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Author Topic:   Tachometer Jumping All Over at Times

jason78 posted 08-01-2014 08:45 PM ET (US)
I have a Mercury tachometer. Other than the needle sitting at 1000-RPM when the motor is OFF, it has worked fairly well. Lately, it has started acting weird. When I cold start the engine, the needle swings all over the place and hovers around 5,000-RPM at idle when it is clearly not running more than about 700-RPM. Eventually it pops back and starts acting right again, then it will start reading about 1,000 over. Any tips other than just replacing it? I feel like I can't trust this tachometer any longer and I am afraid I might over-rev the 1998 Mercury 200XL carburetor two-stroke engine. Thanks Jason
jimh posted 08-02-2014 04:29 AM ET (US)
It is completely normal for the dial pointer of an outboard engine tachometer to not return to the zero mark on the scale when the engine power is off.
There are two area of possible causes for erratic operation of the tachometer
--the signal to the tachometer is bad, or
--the signal to the tachometer is fine, but the tachometer is bad.
Do you notice any change in the battery voltage when the tachometer acts up?
The tachometer signal is derived (typically) from the alternator. If there is a problem in the alternator, the battery charging output will also be affected.
Most outboard engine tachometers have a dial calibration adjustment on the back face of the instrument. It is typically a very small rotary switch. Perform this procedure:
--note the present position in which the switch is set;
--insert a properly sized screwdriver or adjustment tool into the slot of the rotary switch;
--move the rotary switch back and forth throughout its range of adjustment;
--repeat the above about ten times;
--return the switch to the original position.
This procedure will tend to wipe away any discontinuity in the switch contacts. It may restore the tachometer to normal operation.
jason78 posted 08-02-2014 10:32 AM ET (US)
Thanks Jim, I will give that a shot and pay closer attention to voltage.
phatwhaler posted 08-02-2014 06:07 PM ET (US)
You more than likely need new voltage regulators. Look for burnt electrical connectors coming coming out of the regulator wiring.
This problem has happened on both a 1996 150 EFI amd a 1996 200 Carb motor that I have owned. CDI Rapair is a good source of supply for new regulators.
fishinchips posted 08-03-2014 11:30 PM ET (US)
Had a omc johnson 90hp that did the same thing. Turned out to be the voltage regulator.
jason78 posted 08-04-2014 01:03 AM ET (US)
Thanks for the additional suggestions! Moving the selector a few times seems to have solved the problem for now. If I have any other problems I will check the regulator.
jimh posted 08-04-2014 01:15 AM ET (US)
Jason--I am glad to hear my procedure worked to restore your tachometer to good operation. My procedure has an impressive track record of success. I will add your case to the statistics.

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Author	Topic: Tachometer Jumping All Over at Times
jason78	posted 08-01-2014 08:45 PM ET (US) I have a Mercury tachometer. Other than the needle sitting at 1000-RPM when the motor is OFF, it has worked fairly well. Lately, it has started acting weird. When I cold start the engine, the needle swings all over the place and hovers around 5,000-RPM at idle when it is clearly not running more than about 700-RPM. Eventually it pops back and starts acting right again, then it will start reading about 1,000 over. Any tips other than just replacing it? I feel like I can't trust this tachometer any longer and I am afraid I might over-rev the 1998 Mercury 200XL carburetor two-stroke engine. Thanks Jason
jimh	posted 08-02-2014 04:29 AM ET (US) It is completely normal for the dial pointer of an outboard engine tachometer to not return to the zero mark on the scale when the engine power is off. There are two area of possible causes for erratic operation of the tachometer --the signal to the tachometer is bad, or --the signal to the tachometer is fine, but the tachometer is bad. Do you notice any change in the battery voltage when the tachometer acts up? The tachometer signal is derived (typically) from the alternator. If there is a problem in the alternator, the battery charging output will also be affected. Most outboard engine tachometers have a dial calibration adjustment on the back face of the instrument. It is typically a very small rotary switch. Perform this procedure: --note the present position in which the switch is set; --insert a properly sized screwdriver or adjustment tool into the slot of the rotary switch; --move the rotary switch back and forth throughout its range of adjustment; --repeat the above about ten times; --return the switch to the original position. This procedure will tend to wipe away any discontinuity in the switch contacts. It may restore the tachometer to normal operation.
jason78	posted 08-02-2014 10:32 AM ET (US) Thanks Jim, I will give that a shot and pay closer attention to voltage.
phatwhaler	posted 08-02-2014 06:07 PM ET (US) You more than likely need new voltage regulators. Look for burnt electrical connectors coming coming out of the regulator wiring. This problem has happened on both a 1996 150 EFI amd a 1996 200 Carb motor that I have owned. CDI Rapair is a good source of supply for new regulators.
fishinchips	posted 08-03-2014 11:30 PM ET (US) Had a omc johnson 90hp that did the same thing. Turned out to be the voltage regulator.
jason78	posted 08-04-2014 01:03 AM ET (US) Thanks for the additional suggestions! Moving the selector a few times seems to have solved the problem for now. If I have any other problems I will check the regulator.
jimh	posted 08-04-2014 01:15 AM ET (US) Jason--I am glad to hear my procedure worked to restore your tachometer to good operation. My procedure has an impressive track record of success. I will add your case to the statistics.