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Author Topic:   OMC Tachometer, Mercury Engine Controls
mowokicker posted 01-07-2006 07:36 PM ET (US)   Profile for mowokicker   Send Email to mowokicker  
Hi, I have a 1979 mercury outboard on my 1979 Larson Wildfire. I bought the boat back in November and have been rebuilding it since. An OMC tach came with the boat but was not plugged into anything . I am about done with the build and found a plug on the front of the mercury control arm/box which accepts 3 differnt wires. I imagine the 3 wires comming out the the tach goes into this plug but i dont know which ones go where. The plug has a verticle slot with 2 parallel slots that are prepindicular to the verticle slot. Kind of looks like this " I= " Any help would be great
jimh posted 01-07-2006 08:33 PM ET (US)     Profile for jimh  Send Email to jimh     
See the REFERENCE section for the wiring color codes. A tachometer usually has three terminals: signal, ground, illumination.

The tachometer will only read accurately if the pulses from the engine match the calibration. An OEM tach may not have an adjustment switch like a universal after market tachometer normally does. Better look into that before you try to connect it to the engine.

mowokicker posted 01-07-2006 11:18 PM ET (US)     Profile for mowokicker  Send Email to mowokicker     
Yeah, I know that the black [wire] is the ground, the [gray wire] is the [tachometer signal], and the [violet wire] is [battery positive when the ignition switch is in RUN or START], but I don't know which wire goes in what socket on the throttle arm box.
jimh posted 01-07-2006 11:41 PM ET (US)     Profile for jimh  Send Email to jimh     
Unless you are a friend of Superman you won't be able to see what wire is wired to what terminal on the Mercury connector until you either:

--open up the control box and look at the other side of the connector and see how it is wired, or

--use a continuity tester or ohmmeter to check what is wired to the connector terminals.

There is a connecting plug that you can buy from Mercury which should mate with that connector on the control box. It should have color coded wires attached to it. You'll need the plug anyways to connect, won't you?

LHG posted 01-08-2006 02:15 PM ET (US)     Profile for LHG    
You need to purchase, from Mercury or Teleflex, the tachometer harness that plugs into the side mount control box. This typically powers the gauges on the earlier Mercs.
mowokicker posted 01-08-2006 02:43 PM ET (US)     Profile for mowokicker  Send Email to mowokicker     
I have taken apart the control box before to see if i could tell which wire was going to what socket, but they go in side by side so its hard to tell which ones goes where. I bought some slide connectors that fit into the existing plug but if there is a plug that goes into that i will purchase that. Thanks.
mowokicker posted 01-10-2006 12:47 AM ET (US)     Profile for mowokicker  Send Email to mowokicker     
Hello, I had an idea today that helped me find which wire went to which slot on the front of my control box for the tach hookeup. There are 3 differnt wires as you know; ground, power, and pulse. I used a 12v light in order to find which wire is which. (be sure key is on). To find the power i attached the neg. wire of the light to the neg battery terminal then took the pos. wire from the light and just stuck it in each slot until the light came on, thus telling me which one was power. Then i turned the key off and attached the pos. wire from the light to the pos. battery terminal and took the neg. wire from the light and repeated the process until the light lit up. The only one that didnt make the light illuminate is the pulse wire. If i am wrong i would appreciate someone telling me before i hook up my tach. Thanks,
Joe Kriz posted 07-02-2006 04:10 PM ET (US)     Profile for Joe Kriz  Send Email to Joe Kriz     

That sounds correct to me. You will not have the 4th wire in the plug.. Only the 3 wires as illumination comes from a different source.

To correct the statement above that there are only 3 connectors to the tack..... There are 4

Power (Key ON)

If you are not using the illumination, then you do not need to worry about the light hookup...

Sounds like you have it figured out...

FAB posted 07-20-2006 07:36 AM ET (US)     Profile for FAB  Send Email to FAB     
The only problem with using a test lamp is it does not tell you polarity. You can have the positive and negative backwards and the test lamp will still come on. If you use a voltmeter and the needle pegs down you have it hooked up backwards. If you use a voltmeter and the needle swings up to 12 volts now you have found positive and negative. Buy the right harness and you won't have this problem. And I don't think an OMC tachometer will work with a Mercury.
jimh posted 07-20-2006 08:17 AM ET (US)     Profile for jimh  Send Email to jimh     
We have not heard from the originator of this discussion since January. As it is now late July and six months have passed, I think it is safe to conclude he has resolved the problem.
mercuryman1113 posted 10-04-2010 11:54 PM ET (US)     Profile for mercuryman1113  Send Email to mercuryman1113     
[Revived this old discussion after being dormant for four years to introduce a new topic.--jimh]

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