1983 OMC tach operation

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  • September 10, 2018 at 7:50 pm #11147
    johnrude
    Participant

      Gentlemen,
      My OMC tach is beginning to indicate erratically on my 1983 Johnson 70. Just old age or poor connection somewhere? The plug and play connector into the throttle/shift control looks OK for a 35 year old connector.It does not effect the engine running at all. Thanks for your suggestions.
      Jim

      September 10, 2018 at 7:53 pm #82585
      frankr
      Participant

        US Member

        Probably poor connection. Clean your battery terminals as a first step.

        September 10, 2018 at 8:57 pm #82588
        fleetwin
        Participant

          US Member - 2 Years

          Could be a bad connection, or perhaps a problem with the alternator…The tach gets its signal from the alternator windings. So, if the alternator is messed up, the tach could act erractically.
          The simplest thing to do would borrow another tach and plug it into the connector on your control box…I realize it might be tough to find a donor tach, but possible.
          Those rubber amphenol connectors can be troublesome also. Pull the tach plug off and look inside, the three pins should be straight, and make sure none are pushed back in the connector. Try to peek at the socket end of the connector on the control box as well, perhaps one of the sockets has pushed back in its connector.
          Please do not cut the leads or attempt to lube the rubber connector with grease or WD40, this could create a bit of a short. Soak the two halves of the connector with rubbing alcohol before reassembly, the alcohol will lube the rubber and evaporate quickly.
          I suppose I should have began my reply with checking the "pulse setting" on the tach. It may be very hard to see buried behind the dash, but the OMC tachs should be set to position "6" for your engine.
          Like Frank says, check simple things first, like battery connections/cables. You could try wobbling the rubber tach connector while the tach is misbehaving to see if it straightens out. How is the tach misbehaving?

          September 11, 2018 at 9:32 pm #82677
          johnrude
          Participant

            The tach is operating erratically. At dead idle it’s bouncing low while at mid throttle it’s fluctuating perhaps 1000RPM from a constant throttle setting. I do a bit of trolling usually about 1000 so it’s fluctuation is more obvious at idle. It is erratic because at times, it seems to be just fine. I will dig a little deeper in the connection. Thanks!

            September 12, 2018 at 1:46 pm #82716
            fleetwin
            Participant

              US Member - 2 Years

              Sounds like the tach is probably just worn out…Try wobbling the connector on the control box while the tach is fluctuating to see if that makes a difference….Having the tach pole setting incorrectly adjusted would not cause the tach to bounce/fluctuate, it would just cause an inaccurate RPM reading. A loose wiring connection would probably cause the tach to bounce up and down from zero RPM….
              But, as with anything electrical, anything is always possible….It is best to start with the simplest/easiest solutions though….
              The simplest thing to try would be to plug in another tach to your control box….I realize it might be tough to find a plug in tach to "borrow" for a few minutes, but maybe your local dealer can help you out….Again, your tach gets its signal from the alternator….So, if the alternator is not working properly, this may affect tach operation. There are some relatively simple ohm meter tests for the alternator windings and rectifier diodes though.

              September 12, 2018 at 11:50 pm #82762
              billw
              Participant

                US Member - 2 Years

                Believe it or not, I have had some luck, in multiple boats, taking a screw driver and turning the pole adjustment switch back and forth a few times, then putting it back on the correct setting. It’s like the switch contacts were dirty. Your results may vary; but it’s worked for me quite a few times….

                Long live American manufacturing!

                September 13, 2018 at 2:34 pm #82795
                fleetwin
                Participant

                  US Member - 2 Years
                  quote BillW:

                  Believe it or not, I have had some luck, in multiple boats, taking a screw driver and turning the pole adjustment switch back and forth a few times, then putting it back on the correct setting. It’s like the switch contacts were dirty. Your results may vary; but it’s worked for me quite a few times….

                  Sure seems like an easy solution, will only take a few minutes….Worth a try…

                  September 13, 2018 at 8:45 pm #82817
                  johnrude
                  Participant

                    Excellent thought. I would not have thought about that. I will give it a try. Thanks!
                    Jim

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