Erratic tacho

[ArthurWood]

The tachometer on one of my engines has begun to give erratic readings, sometimes showing ~2/3 of actual and sometimes the correct reading (as indicated by the sync indicator vs the other tach). It does not fluctuate rapidly. Is the signal from somewhere on the alternator?Belts seem OK.
Any thoughts on where I shld begin troubleshooting? Engines are 7.4l Merc petrol on 37ft Sea Ray.

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[TonyBrooks]

Without more info, difficult to know.

Petrol engines are likely to pick it up from the coil.

Any engine may use an inductive pickup from something "toothed" on teh crankshaft or camshaft, but in the case of camshaft pickups the guage needs matching to accept half engine revs or the "toothed" item needs twice as many teeth.

Any engine can get a signal from a single phase tap on teh alternator. The erratic nature of your fault leads nme to suspect a loose terminal (first thing to check) or possibley a dry joint or diode fault inside the alternator.

Tony Brooks

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[claudio]

... or maybe a loose fan belt.




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[ex-Gladys]

As said earlier the standard way of picking up a tacho pulse from a petrol engine is from the low tension firing pulse on the ignition. In a marine environment could be the inside of the gauge gungin up. Seen it on motorbikes where the wet gets in...

<hr width=100% size=1>Larry Botheras

Anderson 26 "Amber"

 

[tr7v8]

It will be triggered from the coil. Is it electronic ignition, if it is it could be stray sparks if the plugs are getting a bit old. WOrth swopping the tachs over and seeing if it is the tacho or the wiring. If it is the tacho it would be worth while putting it somewhere warm and dry and seeing if that cures it. At the same time check the connections and see if they're clean especially the earth. If it is the instrument then a local repairer may be able to fix it cheaper than a replacement, try classic car repairers rather than specific marine ones.

<hr width=100% size=1>Jim

Draco 2500

 

[ArthurWood]

Thanks

for all your suggestions. I'll take a look when it gets a bit warmer. Temp hasn't risen above 60F here the past couple of days. Brrrrr.

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[ArthurWood]

Update

Can't easily remove tacho, so reached up behind dash/helm and sprayed CRC Electronics Cleaner on back of gauges. Seems to have cured tach prob and prob w/a temp gauge.

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