OMC Tachometer Help Please

[BigBrud1958]

I have a 1984 Johnson 185HP that I use infrequently. This year for some reason the tach is reading about 60% of what it should read. For example, WOT shows at about 3,300 rather than the 5,500 that it should. Everything else seems to be working correctly and I have not changed anything in the set up except the battery. The speed at WOT is what it has been historically. Operating the boat, I would guess that the engine is performing about as it should.

Where do I start to diagnose this problem? Is it most likely a bad tach or the signal coming from the motor?

Thanks in advance for the help.

[rockman69]

Look at the back, there should be a little dial-doohicky that you turn with a flat screwdriver. Turn and try different settings with it.

[BigBrud1958]

Thanks. This tach was very close to accurate last year when I packed it up for storage for the winter. If it is just an adjustment screw thing, do you have any idea why it would be so far off after being stored over the winter? Also, how do I know the real RPM with which to synchronize it?

[south599]

well those things do go bad. I had a tach from 84 on my boat that I had to replace because it went bad. fastest way would be to find a buddy with a similar one and hook it up to see if it works. then you'll know if yours is bad.

[Forkin' Crazy]

Is it charging the battery?

[JohnR]

If the battery is charging I would not be surprised to find the tach is bad. I swapped the motor on my boat and coincidentally the tach went bad at the same time. I thought it wasn't reading correctly because I went from OMC to Merc but it was the tach itself. A budget Teleflex/Sierra one can be had for under 50 bucks.

[BigBrud1958]

I have every reason to believe that the battery is charging, however, as I use this boat infrequently, and I just bought a new battery, and I had it on my automatic charger until just before use, it is possible that it is not. Is there a voltage that I should see during operation, below which I could have a stator/rectifier/regulator issue? I have no problem replacing whatever needs replacing, I am just looking for an indication where to start.

[JohnR]

I would connect a digital volt meter to the battery, it should be around 12 volts and somewhere near 14 when it's running.

[Forkin' Crazy]

Check the diodes in the rectifier. If they are good, grounds, wires, and connections are good, I say it was the tach.

[JL Sargent]

I had the 150XP '87 model and for a while it did charge the battery but quit sending the tach signal. After several more trips the charging circuit went ahead and layed down too though.