Need to troubleshoot charging problem on Yamaha F115TLR

[HotTommy]

I recently purchased a Yamaha F115TLR, # 68VL1063xxx. I beleive it was sold in 2006. While checking it out I discovered an apparent charging problem. I connected my multimeter to the two lugs that connect the main power cables from the engine to the battery. With everything off I have 12.4 on my freshly charged battery. With the engine running at idle (on muffs), the voltage drops to 12.1. I briefly bumped the RPM up to 2,000 and the voltage did not change, so I assume there is a charging problem. I have worked on car engines and outboards before, but this is my first Yamaha. Here are my questions.

1. Is this a normal voltage reading at idle?
2. If not, what are the most likely root causes?
3. What do you recommend to troubleshoot the problem?

 

[GA_Boater]

First guess is the rectifier/regulator is bad. Voltage should increase to roughly 14 volts.

There may be a couple of fuses in the charging, if so, check them.

Boatinfo doesn't have a manual for 2008, but this one up to 2003 should have what you need to troubleshot.

http://boatinfo.no/lib/yamaha/manuals/1997-2003yamaha.html#/246

 

[HotTommy]

I found a new OEM style rectifier for abour $40 and decided it was cheap enough to try and avoid a trip to the shop. I installed it and saw the following (running on muffs): 12.4 volts with everything off. 12.3 volts with the key on. 12.5 volts at idle and 12.6 volts at 1200 RPM. It's an improvement over before, but less than I imagined I'd see. When this virus thing allows, I'll put it in the water and see what I get at higher RPM. .... In the meantime, do these idle voltage readings match what others have seen with a similar engine?

 

[QBhoy]

Put the meter across the positive and to another earth that will not be affected by going through the battery and see.

 

[HotTommy]

Thanks.