2018 Mercury Optimax Pro XS blowing "Power-on Circuit" Fuse

[mako538]

Hi all,
We have a 2018 Mercury Optimax Pro XS 225hp (s/n 2B534312 model 1225P83BD) that when turning the ignition switch causes the 15 amp "Power-on Circuit" Fuse to (per the "Operation Maintenance and Installation Manual") blow. We had batteries tested, both bad, and have been replaced.

The back story: spent the summer/early fall replacing the fuel tank on our Mako. so the whole console had to come off, obviously needing to disconnect everything. Also took the opportunity to rewire electronics and a new switch panel. So yes, lots of electrical changes. But have checked ENGINE wires both under console and on block, all look new and in good shape. All were plugs and double checked in right place.

All electronics/lights were and are working fine. On the day we were to launch, no response from the engine. Did some research, found the blown fuse. Replaced.
Tested again (and not happy with myself afterwards for doing this) turned over the engine with no gas. (I am praying one inadvertent turnover would not hurt the fuel pumps.)
So we filled the boat with gas, launched, primed the bulb till hard, engine started right up.
Went for a ride - 5 minutes to open water, then on a plane for about 1-2 minutes at 22knots, and the engine stops dead. Fuse blown again. Replace, blows immediately.
We get a tow in (thankfully only couple hundred yards off shore). This is when we had batteries tested and replaced (read about possible cause of an increase in amperage for low voltage).
Now the fuse blows every time. It is immediate and sparks.

In trying to isolate, i have done following:
Disconnected SmartCraft gauges. Fuse blows.
Disconnected the new electronics switch panel off the batteries. Fuse still blows.
Disconnected high pressure fuel pump. Fuse blows. (so i guess pump is ok.)
Also have a service manual on order, hopefully here this week so have for next weekend.

Have read many posts about check wires, check wires, check wires, which we’ve done, as said above. All on block look in great shape, terminals not even corroded and no worn insulation. Could it be possible we put a screw through the conduit and into a wire? Possibly, but I don't think so.
Starting this thread to hopefully generate ideas where to look / next steps.

Thanks,
Mako538

[DewRonInc]

Possible ignition coil problem

[EuropeanAM]

Ignition coils should be on a 20 amp fuse.

I would recommend disconnecting the 14 pin harness and connecting a TRANSOM-MOUNTED (test) keyswitch harness to see if the problem ceases or continues.

This is the simplest means of quickly identifying if the problem is in the ENGINE or in the BOAT WIRING.

[mako538]

Thank you for the responses. makes sense to isolate. I am assuming a test keyswitch harness may be more inexpensive than a full blown harness? any advice where I would be able to purchase one? Is it common for merc dealers to carry them? (or do i need a full blown harness?)

[SilverFox579]

any advice where I would be able to purchase one?

Order one from EuropeanAM - If you want to order or price Mercury Engine Parts or Accessories

[mako538]

I have sourced one locally thanks. timing is part of the issue. Shout out for Marine Sports Center in Bridgeport CT - Larry is lending me his test harness! (I don't even know him!)
Will report back results...
thanks all

[EuropeanAM]

Great. Most any Mercury Servicing dealer should have one in their shop (I know I have several of them).

If it proves the problem to be on the boat side, THEN you inspect at the console... if problem cannot be determined it might be easiest for you to simply replace.

[SilverFox579]

Really nice of them to loan you a test harness!

[mako538]

Finally hooked up with Larry to get the test harness. So connected and turned key, got starting beep and fuse did not blow.

Took multimeter to boat's ignition harness while disconnected from engine. Tested resistance across all wires one at a time. Open circuit on all except what I will call 2 and 6 wires (will upload pics when back at computer, but if latch clip is down, they are top middle and lower right pins). Got .6 ohms there. I assume this shows boats harness is compromised? Or are those 2 wires connected somewhere? Again, 14wire plug disconnected at engine.
Have not started engine as not near water source. That will be next step.

[mako538]

20221119_130902.jpg
its these 2 pins that have .6 ohms across them. the other (and these to others) are all open circuits.

[EuropeanAM]

The connector pictured is not the key-switch harness connector. That is the BOAT sensor harness connector (fuel/oil/paddle harness).

See yellow arrow in picture below for picture of CORRECT connector location:

14-PinHarness-Engine.jpg

[mako538]

Yes, that is the 14pin connector I disconnected at engine to connect test harness. connected test harness there at the engine, turned to ON, got tone and fuse was OK. so this tells me engine is not the causing the fuse to blow. I did not get the engine started, but turned to on multiple times (got tone and fuel pumps whining), and cranked it over (with fuel and water this time!) and fuse held.

so that leaves the boat's harness:
I left the 14pin plug UNPLUGGED and went to the ignition key switch at console (other end of boat's wiring harness from the engine). disconnected 6 pin (in my picture) and tested resistance across each to the others - expecting open circuits on all.

with the 14pin unplugged at the engine, I would assume all those wires go to open circuit. but I got .6 ohms on pin 2 to pin 6, which thinking about it, figured you'd want colors. its the black wire and the purple wire - both solid (I looked hard for a stripe and didn't see any, but it was dark and I was working with a flashlight.)

I did not get a chance to look up the black wire and purple wire - do they go somewhere else other than 14 pin? i.e. I did not unplug the other wiring harness (the other 6pin? - not sure of count of wires into that one) at engine. is it possible that's why i have connectivity across black to purple?

I guess my theory is if all 6 wires at remote ignition (other side of my boats harness from the engine, where I tested connectivity) are separate wires to the 14pin connector, and the 14 pin connector unplugged, then all should be open. and if that is the case, then did we put a screw into the conduit and into the wires (causing the .6ohm ckt)? I kind of cannot believe that - due to gap from fuel tanks support to conduit (would screw even reach?), the SAE covering over the conduit seems solid, and the fact conduit looks to have room in it and would think the screw would push any wire aside.

any way, the fact that the test harness did not blow the fuse (and the boats harness does blow the fuse) tells me its the boat's harness causing the fuse to blow.

So is next steps replacement of the harness? or is there something else I should look for?

thanks for all the help. (and sorry for the long-winded-ness!)

[EuropeanAM]

Black wire is GROUND, purple wire is key-switched 12V power. There would normally be continuity here as there are literally dozens of circuits on that purple wire that have some resistance value to ground. Normally wouldn't be a dead-short, though (look for the instrument harness, unplug it, and see if that made a difference).

If unhooked at the engine, the only items that are normally connected to that circuit are the instrument harness (or Smartcraft connection to J-Box), possibly the lanyard, and in some strangely-wired situations, a jackplate or trim.

[mako538]

20230423_145414.jpg
Just to close the loop - replaced the harness end of April, here is a pic of the old harness with a big nick. I guess we hit it with Sawzall when doing the gas tank work. Sawzall's and harnesses don't mix well!
Engine is now running fine.
Thanks for your help.

[JOE54]

thanks for update /sharp blade

[EuropeanAM]

Ouch! Good choice replacing harness!