1978 Mercury Model 700 70 horse Tachometer

Discussion in 'Outboards' started by Dave T, Sep 17, 2013.

  1. Dave T
    Joined: Sep 2011
    Posts: 250
    Likes: 14, Points: 18, Legacy Rep: 158
    Location: Anamosa Iowa and North Buena Vista on the Mississi

    Dave T Senior Member

    I bought a 1978 70 horse Mercury outboard and I want to add a tachometer so I can better monitor it's performance. The control has 3 wires that exit the front of the control that have been cut off a red, black and white I'm not sure if these are for a tach or not. Can anybody tell me what I need to do to connect a tach to this motor? Any info will be appreciated.

    Thank You
    Dave T :confused:
     
  2. BMcF
    Joined: Mar 2007
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    Location: Maryland

    BMcF Senior Member

    You can buy an outboard motor tachometer from several vendors that simply convert the pulses in the alternator output in to RPM on the tach needle. So the only "hookup" required is power and ground.
     
  3. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Outboards from that era have a coil under the flywheel that acts as a simple generator to charge the battery. The rectifier is mounted somewhere else on the engine.
    On the rectifier input terminal is 14V AC with the same frequency as the engine rpm.
    Check if the white lead on the control box starts at that terminal, red and black are probably power and ground.
     
  4. Yellowjacket
    Joined: May 2009
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    Location: Landlocked...

    Yellowjacket Senior Member

    Mercury's of that vintage have a tach wire in the harness that goes to the control and the wires coming out of the front of the box were wired for a tach. The box has a three wire plug, but they could have pulled that off and you might just have a pigtail hanging out.

    Here is a link to a place where you can get a schematic for your engine.

    http://www.maxrules.com/mercwireindex.html

    If you look at the 1976-1978 50 horse electric start wiring diagram you will see how the control is wired, and the controls are all the same.

    With the two above diagrams you should be able to figure out what plug pin is what, and you can use an ohm meter or test light to chase the wiring and double check. These engines are, for the most part wired with a 12 pulse output to the tach, here is a link to the Farina web site,

    http://faria-instruments.com/site_manuals/IS0086_S_Outboard_Tach_Applications.pdf

    double check, but to me it looks like you have a 12 pole tach output. Of course the tach needs power so you need 3 wires (which you have), a hot lead to power the tach, a ground and the pulse lead. Most aftermarket tach's have a DP switch on the back that allows you to select the pulse input to the tach. Inside the box there are three wires that go to the tach output. They are a brown, a black and a white wire. The white is the pulse output, the black is ground, and the brown is the power for the tach. My guess is that the person running the wires from your box exchanged a red wire for the tach power lead and the others are consistent with the standard Merc diagrams.

    Pretty easy
     
  5. Dave T
    Joined: Sep 2011
    Posts: 250
    Likes: 14, Points: 18, Legacy Rep: 158
    Location: Anamosa Iowa and North Buena Vista on the Mississi

    Dave T Senior Member

    Thanks for the info Yellowjacket looked at the wiring diagrams. The wires from the control are brown, black and white as you said. I'll go ahead and get a tach ordered.

    Thanks a bunch!!
    Dave T
     

  6. Dave T
    Joined: Sep 2011
    Posts: 250
    Likes: 14, Points: 18, Legacy Rep: 158
    Location: Anamosa Iowa and North Buena Vista on the Mississi

    Dave T Senior Member

    Bought a tach from discount marine and installed it. It works great. after studying the wire diagrams I saw that the brown wire is the sender wire for the tach, the white wire is the 12v positive and the black is ground. Thanks again yellowjacket for the link to the wiring diagrams.

    Dave T :)
     

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