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Author Topic:   Replacing old OMC tachometer
cinegamma posted 08-23-2009 10:40 PM ET (US)   Profile for cinegamma   Send Email to cinegamma  
My 1984 Montauk has the original OMC tachometer that still works, but I now have a 1996 Johnson. Currently I have no way to monitor the System Check. Do the tachometers with the System Check indicators also have an audible alarm? Any suggestions on a brand/model? It looks like the tachometer portion of the wire harness is hooked up to my old gauge. Is it as simple as hooking the other wires in the harness to the new gauge?


Salty Tricks posted 08-24-2009 02:14 PM ET (US)     Profile for Salty Tricks  Send Email to Salty Tricks     

The 96 engine should have come with a harness that has a plug for the system check Tach. It is also wired to a small horn to provide audible alarm for the system check. If you have a binacle controller that came with that motor the horn is under the controller housing. You also may decide to use your old tach and purchase a separate system check gauge(I got mine from Ebay). Both type gauges will give you audible alarm.


cinegamma posted 08-24-2009 02:51 PM ET (US)     Profile for cinegamma  Send Email to cinegamma     

I think the controller I have is an older one (at least older than the motor). Someone mentioned you should hear a short tone when the key is turned on. I don't hear anything, so maybe I don't have the alarm horn or something else is wired improperly. Until I get the system check gauge, I would at least like to have an alarm horn to indicate a problem. Is the horn easy enough to spot under the controller?


conch posted 08-24-2009 05:48 PM ET (US)     Profile for conch  Send Email to conch     

This should be straight forward for you. Locate the eight wire plug from the harness at the console,it will have to (1)plug into a system check tach or (2)a system check gauge or(3) a black box which will enable the horn to function. Number (3) is rare and I have not seen this in years. Having a alarm horn is another issue ,look for it. The horn is connected by another two wire plug from the harness. Your eight wire plug is most likely just hanging in the console. The tach or gauge are both on E-Bay. I like the OMC tach as it has 100 rpm increments,you can then use the purple and black leads for a voltage gauge ,(capping) the unused gray wire. This is also a good time to add a water pressure gauge. The diagrams for the harness should be in the Accessories service manual for your engine but it is pretty much plug n play. I hope this helps.


jimh posted 08-24-2009 10:32 PM ET (US)     Profile for jimh  Send Email to jimh     
There was a sea change with OMC outboard engine wiring harnesses in 1996. You have pre-1996 boat rigging and a 1996 motor. There is an adaptor kit made specifically for the purpose of adapting a 1996 motor to a 1995 or earlier rigged boat.
cinegamma posted 08-25-2009 10:50 AM ET (US)     Profile for cinegamma  Send Email to cinegamma     
What do I need to look for? What are different about pre-1996 and post-1996 wiring harness?

jimh posted 08-25-2009 11:37 PM ET (US)     Profile for jimh  Send Email to jimh     
In c.1996 OMC changed to the Modular Wiring System (MWS) harness. Different connectors were used in their wiring harnesses. The System Check gauge was introduced and a connector was provided for the gauge in the MWS harness.

The pre-1996 engine wiring harness was characterized by a large red round Amphenol multi-pin connector with a rubber body, as well as several smaller round rubber body Amphenol connectors. The MWS harness uses Deutsch connectors, which are typically rectangular and have a nice rubber or vinyl bellows for making a seal.

You can see details of the adaptor kit on the website


Locate the parts drawing section. Go to OB ACCESSORIES. Got to year 1996. Locate the 1995-BOAT to 1996-Engine Adaptor part kit.

jimh posted 08-25-2009 11:39 PM ET (US)     Profile for jimh  Send Email to jimh     
If the original electrical harnesses in your boat have been modified to accommodate a System Check gauge, there is no telling precisely what has been done. I suggest you make a detailed drawing of all the electrical circuits in your engine, harness, and dashboard wiring. This will give you the necessary understanding of precisely what configuration you have now. It is impossible to speculate about how your particular motor is wired if there have been modifications performed to the original wiring harnesses.
jimh posted 08-25-2009 11:42 PM ET (US)     Profile for jimh  Send Email to jimh     
In System Check rigging the alarm sounder is controlled from the System Check gauge. The alarm signals are sent to the System Check gauge. The gauge monitors the alarms. It sounds the aural alert sounder when there is an alarm condition. This is quite different from the older alarm sensor and alarm sounder wiring, which was a simple WIRED-OR arrangement in which all sensors pulled a common alarm circuit to ground.
cinegamma posted 08-25-2009 11:54 PM ET (US)     Profile for cinegamma  Send Email to cinegamma     

Thanks for the help and explanation. I have the old harness hooked-up via the adapter. Will this configuration sound the old style alarms? I'm guessing if I want the system check gauge, I'll need to run the newer style wiring harness?


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