100HP - won't charge battery

My dad has a '99 or '00 model 100hp 4-stroke Yamaha. His battery would not stay charged, he thought it was bad (5 years old anyway) so he bought and installed a new one. After using the boat for about a day, the fishfinder gave him a "low voltage alarm" at 10.5 volts. Later, while a 3/4 throttle the engine sputtered some and electronics shutdown. We headed home and I checked battery voltage with my meter - 10 volts. Disconnected everything except motor leads and jumped off with battery charger. Ran in neutral at about 3000 rpm and voltage a battery only went down, never up. I figure the motor should put out at least 13-14 volts. When I lifted positive lead, motor would shut down. Shouldn't it stay running? I figure it is either stator or voltage rectifier? Any help on checking? Thanx!

Fish like it ain't no tomorrow!

Re: 100HP - won't charge battery

I have the same motor 2000 F100 Yamaha, and mine had the same problem. For me it was the rectifier. It is on the front of the motor by the air intake i think. It is about a 3-4 inch metal box with fins on the outside. Anyway, It looked fine from the outside, but when I removed it the backside had completely melted. I heard this was a common problem for this year motor. I think they upgraded the part when you order a new one. It is an easy change and will fix your problem completely. Hope this helps.

Re: 100HP - won't charge battery

Thanx Hawaiin Hunter and Mahalo,
My dad is going to check it this friday. Boat is about 1 1/2 hr away at their riverhouse. I have had a similiar failure with a 4-wheeler, so I kind of suspected it. Always good to have some backup from someone with same motor. Do you live in Hawaii? I was there for 4 years in the Navy. It is beautiful, just expensive. People were great and I still have a friend or two in Aiea and Waipahu. Shaka!

Give me Fishing or Give me Death!

Re: 100HP - won't charge battery

the new Yamaha RECTIFIER & REGULATOR part is 67F-81960-11-00. It replaced the old one 67F-81960-00-00.

Re: 100HP - won't charge battery

I can almost guarantee it is the rectifier,
I had that go out on my engine, and Even when I got a jump, the engine would not run long at all, cuz there was just no juice in the battery, and with a bad rectifer, you need the battery to keep the engine running.
I Think the engine should keep running with no battery with a good rectifer.

Re: 100HP - won't charge battery

Rectifier !!!!!!

Re: 100HP - won't charge battery

Indeed it was the regulator/rectifier. Thanx for the help. It is, however, quite a pain to remove. you have to disassemble much of the throttle and transmission cabling going down the port side of the engine just to gain access to one bolt on the rectifier. Took me about 4 hours to do what should be a 10 minute job. Checked with a local outboard repair shop and he confirmed the requirement for disassembly. So, I figure I still save a couple hundred. Thanx,