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Author Topic:   Erratic Tachometer Readings
Local Boy posted 02-29-2008 01:09 AM ET (US)   Profile for Local Boy   Send Email to Local Boy  
I have an Outrage with twin 1998 135-HP OptiMax motors. Lately, the tachometer reading on the port engine has been erratic as compared to starboard engine. At troll speed of 1,200-RPM on both engines the port engine [tachometer reading] will jump to as much as 3,000-RPM for an hour or so. Then [the tachometer reading] returns to normal. During these times the engine shows no noticeable change in power or RPM. Mercury suggests that I take it a dealer. If I could I would. I live on Molokai and nearest dealer on Oahu. So far I have been lucky in doing my own maintenance. The problem seems like a wiring problem to me, that is, connections or grounding. I have not found anything in this area yet. These are the old analog type gauges.
Has anyone experienced similar behavior??

Thanks Gary

jimh posted 02-29-2008 09:14 AM ET (US)     Profile for jimh  Send Email to jimh     
I agree that an erratic reading on a tachometer is often due to a poor connection. Check the back of the tachometer gauge to see if there is a calibration control. Follow this procedure:

--note the current position of the calibration control setting

--using an appropriately sized screwdriver or adjustment tool, rotate the calibration control back and forth throughout its range of adjustment several times

--return the calibration control to the original setting

This procedure often restores the tachometer to normal operation. If the procedure does not succeed, try spraying a small amount of an electrical lubricant and contact cleaner (such as WD-40) onto the calibration control in such a way that it will be likely to penetrate to the actual contact surfaces of the control. Repeat the procedure above.

jimh posted 02-29-2008 09:17 AM ET (US)     Profile for jimh  Send Email to jimh     
I have seen this procedure restore normal operation to a tachometer which has been used in a saltwater environment.
ivansfo posted 02-29-2008 06:40 PM ET (US)     Profile for ivansfo  Send Email to ivansfo     

Do you have Smartcraft gauges? And how old are your batteries?

I experienced the wild tach needle a few years ago with my pair of 135 Optimax. It was certainly an annoyance but this problem alone wasn't enough for me to spend the effort to take the boat back to my dealer.

My problem was cured when I swapped out my pair of aging batteries. Nothing else on the boat was done to fix the problem. With a fresh set of batteries, all my weird problems with the Optimax went away.


Local Boy posted 02-29-2008 07:33 PM ET (US)     Profile for Local Boy  Send Email to Local Boy     

I have the old analog gauges. The batteries are about a year old and show no signs of weakness. But I will put this on the list. I would suspect that since reading jumps higher, if it is the battery it could be a battery grounding problem. I have taken Jim advice a played with calbration pot. As Jim rightly pointed out, this low voltage signal could easily be effected by eratic pot resistence.

I went down to harbor this AM and turned pot back and forth a couple of times. Could not turn pot all the way because of too long of screwdriver. If the wind lays down tomorrow, I will take the boat out and try. At ldle no signs either way.

Thanks everyone

Local Boy posted 03-01-2008 05:58 PM ET (US)     Profile for Local Boy  Send Email to Local Boy     
Thanks to Jim it appears that my Eratic Tach reading has been fixed. After 6 hours on the water this am, both tachs were rock solid and equal.

It does a appear that the problem was in the calibration pot. After working on this I realized that further isolation of the problem could have been done by placing a voltmeter across the tach input terminals. A note for anyone having similar problems inthe future.


swist posted 03-02-2008 07:31 AM ET (US)     Profile for swist  Send Email to swist     
Just another aside - I have an electronics background, and after my tach failed (consistent very high readings regardless of calibration, connections, etc), I replaced it and took the old one apart out of curiosity - I have to say I was underwhelmed by the quality of protection against salt-air or water incursion, or against vibration. This was a Teleflex Premier-Pro style analog gauge. I don't know if there are other models (or other brands) that are built to be more environmentally robust.

I am sure that Jim has led you to to the solution for your particular problem, but in the future I will never assume that suspicious tach readings are necessarily indicative of the actual engine speed.

jimh posted 03-02-2008 08:44 AM ET (US)     Profile for jimh  Send Email to jimh     
I am glad to hear that my repair procedure provided a remedy to your erratic tachometer reading problem. So far, as best as I can recall, this procedure has been one-hundred-percent successful in restoring tachometer calibration to tachometers which had sudden onset of erratic readings.

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