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  1. #1
    Seaman Apprentice sc00ter78's Avatar
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    Default 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
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  2. #2
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    Default 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.

  3. #3
    Seaman Apprentice sc00ter78's Avatar
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    Default 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
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  4. #4
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    Default 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?

  5. #5
    Seaman Apprentice sc00ter78's Avatar
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    Default 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?

  6. #6
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    Default 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.

  7. #7
    Seaman Apprentice sc00ter78's Avatar
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    Default 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."

  8. #8
    Seaman Apprentice sc00ter78's Avatar
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    Default

    ...

  9. #9
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    Default 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.

  10. #10
    Seaman Apprentice sc00ter78's Avatar
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    Default 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?
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  11. #11
    Seaman Apprentice sc00ter78's Avatar
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    Default Re: Tach install on Merc 500 Thunderbolt

    Anybody???

  12. #12
    Seaman Apprentice sc00ter78's Avatar
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    Default 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).

  13. #13
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    Default 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.

  14. #14
    Seaman Apprentice sc00ter78's Avatar
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    Default 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?

  15. #15
    Seaman Apprentice sc00ter78's Avatar
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    Default Re: Tach install on Merc 500 Thunderbolt

    Anybody???

  16. #16
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    Default Re: Tach install on Merc 500 Thunderbolt

    Had I not experienced the same problem with mine at one time I would not believe your readings. However, I also experienced a rough idle condition and weak spark. My solution was to go with the hat trick and replace the stator, trigger and rectifier. That solved my problem but I can't tell you for sure which was at fault because they all can cause very similar running conditions. My stator did have a bad spike condition so I just assumed damage to the rectifier and trigger. Expensive way to go but it saves a lot of headaches.

  17. #17
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    Default Re: Tach install on Merc 500 Thunderbolt

    dont use the wire to switchbox .... go from either yellow wire on the rectifier to the sender connection on the tach, black from tach to a ground
    power/+ should go to a ingnition ON connection if done this way it should work with the proper 1 to 6 settting. if not you got a bad rectifier or stator
    most likely would be rectifier than the stator

  18. #18
    Seaman Apprentice sc00ter78's Avatar
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    Default Re: Tach install on Merc 500 Thunderbolt

    Thanks Jimg984 and sschefer,

    I will be looking at that this weekend. Just bought a rect. off Ebay and awaiting the guy to reply and mail it off.

    I wil keep you guys posted.

  19. #19
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  20. #20
    Seaman Apprentice sc00ter78's Avatar
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    Default Re: Tach install on Merc 500 Thunderbolt

    Been there...

  21. #21
    Seaman Apprentice sc00ter78's Avatar
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    Default Re: Tach install on Merc 500 Thunderbolt

    Had the boat in the water yesterday and tested out the new rectifier along with my old one and both work.

    Tried the new tach on setting #2 and the needle jumps around and on setting #6 the needle flutters at around 100 to 200 RPM's above normal.

    Lastly I put in the old origanal Tach and the needle stay level with all RPM's.
    So I guess the tach may be the problem although it is new from Mercury.
    The rectifier charges the moter fine.

    Depth sounder works too, evn put it on it's own switch.

    Thanks to everyone who helped me with this problem. I have learned lots throughout all this.

    I think that will be the last of the posts from me on this issue. Time to look for a new/used boat. Been looking at the 50HP Evinrude E-Tech on an aluminum. (Starcraft, Polar Craft, Misty Harbor, Alumacraft, Smoker Craft)

  22. #22
    Seaman Apprentice sc00ter78's Avatar
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    Default Re: Tach install on Merc 500 Thunderbolt

    After talking with the staff at Mercury I found out the Tach I have is for a Lightning ignition, not the Thunderbolt ignition. That was the cause for the jumping.

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