Alternators not charging

[Sideskraft]

My new to me '81 Formula 255 Liberator w/twin 260 Mercs "went dead" a week ago on its first venture on the water. After running for about 30 minutes with the battery selector on "both", one engine stalled and then 15 seconds later the other stalled (this happened at idle). While running on the water we noticed a burning smell -- like plastic or something, not burning oil. This may or may not be related to the electrical issue. The batteries didn't have enough juice to start either one once they stalled. I've since bought new batteries and using one battery, I fired up each engine separately on the trailer. When the battery was sitting new it read 12.76V -- when either engine was running it read 12.45V. They both started and ran well, but it's apparent that neither alternator is putting out voltage, or if they are, something else is stealing it away. Both ground cables are secure to the blocks, but I haven't had a chance to check the wires from the battery to the starter. I assume if they were loose there would be difficulty starting. Doesn't the alternator wire run to the starter to charge the battery? I also checked the B+ cables for each engine where they connected to the battery switch to insure the switch wasn't the problem, and I got the same readings there. So it appears that neither alternator is functioning -- or at least any voltage being generated there is not making it to the B+ battery cables on either engine. Something else interesting happened last week when I wiggled the ground cable for the port battery while the star battery was connected. The sink water pump turned on! I have not been able to get that pump to run (it ran when I picked up the boat), and also some of the cuddy lights do not work. I have no idea if any of this is related to the alternator issue, but I'm guessing that it might be. Any suggestions would be appreciated.

[scarab300guy]

Should be four wires coming off each alternator - 1) the ground wire (goes to block or - batt terminal; 2) Charge wire - thick and goes to the + batt terminal; and 3 and 4, which are the 2 thin wires that come out of the alt chassis. One is the alt light/exciter wire (in a car it would run to the alt light on the dash, in most boats it just gets crimped to the thick charging wire, which is fine). The other goes to switched power. What you're describing sounds like these last 2 are reversed. Common mistake.

[US1 Fountain]

Check the fuse at the starter for the alt.

[Sideskraft]

Just had the alternators tested and they both work. One is putting out excessive voltage, so the regulator will be replaced. It looks like the problem of the alternators not charging is that the purple wire is not putting out B+ to the battery sense lead, on either engine. Since these are totally independent systems, it seems extremely unlikely, but that's the case. I've already looked at the harness going from the main connector and it's fine. The electric choke and coil both get B+ from the same circuit and they both function, so something's wrong inside the harness with that one lead, on both engines, again highly unlikely, but it's true. I will probably end up running a new individual wire from the B+ coil terminal to the battery sense terminal input on both alternators. The only reasonable explanation is that the wire harness builder did not connect this purple wire properly on either engine harness, so it's just a dead wire sitting there. That means the alternators have not functioned since new, so both owners had to do regular battery charging. If anyone can come up with a better explanation for this, I'm all ears!

[ThisIsLivin]

Check for an inline fuse. I came across the same problem on a friends boat. He had taken it to three different mechanics and they couldn't fix the problem. I checked voltage at both ends of the wire and it was different, so the wire was broken somewhere. I traced it to an inline fuse.

[Sideskraft]

Thanks Livin. I actually discovered that, probably 15-20 years ago, the first owner converted to pointless ignition, and in doing so he wrapped the B+ ignition wire/connector that ran from the coil post to the alternator around the main engine wire harness and wrapped it in electrical tape. I unwrapped it, ran a jumper wire to the coil, and now they are charging great!!