Tuesday, March 4, 2008

Replacing alternator

The original Motorola alternator on Aeolus started acting up a bit last week when we did a day trip to nearby Jones Island State Park. I noticed that after starting the motor that the alternator was not producing any charge. After a few minutes it finally kicked in. Figuring it was probably an early indication of the inevitable failure of the 20 year old alternator, I decided to give my back up a try.

About a year ago I purchased a back up alternator from DB electrical. I got a Mando marine internal regulator 50 or 55 amp, I can't remember. From everything I've read these are pretty good alternators, though they are Korean, and are OEM on Mercruiser I think. Anyway, I had to do some research to figure out how to wire the thing because the wiring is not identical to the Motorola. Fortunately, I have no fear of anything on engines as I grew up under the hood of a car.

After a lot of reading, which I always do, I decided to not ground to the case as the Motorola had been but to run the ground to the block directly. I used 6AWG boat cable for the short run.
When I fired her up she worked great but put out a fairly high 14.6 volts. Alarm bells. I called DB electrical to find out the regulator setting, and they said anything below 14.9 is normal. Called Trojan battery since I have their batteries and they said anything below 14.9 is great. In fact, they recommend 14.8 for their flooded batteries, which I have. Never mind that Casey says anything above 14.4 is bad...I do think, however, that Casey's suggestion of putting a switch in the excite circuit to shut off the alternator on long motoring trips is a good idea. You have to stop the motor and restart it for this to work, but that's OK. Up here, where the wind is fickle in summer, you often motor for 6 or more hours a day when you are trying to get somewhere. That's a long time to pump more than 14 volts into a battery. So I went and got a high quality Cole Hersee single pole single throw switch and built a little wooden house for it and have mounted it on a reinforcing bulkhead in the engine compartment near the alternator. I connected the excite wire to it, and it works perfectly. For use on those occasions when I'm motoring along all day.

Have to say, this Mando alternator cost me about $100 and to rebuild my Motorola is estimated at $150. Why bother. Time will tell if it holds up, but at that price, sure beats a Balmar.


Yuri said...

Great! Last week I replaced the same Motorola with the same Mondo on my own boat. Glad to hear it's working out well; in my case it was simply that the Mondo was the only other alternator on the island that would "bolt on" to my M-25XP.

Brian, Amy, Elliott and Owen said...

Yes, so far so good. She pumps out plenty of voltage and runs smoothly. I plan to buy another one as my emergency back up.

Anonymous said...

Wiring in an excite circuit cut-off switch is something I need to do. We sail on the Chesapeake, and like in your area, there are sometimes long days of motoring - boiling out the electrolyte is a reality when motoring for that long.

Great blog here, I've just discovered it. I'll be making my way through your posts, and looking forward to the reading.

Rick s/v Cay of Sea

Brian W. said...

Glad you find it useful. It has turned out that I have not used this switch as much as I had expected. I add water to my batteries every 6 months or so, which is OK with me. But I have it in case I am motoring long ways without drawing much power. I haven't checked my voltage when motoring and running GPS and such recently, but I am sure it drops it down below the 14.5 or so output of the alternator.

Anyway, I'm not posting as many repair articles as I once did because quite honestly there is not as much to do!