Lund/Merc 2 stroke tach troubleshooting

[JayM5209]

The boat comes out of storage this weekend so it's time to tap into the wealth of knowledge on WC. I've done some searching in the numerous tachometer threads here but need some advice on where to start my troubleshooting. Here's the situation -

2008 Lund Rebel 1625 XL SS with 2008 Merc 50 hp 2 stroke purchased new in April '08. The tach has never shown any erratic or unusual behavior. During my last week long trip of 2016 to NW Ontario - late August - it suddenly and then consistently read 600 - 1000 RPM high. When I shut down the tach needle stayed at 600-1000 RPM. If I turned the ignition on (without starting the motor) the needle dropped to 0, but then when the motor was started it read high again. For the remaining 3 or 4 days of the week this behavior was very consistent - 600-1000 RPM high while running, needle did not drop to zero when shut down, dropped to zero when key was turned on w/out starting, read high when started again.

No one has been under/behind the dash to mess with the setting, no electrical work has been done on any wiring in the boat, no new electronics or power functions have been added. Loose wire connection? Short somewhere? Bad tach? . . . . ?

Any suggestions as to where I start will be greatly appreciated. TIA.

[REW]

Jay,
A two cycle motor tach works by counting the number of pulses that are coming down the alternator line from the motor.

The fact that the alternator reads something and changes, likely indicate that the wire going to the motor alternator is in good shape.

The quickest and easiest way to check the tach, is to take a new, or known good tach - hook up voltage ground and tach wire to the new or known good tach and check the readings.

If the readings are good, then you know that the problem is the tach itself.

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Also check the "pole setting switch" on the back of the tach and verify that the switch is in the correct position.
This switch is set to match the number of "poles" that are on the alternator in your boat. If the switch is in the incorrect position - the tach will read either high or low.
Also, sometimes this switch can get dirty - so you could switch the pole switch to all of the different positions several times to essentially clean the switch. Then, return the tach to its correct setting and retest the tach reading.

http://www.gandermountain.com/assets...plications.pdf

For your motor - the pole switch should be set on "12" which corresponds to the # alternator poles on your engine.


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The other simple thing to do, is to measure the voltage going to the tach, and the ground wire of the tach, to verify that you are getting full battery voltage between the tach voltage line and the tach ground line.

The tach voltage wire comes off of the ignition switch, and could be low if there is a connection issue in the boat's ignition switch, or if the ground wire going to the tach, has a bad ground connection.

So, just go to the back of the tach, and read the voltage between the voltage and ground connections and find if the voltage measured here - with the ignition switch on - is the same as the starting battery voltage as measured at the starting battery terminals.

Good luck

[yarcraft91]

It's worth doing an experiment with your tach. Note its current setting, which is probably 6 or 6P. Rotate the switch through all possible setting 4 or 5 times, then return to the original setting. This will clean off the switch contacts and may stabilize your tach readings.

If your tach has a 6P switch setting, that is correct for the 12-pole alternator in your outboard. However, there are other switch setting codes used in some brands of tachs that can make it confusing to find the right switch setting.