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Various Mercury Wiring

Well, its not as easy as it sounds. There is a lot to consider when it comes to wiring. The basics. You need fuses and distribution blocks. Make sure the wire that you use after the fuse is rated for it or it will melt because of too much amperage coming through. In other words, you place a fuse in line, it will only let so much amperage through, make sure the wire after the fuse can handle the amperage passing through or you risk a fire hazard (more to come).

As far as remote wiring schematics, take a look at the above picture. There are industry standards. I have found that the older Mercury's don't follow the standard. Example: The tachometer wire should be gray; on Mercury's, it is Brown. There are other differences and I will go into them. For now, I will limit this page to some facts. As I get time, I will place a write up on common boat wiring diagrams for accessories, lighting, gauges, etc. I have successfully wired my boat, placed gauges and working lights on it. The process can be time consuming and tedious but it needs to be correct and can get a little expensive but so can buying a replacement gauge or replacement hull if you catch fire.

I will make an honest effort to post some useful schematics for wiring your boat for various accessories as I come across some info and put it to the test. I have wired batteries, stereos, fish finders, lights, gauges; all with success, just need to create schematics and post.

Wire - make sure it is marine grade wire...meaning it has non-corrosive coating such as tin; Use tinned wire.. tin plated wire is better than just plain copper; copper will corrode / oxidize with very little time. Wicking is the process by which moisture reaches all unexposed copper so even though you think it is shielded, moisture will travel through the wire underneath the plastic layer and cause problems. In time the corrosion will go deep into the copper and as a result, a decrease in conductivity will occur causing problems with wiring. We are always near water and moisture, and even though tin plating does corrode, it has better conductivity properties when oxidized, much better than copper oxidization (can you say voltage drops). Your connections will remain stable and hardy for longer periods of time. Tin acts as a sacrificial anode in the sense that tin will oxidize before copper. SO... before the oxidization process begins to go deep into the copper, the tin must be completely oxidized (but remember tin oxides have better conductivity than copper oxides).

Tin does have initial lower conductivity than copper and you will see voltage drops but given time, copper by itself will lose conductivity at a much higher cost and voltage drops it is a compromise...and that will cause less work down the road.

Tin coated wire also accepts tin soldering much better initially and over time. Also, the corrosion resitant layer is complete, covered in tin. Copper on the other hand is resistant to soldering.

Fuses - always use fuse boxes and rated wire for the application you are using it for. Fuses protect you from blown accessories, fires and such. Why do fuses blow / melt? Improper connection, too much amperage flowing, shorts, etc. Before replacing a blown fuse, inspect for problems that caused the fuse to blow.

Switches - again, make sure you get marine grade switches. They have tin connections rather than copper..see pic to left.


Tachometer - if you no longer have the original tachometer and have purchased an aftermarket tachometer, chances are that it will not register. To make the tachometer work on the Mercury 650Electric start, you need to move a

brown wire from the switch box to the rectifier terminal. The rectifier has two Yellow / Red terminals. The brown wire should go to any of those two.

For this engine (Merc 650 1975), the tachometer should have the selector set to 5 in order to read the 12 pole engine. I used a Faria #TC9120A model number and it works great. Any new tachometer you buy for this particular engine should be able to be adjusted to read 12 pole alternators. If you have the incorrect pole settings, it will probably not register or register incorrectly according to Faria. I have heard of some people frying their tachometers, just be sure you have yours set correctly before powering it up.


Try it without making the wire switch and it probably will not register. It is a simple change.



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