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Author Topic:   Fluctuating RPM Indication
Jaybld posted 08-03-2007 08:30 AM ET (US)   Profile for Jaybld   Send Email to Jaybld  
While cruising around 4,000 RPM's ( 2003 90 hp Merc 2S) I noticed the RPM gauge would occasionally move immediately to 5,000, stay there for some time, and then move immediately down again. Sometimes it would also go to 3,000 and then suddenly move back up. All the while I never touched the throttle nor did the engine actually change RPM's. The RPM gauge appears to work normaly, otherwise.
Anyone see this before ( I tried a search and could not find anything exactly like this)? Could it be a retifier, bad gauge, loose or corroded wiring?
Henry posted 08-03-2007 10:36 AM ET (US)     Profile for Henry  Send Email to Henry     
Yes I had this problem on my 2005 Mercury 2 stroke Saltwater. The tach was old and I attributed the problem to the tach. I installed a new tach and the problem disappeared.
bluewaterpirate posted 08-03-2007 10:59 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Check the electrical connections on the back of the tach for corrision. My tach lasted three years then I had to replace it. Sounds like it's time to replace it.
Jaybld posted 08-03-2007 04:27 PM ET (US)     Profile for Jaybld  Send Email to Jaybld     
Thanks Master Chief, and Henry. I'll pursue those leads.
jimh posted 08-05-2007 08:17 AM ET (US)     Profile for jimh  Send Email to jimh     
Most tachometers have a multi-position switch used to calibrate the tachometer to the frequency of the pulses supplied by the engine. Locate the switch on your tachometer. Make a note of the current position. Using an appropriately sized screw driver, rotate the switch back and forth throughout its range of adjustment at least ten times. This will help to wipe any corrosion from the switch contacts. Return the switch to the initial position. This procedure often cures problems like the one you have described.
Jaybld posted 08-05-2007 09:25 PM ET (US)     Profile for Jaybld  Send Email to Jaybld     
Thanks to all. I removed the gauge and cleaned all connections with salt away and then Corrosion Block. I also rotated the switch JimH referred to, many times. I did not see much corrosion and no rust, but there was some of the tell-tale white residue which I assume is from dried salt. By the way, that switch does not have very forceful detents, but I double checked to ensure it was in the right (6P) position.
As I could not take the boat out I was not able to check it to see if the fix worked, but it was certainly worth the try. I will follow up this week after I take her out again.

Thanks again to some great advice.

Jaybld posted 08-12-2007 10:30 PM ET (US)     Profile for Jaybld  Send Email to Jaybld     
Boat and tach tested fine over the last several days while in the Harker's Island ( NC) area. Thanks again.
jimh posted 08-12-2007 11:03 PM ET (US)     Profile for jimh  Send Email to jimh     
Great news. Moving the switch was much less hassle and expense than replacing the tachometer. I have seen several intermittent problems with tachometers cured using my technique.

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