'78 Mercury 900 90hp draining battery while sitting in driveway

[talbee]

I have a '78 Mercury 900 90hp that's draining my brand new 29DC battery I just bought a few days ago while sitting in driveway, then will not allow to start my engine. I have absolutely nothing hooked up like fish finder (sits in the house), there's no radio, and there's no bilge pump. This has happened with 2 other batteries. I keep having to charge it. I'm not understanding why this is happening.

Would I need to go through and check all my ground wires?

 

[Chinewalker]

My first bet would be the rectifier/regulator is burned out and shorted. It converts the AC current of the stator to the DC of the battery. Sometimes they get cooked when someone reverses the battery cables. Most of the time they simply burn up to an open circuit, but I've had several over the years that short out in the process.

 

[Chris1956]

Chine is likely correct. Remove the wrap cowl and find the black, square rectifier. If original it will have three ring terminals (yellow, yellow, red) on it. You can test it with an ohmmeter. It is simply a 4 post, full-wave rectifier, with the -DC post tied to ground, via the mounting bolt.

 

[talbee]

Yesterday, after I finished timing the engine, I took it on the water for final adjustments, it died. When I tried to start it, I smelled like a burnt musty smell, no smoke. The smell could be from me trying to start up many times, idk. I wasn't able to locate where it was coming from. Fortunately, I wasn't far from the dock and was able to troll back.

If it is the rectifier, can I rebuild it?

Please keep in mind I'm fairly new to working on boat engines.

 

[Chris1956]

The rectifier is a sealed unit. It cannot be rebuilt. They are avail aftermarket. If you are handy with electronics, you could substitute a $5 25A full wave rectifier, grounding the -DC post to the engine frame.

 

[talbee]

The rectifier is a sealed unit. It cannot be rebuilt. They are avail aftermarket. If you are handy with electronics, you could substitute a $5 25A full wave rectifier, grounding the -DC post to the engine frame.

I'm sorry, I'm not sure what you mean. I'm kind of a noob when it comes to working on engines. I've been learning from forums and videos.

 

[stevehendo34]

Here is one I bought on ebay for my 90 1978
http://www.ebay.com/itm/New-Rectifie...c94b73&vxp=mtr

It is 4 diodes in a single package. It take the alternating current from the stater under the flywheel and converts it to DC current to charge the battery.

So its like the little thing you plug into the wall to charge your phone, but the stater supplies the alternating current instead of the wall outlet.

 

[emckelvy]

You might consider an exact replacement, just click on the "Boat Parts & Accessories" tab at the top of the page and plug in "18-5707" into the search box.

This Sierra Marine aftermarket rectifier mounts exactly as the original and is a wire-for-wire replacement for your bad rectifier.

HTH.........ed