Tachometer Readings Intermittent on Mercury FOURSTROKE

Electrical and electronic topics for small boats
jheiii
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Tachometer Readings Intermittent on Mercury FOURSTROKE

Postby jheiii » Thu Oct 25, 2018 1:29 pm

On my 2006 Mercury 90 HP EFI FourStroke engine (with 375 hours of running time) the tachometer gauge is the standard analog, non-Smartcraft gauge.

The last time running the boat, I noticed the tachometer reading would drop from say 2,500-RPM down to 1,000-RPM, then back up to the 2,500-RPM.. The engine ran fine.

After a couple hours, this behavior went away and did not come back again.

The voltmeter and water pressure gauges both were normal the entire trip.

Today, I went out to clean up and check the connections on the gauge. I tested the ground for continuity and cleaned the terminal. Ground tested good. I tested the ignition post off the back of the gauge. Battery switch on, key on, engine off: 12.2 to 12.4-Volts. With engine running, 14.2 to 14.4-Volts. The voltage didn't fluctuate or spike.

The tachometer was showing steady low-RPM idle, then started dropping to 0. After a couple minute,s it dropped to 0 and that was it. Engine runs fine, no change in voltage. I put the multimeter on the sender post, the gray wire out the back of the tachometer and id not get any reading : 0-Volts.

Q1: What would be the next step for testing that sender signal on the gray wire out the back of the tachometer?

Thank you
2007 Montauk 170

jimh
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Re: Tachometer Readings Intermittent on Mercury FOURSTROKE

Postby jimh » Fri Oct 26, 2018 9:04 am

Does the tachometer gauge have a rotary selector switch on the back panel of the gauge that allows its calibration to be adjusted for engines of different cylinder or alternator pole configuration?

jimh
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Re: Tachometer Readings Intermittent on Mercury FOURSTROKE

Postby jimh » Fri Oct 26, 2018 9:26 am

Generally the tachometer electrical signal (on the gray wire) is a fluctuating signal, consisting of pulses whose frequency varies in proportion to engine speed. If measured with a DC Voltmeter, predicting what sort of reading will occur is difficult. You might try reading the voltage with the meter set to AC. That may produce a reading.

I am not familiar with how the tachometer signal is generated on a Mercury FOURSTROKE, so I can't offer much advice on how to investigate it. Because the signal is likely a series of pulses, the best tool to analyze it would be an oscilloscope--but that's probably not something you have in your tool box (nor do I).

jheiii
Posts: 38
Joined: Tue Feb 07, 2017 11:39 am
Location: South Florida

Re: Tachometer Readings Intermittent on Mercury FOURSTROKE

Postby jheiii » Mon Oct 29, 2018 11:10 am

Jim,

Thanks. In researching this concern, I noticed you recommended a possible fix by rotating the rotary selector switch and in many cases it fixed it. I did try that when cleaning/checking the connections, no luck. I was hoping it was that easy.

The next step seems to be testing that gray wire. First, completing a continuity test from the tachometer signal to the signal from the outboard. I'll pop the cover off and have a closer look, if I can get to that gray wire on the outboard without pulling it apart then we'll give it a shot.

If that checks out, then like you said I have to find a way to test the resistance (Ohms) between the gray wire and the rectifier metal case. If the reading isn't in line with 10K-50K, then the rectifier is the concern. However, our multimeters don't seem to have that capability.

However, the multimeter does have a Diode function, and there are Youtube videos where people are using that diode function between the ground and signal wire, if it produces a reading then the rectifier is good.

Have an appointment with the mechanic for the 12th of November.
2007 Montauk 170

jimh
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Re: Tachometer Readings Intermittent on Mercury FOURSTROKE

Postby jimh » Tue Oct 30, 2018 3:48 pm

jheiii wrote:...then like you said I have to find a way to test the resistance (Ohms) between the gray wire and the rectifier metal case.


I do not see where I made that suggestion.

If you want to test the diodes in a rectifier, the best method I know is to disconnect the rectifier from the other components. Because the rectifier is probably connected to some coil windings (in the stator of the alternator), the resistance of the stator windings will confuse you about the resistance readings of the diodes if you try to make measurements of the diode while it is connected to the coil windings.

jheiii
Posts: 38
Joined: Tue Feb 07, 2017 11:39 am
Location: South Florida

Re: Tachometer Readings Intermittent on Mercury FOURSTROKE

Postby jheiii » Tue Nov 13, 2018 2:47 pm

Afternoon Jim, my apologies if I misinterpreted or misquoted your prior post. Updating the thread with the solution, I just got the call from my mechanic - boat will need a new tachometer. 1 hour of labor, and part is under $100 from Mercury.
2007 Montauk 170

jimh
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Re: Tachometer Readings Intermittent on Mercury FOURSTROKE

Postby jimh » Wed Nov 14, 2018 9:24 am

JHEIII--thanks for the update. Ask the mechanic to give you the old tachometer. Then, if you don't mind, perhaps you could send it to me. I'd like to disassemble it to see what is inside. I have been curious about outboard engine tachometers and how they're made for some time.