I'm currently putting a 10 pin harness to the dash in an 88 formula,started project last week noticed the harness part #84-812475a20 has a seperate ground for all the accessories ,and the original had a ground and purple jumper to each accessory . I'm no electrician but Any ideas or thoughts would be appreciated . Should I wire up like the old or just go with it ? Fuel gauge is going to be left out of the black and purple which makes me wonder . -thanks

Did you order the new harness to match the original? If not, it might be easier to get the correct Merc harness to match the existing. In our case, we have added so much stuff at the dash vs. factory we have added an additional 14-16 wires from the dash to the engine/bilge. We used the 4&6 wire GM WeatherPacks placed close to Merc harness connection that allow us to disconnect/reconnect all the electrical in minutes when we R&R the engines. All connectors only fit one way....good winter project!

the merc 10 pin (actually 9) harness runs the gauges and ignition also buzzer. All accessories are wired independent of the harness, though the harness does have a large power wire and ground wire.

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I wanted to isolate all my switches for accessories from the engine gauges, so I ran a separate fused heavy gauge wire up to a bus bar that has its own set of fused circuits. This cleaned up the wiring quite a bit, and kept the gauges from jumping around when I turn on bilge blowers, trim pumps, etc. Also corrected my issue with the gauges reading off when running the nav lights at night. I also ran a matching ground back to the battery.

Same here, Im running a 10ga power and ground wire seperate from the harness to a blue sea 12 junction fuse box with a neg. bus. The merc harness is only for ignition and gauges.

I think what you mean is the new harness has 3 wires for each gauge. The hot (purple), ground (black) and sender (varies).

The drawing Phragle posted is the way it was probably wired originally, with 1 hot and 1 ground coming out of the harness and then all the gauges daisy chained. That is OK but a bad connection anyplace in the chain will cause the gauges "downstream" to act up.

So what Merc has done in that particular harness is provided a bunch of grounds and hots that are already tied together inside that tape. For a boat builder this makes life easy as there is no need to make up all those jumpers and all the leads are labeled. It also means a lot more wires behind the dash and you are stuck with the lengths Merc provided unless you want to cut and put new connectors on it. Which is fine also btw.

Many riggers buy the extension harness that is not terminated at the dash end, in fact it has a female connector that gets cut off and then wire from scratch.

I also like the separate feed/ground for the accessories also but that harness will do just fine for normal lighting and pump loads. If you do run the additional 12V for accessories make sure to fuse it or put a breaker on it at the source. And hook all the grounds up to the same place back at the engine.

Yes mark thanks this is what I was wondering new harness won't need jumpers , but the fuel gauge did so I was wondering where to jump it from ?also where to wire the big red and black ? Thanks

You can grab the hot & ground from any of the other gauges.

As far as the big red & black I'm not sure. Normally that would go to a negative (ground) bus bar and a fuse or breaker panel.