I have a pair of 1999 Mercruiser 7.4 ltr motors. On the top and back of the motor's is a assembly with a blue box labeled "Mercathode"
What is this, what's it do and should it always have 12volts to it even when batteries are off?

Any help is appericated. Thanks.

From what I've read, It's an electronic corrosion inhibitor. It gets hooked to the cathodes (anodes) external to the boat and the current running though it prevents electrolysis or corrosion (I think). My boat doesn't have it and I asked a mate about them and he said it was mainly for boats that are mored rather than trailered. (more time in water = more corrosion) I would think therefor that it should have power 24/7 regardless of ignition status but if your battery switch is in line before everything else you may interupt that.
I'm sure people vastly more knowledgeable than me will chime in, but at least you now know what the little blue box is Supposed to do!
Hope that helps!

Here is a link to your Bravo Service Manual. It's about 12 Mb in size, so hopefully you have a broadband connection.

See section 7 for all the components. The Mercathode starts on page 7A-12.

Ghostrider, looks like you were on the money, Thank You.

Zackman, Thank You, I took a look and am now know all about the system. I saved the PDF of the service manual, so next time I have a question of this type I know where to look.

Thank you again

Hey Zackman,

does the bravo service manual in your link cover XR drives?

Thanks, Mark

No, but this should Service Manual #28 2001- newer Bravo drives. ALL

it is a device to aid with the bonding system to fight elect. as goserider said, they are really designed for boat that stay in the water....don't do much for trailer boats, if you look around the engine area you should see extra ground wires running from all metal fittings, all tied together and into the black/blue box, and have seen them go bad and start drawing lots of voltage....Rob

Zackman,

You wouldn't happen to have a service manual for 08' 377's ( 6.2's) would you??

Autin, service manual #31 here.

My boat is a trailer boat. I think I'll leave the system in place, but not wire it directly to the battery since there is no need for it work as the boat sits on the trailer. I'll wire it to the on side or the perko so it will function only when the batteries are on and the boat is in the water. Thanks again to all for the help.