2022 Mercury pro xs 200hp large electrical short when wet

[supra360]

Hello,

I'm experiencing a recurring issue with my boat's motor that leaves me stranded, most recently during a trip in the rain. This also happened about two years ago. I suspect there's a significant electrical short somewhere in the motor or wiring that only presents itself when the system gets wet.

Here’s a summary of what happened:

- I’m using a fully charged Ionic 125Ah battery.

- My electronics are isolated with a kill switch.

- The commute to the lake was about two hours.

- Upon arrival, I used the tilt to remove the motor support—it was very sluggish.

- The motor would not start. The battery entered Guardian Mode.

- This is the second time this has happened, so I brought a fully charged AGM backup battery, which tested at 13V.

- I connected the AGM (motor cables only).

- When trimming the motor down, it was again very sluggish. Trying to start the engine resulted in just a click—no turnover, as if the battery was weak.

- I drove back home (2-hour return trip). The rain had stopped and it was sunny.

- Back home, both trim and engine worked perfectly.


It seems like something is drawing significant current only when the motor or wiring is wet—enough to prevent the trim and starter from operating properly.

What kind of electrical short could be causing this much load when wet?

Serial Number: 3B177373

Thanks

[mactlman]

water getting in a trim button and trying to trim motor would draw a lot of amps? maybe try wetting switches and controls with a hose or spray bottle full of water? has to be something exposed to rain - below deck and engine sealed mostlyl

[supra360]

I'm leaning towards one of the four trim switches also.

[alli ss]

Unplug the red wire from each switch and isolate it

[EuropeanAM]

- My electronics are isolated with a kill switch

Assuming this meant a Battery Disconnect switch.

- The motor would not start. The battery entered Guardian Mode.

Guardian is an engine feature- what information (specifically) did you receive that told you the "battery entered Guardian Mode"?

Just a hunch: Since the engine wouldn't start AND the trim was acting up, INTERMITTENTLY, even after you connected a known-good AGM directly to the battery cables:

I suspect there is a connection problem at the ENGINE end of the battery cables (or the cables themselves have a problem).

[supra360]

-14.4volts when motor is running.
- I sprayed water on all the trim switches and followed the lines back. Could not replicate the issue.

Don, this is Aluminum boat, Do you know if they're supposed to be continuity between engine ground terminal and the Hull. I am reading 4.4 ohms. This is how the factory set it up.

I have been doing research and some people with similar problem said they had defective engine cables. Is there a procedure to Megohmmeter test them? Would these cables be from Mercury or from the rigging process from manufacture (Crestliner)?

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[EuropeanAM]

If the engine is directly mounted to the hull, there will be continuity between battery ground and the hull (as there is a direct connection between the engine and battery ground).

It's NEVER impossible to have a bad battery cable. It's more common to have a loose connection where they attach at the front of the block (hidden just behind the #1 air runner). I've also seen cases where a lockwasher was pointed "gap up" and allowed the whole connection to fill with liquid electrical tape.

[supra360]

After series of diagnostic test have came to the conclusion. That is a faulty power cable.

Visually inspected both the negative and positive power cables. No sign of defects.

Performed a insulation resistance test with a Megohmmeter in water.

-Positive cable has less than 1M ohms which is well below industry standards.
- Negative cable 742M ohms

Performed a insulation resistance test with a Megohmmeter out of water.

-Positive cable has less than 3M ohms which is well below industry standards.
- Negative cable Infinity ohms

This would explain why the issue only happens in the rain.

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[EuropeanAM]

Would seem there's a break in the insulation somewhere, at the very least.