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Author Topic:   Erratic Tachometer, Two-cycle Outboard Engine
usafretired posted 08-24-2012 01:44 AM ET (US)   Profile for usafretired   Send Email to usafretired  
Lately my 2003 Mercury 60 Bigfoot two-cycle outboard engine tachometer has been reading high at idle. Sometimes [the reading on the tachometer] will jump a bit through the [engine speed] range. [O]nce [the engine] gets to full speed [the tachometer reading] smooths out. [Seeks] any ideas on [the cause of] the problem and what to look for [to remedy the problem]. Thanks
jimh posted 08-24-2012 09:05 AM ET (US)     Profile for jimh  Send Email to jimh     
Check the calibration control on the tachometer. Usually there is a switch to select the proper number of poles in the alternator supplying the signal to the tachometer. Perform this procedure:

Tachometer Calibration Control Cleaning

--make a note of the current position of the tachometer calibration control

--using an appropriately sized screwdriver, move the tachometer calibration control setting back and forth throughout its full range of adjustment, repeating this several times

--return the calibration control setting to the original position noted above

This should clean off the contacts in the calibration control setting switch. This often restores proper operation to the tachometer.

If this procedure does not fix the problem, let us know and we will offer additional suggestions.

Dick E posted 08-24-2012 08:32 PM ET (US)     Profile for Dick E  Send Email to Dick E     
Check [the connection to] ground.
L H G posted 08-25-2012 01:24 AM ET (US)     Profile for L H G    
Generally with an erratic tach reading on a Mercury, you need a new rectifier.
jimh posted 08-25-2012 04:09 AM ET (US)     Profile for jimh  Send Email to jimh     
I don't believe that the tachometer circuit on a two-cycle outboard which uses a permanent magnet alternator is particularly different on a Mercury than on other brands. The tachometer signal is typically taken from a half-wave rectifier tap in the permanent magnet alternator. If the battery charging circuit rectifier is malfunctioning, it can affect the tachometer circuit. This occurs in outboard engines of all brands. Accompanying the erratic tachometer readings will also usually be very poor battery charging current output, if the rectifier is the cause of the problem.

The tachometer is driven by the tachometer signal, but it also operates from the boat 12-Volt battery, and thus any variation in the connection of the tachometer to the 12-Volt power distribution can affect the operation of the tachometer. The connection of the tachometer power leads to the 12-Volt power distribution on the boat should be checked.

fishinchips posted 09-04-2012 10:59 PM ET (US)     Profile for fishinchips  Send Email to fishinchips     
I agree with LHG. I had a same problem. Turned out to be a rectifier


jimh posted 09-04-2012 11:48 PM ET (US)     Profile for jimh  Send Email to jimh     
Ken--Are you saying that there is something unique about Mercury? I just don't understand this. I had tachometer problems on a couple of engines--not Mercury engines--and the cause was the rectifier. I don't think there is anything unique to Mercury that makes the rectifier cause tachometer problems. All the older outboard engines use this arrangement, and all of them will cause problems with the tachometer if the rectifier is bad. This is not something only true about Mercury. The notion that a rectifier can cause tachometer problems is not something that happens only on a Mercury, as far as I can tell. Maybe L H G means that the rectifiers on Mercury outboards are prone to fail, and that's why there is this link between them and tachometer problems.
jimh posted 09-04-2012 11:51 PM ET (US)     Profile for jimh  Send Email to jimh     
By the way, if our original poster is still participating in this discussion--we have not heard from him in almost two weeks--I recommend he follow the advice given and in the order it was given. I would not run out and get a rectifier until I checked the calibration control and the general wiring to the tachometer.

If he needs advice on how to test the rectifier, I can give that, too, but I wait to hear how his checks of the other likely causes turned out.

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