2021 250 ProXS - Alternator Issues

[Flint River Nut]

2021 Mercury 250 Pro XS
Serial: 3B007977


Has anyone had any issues with an alternator overcharging?? I am currently on my 3rd alternator with less than 30 hours. The regulator is not functioning properly and sending too many volts (16 plus) back through the system at 2000 rpms or more. Cranking Battery (31 AGM) checked out fine on a load test. The certified mechanic, who is very good by the way, contacted Mercury and they recommended other tests. All tests checked out and the only option is to install a 3rd alternator, which I am skeptical of. Just like every other part and supply chain in the world, the alternator is backordered until mid-July. This must be an ongoing issue if the alternators are on back order. Any thoughts or opinions welcome.

[Bob_Fish]

A lot of parts on back order. I'm waiting on a shift positioning sensor for a 225 4 stroke. All I can say is good luck. Crazy times.

[EuropeanAM]

I would think that the odds of you seeing 3 faulty alternators (BTW... I've not seen ONE yet) on the same engine would be microscopic.

My guess would be that there is a connection or device somewhere between the alternator and the battery that is not providing a proper connection (poor connections at engine, BAD CABLES, or a battery switch with problems).

Recommend complete bypass of the battery switch with direct connection of the engine and clean power harness to the cranking battery. Test run and determine if the problem persists.

[Flint River Nut]

We are now on the 4th alternator with same issue, overcharging. This rig definitely has a gremlin in it.

[EuropeanAM]

Are the engine cables and clean power harness connected DIRECTLY to the engine cranking battery?

GROUND for alternator been confirmed?

Make sure that the cranking battery is completely isolated from anything that ties multiple batteries together (chargers, battery switches, onboard "charge on the fly devices", etc.

[Flint River Nut]

Thank you Don, I will share this with my service technician. I do know Mercury had him perform several tests and diagnostics before the last alternator was installed and they all checked out. I am at a loss. It does not happen immediately and there is no rhyme or reason when it finally goes. I am hoping the engineers get involved now so we can determine the issue. I am beginning to think it is boat related and not engine related.

Are the engine cables and clean power harness connected DIRECTLY to the engine cranking battery?

GROUND for alternator been confirmed?

Make sure that the cranking battery is completely isolated from anything that ties multiple batteries together (chargers, battery switches, onboard "charge on the fly devices", etc.

[EuropeanAM]

And since it's such an unusual problem- make absolutely certain the battery is secured in the boat (can't move or bounce), AND that there is nothing in that compartment/area that might be able to bounce around and create any type of short-circuit event.