanding on the physics.
 
The stator is the same as a alternator it puts out A/C current before the regulator but i believe the tach uses a diode to turn it into a D/C pulse same as the coil neg on a inboard engine.
 
DC pulses. They tap in to a point that gives half wave rectification; the voltage pulses from 0 to some voltage (9? or whatever) but always the same polarity. That makes DC pulses that the tach "counts".
 
I was told years ago by an instructor in OMC school that tachs hate DC voltage but function on AC, so always check your wiring if you have a tach problem. O'well, I think I'll just give up on this. !!!!!!!!!!!!
 
Well I guess then that the AC current that runs my refrigerator, furnace, stove and ect is nothing but a bunch of half sine waves ????
 
Half-wave trace 1.jpgTop trace is your household AC, bottom trace is half wave trace. You can see how the half wave creates a series of pulses. In an outboard, the faster the flywheel magnets go by the stator coils, the closer together the pulses become. The more pulses per second, the higher the tach reads.
 
If you have a ohm meter that measures Hz you can use it to read rpm. Just put a probe directly in the pos terminal and hold it close to a plug wire. For a two stroke multiply it by 60 and for a four stroke multiply it by 120 i tried it and it works very accurateiy.
 
No body has told me yet it is AC current the tach runs off of. Soooooooooooooooooo I give up !!!!!!!!!!!!!!!. Thank you all for your time.
 
No body has told me yet it is AC current the tach runs off of. Soooooooooooooooooo I give up !!!!!!!!!!!!!!!. Thank you all for your time.

All I can say is that the voltage regulator on my 90 hp Johnson went bad - so I bypassed it, and ran directly from the battery for a couple of seasons.
I had hooked up my tachometer lead directly to one of the stator leads, and I grounded the other stator lead with a jumper wire.
Tachometer worked perfectly during that time.
I reconnected everything in the normal way once I had a chance to replace the voltage regulator, and the battery is charging and the tachometer works fine.

So yes, the tachometer will run off of AC current.

CarMech1969
 
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Thank you Joe ! And CarMech ! CarMech. It makes me feel good for what you said in your come back. Proves the tachs run off of AC voltage.
 
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