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Tach install on Merc 500 Thunderbolt

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  • Tach install on Merc 500 Thunderbolt

    I have a New Merc Tach (PART # 79-883620Q2) to replace an old one for my 1975 Mercury 500, Thunderbolt. 4 Cylinder, 2 Cycle.(Serial # 7052814)

    My questions is:

    What number in Column #1 (Position) do I turn the dial to? (There is a turn dial to select 2 to 6). I have no alternator/charger.

    The tach has connections for 12volts, GND, SEND, and a light.

    The istructions:

    4 Cycle gas Engine, Outboard engine
    Position Pulses/revolution Standard Ignition with alternator with

    2 2 4 cyl 4 Pole
    3 3 6 cyl 6 pole
    4 4 8 cyl 8 pole
    5 5 -- 10 pole
    6 6 -- 12 pole


  • #2
    Re: Tach install on Merc 500 Thunderbolt

    Position 2 should work but it will probably work on postion 6 also. FYI, you have an alternator its the stator and magenets under the flywheel. I think you mean you don't have a rectifier. How are you connecting the tach to the engine.

    Comment



    • #3
      Re: Tach install on Merc 500 Thunderbolt

      I already have one on the boat. Reason for changing is for a cleaner looking one. As I recall there is are red, black, and brown wires. Is the idling range 800-1000?

      I was told by a nearby mechanic that there was no alternator on it and therfor have to charge the battery after everytime I take the boat out. Do you know roughly how many amps it puts out?
      I will look for info on the "stator" you mentioned and more about the rectifier.

      Would you happen to know why the Tach needle drops back to zero for a few minutes when running on the water? RPM ranges are various when it happens.

      Thanks for all your help. Much appreciated.

      Scott

      Comment



      • #4
        Re: Tach install on Merc 500 Thunderbolt

        Red would be the ignition +12V, black is Gnd (-12V) and brown would be the tach signal from the motor. Chances are with the gauge you have, the tach signal is taken from the brown terminal on the switchbox. With a new style tach you will need to remove the brown wire from the 4 terminal side of switchbox (don't move any wires on the 3 terminal side) and connect it to either of the yellow wires on the rectifier. The rectifier will have three wires connected to it. Two yellow wires from the stator, and a red wire that runs out to the battery. This is the +12V that charges the battery. I think the stator puts out about 9amps at full rpm.

        As for the idle RPM range, 800- 1000 is a little high if in the water. On muffs it would be OK. Would shoot for 600-700 in the water.

        Would you happen to know why the Tach needle drops back to zero for a few minutes when running on the water? RPM ranges are various when it happens.

        Could be bad connection at the switchbox, gauge, tach harness connection at the controls. Maybe a broken wire that intermittently make contact. Perhaps a defective gauge?

        Comment



        • #5
          Alternator test/problems?

          Thanks for your help guys "sschefer" and "Mody Blue".

          The info really gives me a direction to go in. It's difficult finding info on this motor.

          Next will be to install a Uniden Depth Sounder (QT206).

          After spending a whole day on the water I've never seen the battery hold a charge or stay topped up. I have always had to recharge it at the end of the day. I am going to get my battery tested (3 yrs old).

          Would either of you know if the alternator has problems charging and if so how to test it?

          Comment



          • #6
            Re: Tach install on Merc 500 Thunderbolt

            http://www.maxrules.com/oldmercs/Wiring/1966ona/26A.jpg

            stator,and rectifier will determine charging.You can find trouble shooting stuff on that maxrules.com site too.

            Comment



            • #7
              Re: Tach install on Merc 500 Thunderbolt

              sschefer - Looking at your restoration of your boat reminds me of when I did mine. You did a great job. I will post some pic's of mine soon.

              I know the feeling of being half way done and thinking - "wow, I didn't expect this much work."

              Comment



              • #8
                ...

                Comment



                • #9
                  Re: Tach install on Merc 500 Thunderbolt

                  Either the battery or the rectifier is no good. Do a search here on how to test the rectifier. Many auto supply places will load test the battery for free.

                  Comment



                  • #10
                    Re: Tach install on Merc 500 Thunderbolt

                    Ok, I've been reading some of the rectifier and tach post on here and looked at my boat pic's and saw that I do have a rectifier.

                    1st
                    Looking at the wiring drawing posted from earlier, it looks possible to take the "E" Tan wire off the Switch Box and connect it to either yellow wire on the rectifier.

                    The are a lot of posts saying to run a seperate wire. Shouldn't both ways work? Cause the Tan "E" only connects to the Tach and if disconnected it won't go anware and just hang losely. Why not use it to connect to the rectifier?

                    2nd
                    After looking for ways to test the rectifier I only came across testing the voltage on the battery leads from the motor with it off and then on. I thought that because you can pull start the motor without the battery connected and the key "on" you should be able to test the voltage comming off the rectifier or cables going to the battery.

                    Prior to me I don't know but say the motor was connected backwards (reverse polarity) to the battery, wouldn't that blow the rectifier and possibly the stator if windings were thin and not a heavy guage?

                    3rd
                    Looking at the pic I uploaded. It looks like someone has already connected a Green wire to the rectifier and left the Brown wire attached. Does anyone agree?

                    Comment



                    • #11
                      Re: Tach install on Merc 500 Thunderbolt

                      Anybody???

                      Comment



                      • #12
                        Re: Tach install on Merc 500 Thunderbolt

                        Tach
                        testing tach now, tach is jumping on all positions (2-6) when connected to either yellow wires on rectifier as well as the switch box.
                        NOTE: new wire ran directly from rectifier and switch box to the tach (no loose connection).

                        Rectifier
                        voltage jumps 12.59v for 2 seconds and then drops to 0v for 1 second and then back up to 12.59v (repeating).

                        Comment



                        • #13
                          Re: Tach install on Merc 500 Thunderbolt

                          Rectifier
                          voltage jumps 12.59v for 2 seconds and then drops to 0v for 1 second and then back up to 12.59v (repeating).
                          Is this while its connected to the battery?If it isn't don't run it while taking any wires off....you could do some damage.

                          Comment



                          • #14
                            Re: Tach install on Merc 500 Thunderbolt

                            This is while the engine is running and tested at rectifier.

                            Why does it matter if the battery is connected other than charging it?

                            What would cause the repeating drop in voltage?

                            Still looking.....does anyone have an answer for the tach question posted above?

                            Comment



                            • #15
                              Re: Tach install on Merc 500 Thunderbolt

                              Anybody???

                              Comment

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