1979 Mercury 90 Inline 6 Wiring for Gages

[kmk89]

I just got a used 1979 Mercury 90 Inline 6 and I am looking to install some gages. Specifically, a tachometer, water pressure gage, and an engine temp gage, but I have no idea what wires go where or if I have to move any wires around on the engine. Installing the gages is easy, but the wiring on the engine to the gages is what I don't know much about. I'd like to do it myself, as the mercury dealer wants $100/hr. Any help would be great. Thanks

 

[Georgesalmon]

Re: 1979 Mercury 90 Inline 6 Wiring for Gages

there should be a plug on the front of the remote control that will have three wires. Ground black, purple ignition (to power the gauges) , and a tan or gray tach signal. You'll have to find some kind of after market senders and gauges for the water pressure and engine temperature, that was not something standard or even available on a '79 merc in '79. Also water pressure is very low by design, maybe only a couple of pounds, and with no thermostat the engine temp is also very low and dependent on the water temperature your running in. Although I think the tach is a necessary gauge, I also get all the information I need about engine temp and water pressure by turning my head and looking at the telltale.

 

[Chris1956]

Re: 1979 Mercury 90 Inline 6 Wiring for Gages

KMK, A water temp gauge will not show any temp reading unless the motor overheats, and then it will be too late. Go with a 0-15 PSI water pressure gauge. Remove the plastic elbow that is the telltale. Put a brass tee where the telltale was, and replace the plastic elbow and install a thread-to-hose barb for the water pressure gauge tube.

That motor may have the old wiring colors. In that case ign accessory power is white and the tach signal is brown. Black is ground. Tach signal is grey on the newer models, but never tan....

 

[kmk89]

Re: 1979 Mercury 90 Inline 6 Wiring for Gages

Ok, I will go with the 15psi gauge. One more question about the tachometer...There are warnings in a few places on the engine stating only to use a certain kind of tachometer. I tried to make out what is says exactly, but the lettering is really faded. Any ideas on what kind of tachometer works and will not mess up the ignition system? Thanks

 

[Chris1956]

Re: 1979 Mercury 90 Inline 6 Wiring for Gages

Any modern OB tach for mercury products will work fine. Older tachs for OMC are not compatable.

 

[Georgesalmon]

Re: 1979 Mercury 90 Inline 6 Wiring for Gages

On my '79 115 the ignition wire is purple. The tach sender is a very, very light brown. Looks tan to me but probably just faded after 34 years. sorry, no wiring diagram, just went and looked at it.

 

[Chris1956]

Re: 1979 Mercury 90 Inline 6 Wiring for Gages

George, You have the exception to the newer standard wiring color. Only the '79 IL6 has the brown tach signal wire and the purple ign power wire. The '78 has brown tach signal wire and white ign wire, like the older models. The 80 IL6 has the purple ign wire and the grey tach signal wire, like the newer models.

I hope the OP has the correct year of his motor, and it has not been altered by some PO.