Mercury Tachometer

Electrical and electronic topics for small boats
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Joined: Sat Jul 09, 2016 12:02 pm

Mercury Tachometer

Postby Redmanf1 » Tue Aug 28, 2018 12:28 am

The tachometer on a Mercury OptiMax 135 just started acting up. At times at idle [the Mercury tachometer will read] 4000-RPM, and then will settle down to 900-RPM. At times you can tap on the tachometer, and [the reading will return] to normal.

[Is the remedy to this problem to discard the tachometer and replace with] a new one?

Thanks for any help.

Posts: 8810
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula

Re: Mercury Tachometer

Postby jimh » Fri Aug 31, 2018 5:37 pm

Before throwing out the tachometer, locate the calibration adjustment on the rear. Usually the calibration is a slot in a rotating knob.

Note the position of the slot. Often there are graduations on the calibration scale.

Using the proper size screwdriver, rotate the calibration control back and forth throughout its range of adjustment.

Return the calibration to the original position.

Test to see if this gives a remedy.

Also, the tachometer could be good, and the real cause of the problem could be in the tachometer signal from the engine.