Alternator/Wiring Issue

[390Express]

I had both alternators rebuilt last spring, but the port motor is not charging. Figured it out when I lost power in the starboard motor due to my bad fuel issue out on the lake, and was coming back on the port motor with the lights, ref. and radio on. When I was pulling into the marina everything started getting dim, and the gauge was reading around 10v. I confirmed that the alternator was not charging by starting both motors, and shutting off the battery switches. When I do this the starboard motor stays running off of the alternator, port motor cuts off immediately. I took the alternator off and brought it back to the alternator rebuild place, and it checked out fine. They bench tested it, and put a new regulator on it for good measure, no charge. (good guys, for sure)

I wired the alternators the same way they were wired when I took them off. The port motor has one additional orange wire that was not used previously, it was folded back and taped off. To the best of my knowledge, the wiring is fairly simple - black (ground) to the case, purple to the excite post, and the fat orange wire to the starter/battery. Any idea what could be wrong?

I talked to the guy at the alt. rebuild place about my issue. He said the issue seems odd to him and thought it may be a loose belt. According to him, even if the excite wire is off, the alternator will usually self-excite around 2,000 rpm. The red wire goes straight to the battery, does that leave the ground as the potential problem source? Any idea what the additional red is for? The purple and black are around 14 gauge if I had to guess. The fat orange/red seems around 8-10 gauge, and the unused orange/red seems around 10-12. The belt is definitely not loose, and it's not slipping. The alternator isn't charging at any RPM.

[Viper]

Seems like it's not getting excited. Check the excite wire and the ground. Do you have battery isolators on board? The orange wire should be the original charge wire but usually gets cut when a battery isolator is installed. Is the alternator getting hot?

[390Express]

Source of the post Seems like it's not getting excited. Check the excite wire and the ground. Do you have battery isolators on board? The orange wire should be the original charge wire but usually gets cut when a battery isolator is installed. Is the alternator getting hot?

Alternator isn’t getting hot, just spins and doesn’t charge. Should the excite wire be 12v when the key is hot, or just when the motor is running? The boat has the two factory perko shutoff switches, but nothing additional by way of battery isolators. Doesn’t seem like adding the additional hot will do anything. Plan on tearing it apart again this weekend.

[bud37]

Your alt should self excite like your guy said.......I dont believe any wiring from the alt should go directly to the battery if that is what out are saying. What ever you do , make sure there is a circuit to battery while the engine is running or you will damage the alt by dead heading it. You probably know this already just cant think of anything else right now.

Just curious, have you checked the alternator when running with your multi meter ?

[bud37]

To add, I think the giveaway that there is a wiring problem here is the fact that you shut off the battery switches and one engine continued to run and the other went off but neither alternator was damaged......doesnt seem right ....there should be no power to either engine with the switches off.

[km1125]

I would not recommend shutting the batteries off EVER on a running engine. You can easily blow the diodes out on most alternators when/if they loose field without a load like a battery connected. Just look at voltage at the battery, with and without a large load (like the blowers) to see if the alternators are charging. Start each motor one at a time and check that you're getting >13.2 VDC at the battery.

I would agree that the alternator doesn't sound like it's putting anything out. It should see voltage on the excite wire anytime the ignition is on, whether the engine is running or not. That feeds the regulator that powers the field.. no field, no output.

Btw, are you using Bill's Suburban as your go-to place for alternators and starter issues? I've used them for DECADES and they are always a great place to get equipment checked out.

[km1125]

Source of the post
..the fat orange wire to the starter/battery.
...The port motor has one additional orange wire that was not used previously..

then:

The red wire goes straight to the battery, does that leave the ground as the potential problem source? Any idea what the additional red is for?

That gets confusing. There should be a "fat red" wire which is the main output wire from the alternator to the battery (many times connected on the starter solenoid battery connection). Not sure what the other wire could be, but it could be for an isolator to connect to another battery bank.

I don't think I've seen any alternator "self-excite" over 2000 RPMs. It may put out some current due to residual magentism, but I can't imagine it would put out full current unless the regulator is properly driven.

[bud37]

Source of the post
I don't think I've seen any alternator "self-excite" over 2000 RPMs. It may put out some current due to residual magentism, but I can't imagine it would put out full current unless the regulator is properly driven.

Used em for years racing, nothing special, right from the scrap yard......internal reg, connected single wire only , they would kick on at about 1500 rpm, just above idle.. ..... I have an idea his alts are internal reg based on what the repair guy said...

[390Express]

Source of the post

Source of the post
..the fat orange wire to the starter/battery.
...The port motor has one additional orange wire that was not used previously..

then:

The red wire goes straight to the battery, does that leave the ground as the potential problem source? Any idea what the additional red is for?

That gets confusing. There should be a "fat red" wire which is the main output wire from the alternator to the battery (many times connected on the starter solenoid battery connection). Not sure what the other wire could be, but it could be for an isolator to connect to another battery bank.

I don't think I've seen any alternator "self-excite" over 2000 RPMs. It may put out some current due to residual magentism, but I can't imagine it would put out full current unless the regulator is properly driven.

Traced the 2nd fat red wire, and it went to the starter. I think the alternator may have been charging prior, but not reading at the gauge. Hooked up the 2nd fat red wire, bingo, 14v at the helm gauge.