What is this yellow wire on my alternator?

[ericson29]

Hello

I just purchased my first sailboat and I chose a 1977 Ericson 29 and she needs some TLC lol. Last week I attempted to sail to Catalina and learned (with no shore power at night) my charging sstem is not charging my batteries. My trip ended very quickly and sailed her back in to the slip. I have not yet had time to diagnose the problem but in the mean time does anyone know what this yellow wire coming from my alternator is for? Its not connected to anything. Also does anyone know what brand or size alternator this is? Thanks for any input.

[smosher]

I believe it is the exciter wire which is attached usually at the + side of the coil. There may have been a reason it was left off. This tells the regulator what your battery voltage is at and how much to output. I would connect it
start the engine and measure the output with a meter. Anything above 13 and less than 15 volts dc means the alternator is working.

Steve

[roadnsky]

Welcome to the forum!
As Steve said, that is the "exciter" wire to the coil.
Looks like you have the original MOTOROLA 35.

Here are a few pics of my old M35 for comparison.
You can see the same yellow wire heading back towards the coil in both shots.

Hook it up, measure the voltage and report back to us.
We'll get you good to go in no time...

[ericson29]

That's what I was thinking but not sure. I have heard from others that it could also be for a tachometer for diesel motors? I should have time to go down to the marina by this Sunday and attempt to diagnose it lol. Thanks for the help. I will report back.

[ericson29]

Also does it matter if I connect it to the positive side of the ignition coil or just 12 volts period?

[ericson29]

Roadnsky

I have to give a bow of respect and envy to how clean and neat your engine is! It is my goal to rebuild my motor / boat to make it that clean. First I'm trying to address the pressing issues and then focus more on making her pretty.

I noticed you have the same or similar oil pressure sensor on the bottom right of your picture. The wires on mine are also not connected to anything, where should the wires be going?

[brewgyver]

It goes to your oil pressure gauge. If you have more than one wire on it, one goes to the gauge, the other most likely to an audible warning device.

[roadnsky]

Also does it matter if I connect it to the positive side of the ignition coil or just 12 volts period?

You should connect it to the coil +
See the attached wiring diagram to help you see some of the basics.

Roadnsky
I have to give a bow of respect and envy to how clean and neat your engine is! It is my goal to rebuild my motor / boat to make it that clean. First I'm trying to address the pressing issues and then focus more on making her pretty.

Thank you. I have to admit, thats a really old picture.
I've since cleaned up the wiring and replaced the Motorola 35A with the 55A.

I noticed you have the same or similar oil pressure sensor on the bottom right of your picture. The wires on mine are also not connected to anything, where should the wires be going?

Yes, the wire from the oil sensor should go to your oil gauge. (See diagram)
Note that they are a matched pair.
If you don't have the gauge, Moyer sells a nice setup on this site. Just give Ken a call...

[ericson29]

I finally made it to my boat and started diagnosing my charging system. My alternator is putting out almost nothing with the mystery yellow wire (exited wire) not connected to anything. I connected the yellow wire to the + post on the ignition coil and the alternator cranks out 14.5 volts at 1,500 rpm but will only idle for a few moments before stalling out. I found this strange so I disconnected the exited wire and started the motor to check idle operation. I found it would now hold a steady idle but produce no power to charge the batteries. I then connected the exiter wire to the positive cable on the starter motor. The motor will now run and idle ok but the alternator will only put out 13.9 volts at 1,500 rpm. Is this normal and more importantly sufficient to charge my batteries? I turned my main battery switch off while the engine was running to see if my motor could now run off its own power and it did ok with a minor stumble. Which brings me to my next question! Should I be able to do this or is it bad to turn my batteries off while the motor is running? Thanks for the input : )

[smosher]

What it means is that your battery needs to be charged and the alternator is putting a high demand on your engine.

After the battery is charged the demand will diminish.

I would charge the battery and measure the vdc at the battery.

Steve

[hanleyclifford]

I finally made it to my boat and started diagnosing my charging system. My alternator is putting out almost nothing with the mystery yellow wire (exited wire) not connected to anything. I connected the yellow wire to the + post on the ignition coil and the alternator cranks out 14.5 volts at 1,500 rpm but will only idle for a few moments before stalling out. I found this strange so I disconnected the exited wire and started the motor to check idle operation. I found it would now hold a steady idle but produce no power to charge the batteries. I then connected the exiter wire to the positive cable on the starter motor. The motor will now run and idle ok but the alternator will only put out 13.9 volts at 1,500 rpm. Is this normal and more importantly sufficient to charge my batteries? I turned my main battery switch off while the engine was running to see if my motor could now run off its own power and it did ok with a minor stumble. Which brings me to my next question! Should I be able to do this or is it bad to turn my batteries off while the motor is running? Thanks for the input : )

You should never run the engine without battery connected - that is how alternators get fried. I have a feeling there was an issue with your altenator from the PO - the wire was disconnected for a reason...

[ndutton]

The different voltages depending on where the exciter wire is connected is troublesome to me. I would repeat that test making sure to measure the alternator output voltage at the same point and getting the probes on good clean metal. The output voltage should be the same.

Before commenting on the engine running with the battery switch off I'd like to see an accurate drawing of the battery wiring. Generally speaking switching the battery off while the engine is running spells death to the alternator's internal diodes.

For typical flooded lead/acid batteries, charging voltages of 13.8 ~ 14.2 are normal.

[Trés 30]

Iginition Wire Pigtail

As a sidebar:

Where can I get the female end to make a new connecting wire to the coil? I was thinking of just doing quick connect or continuous wire, but the blade in the pigtail looks pretty robust.

[sastanley]

Tres,

A standard female blade connector will fit that exciter, even though it doesn't look like it. I put heat shrink over mine so that it is all covered up & not exposed to any other wires for a possible short.

[JOHN COOKSON]

As a sidebar:
Where can I get the female end to make a new connecting wire to the coil? I was thinking of just doing quick connect or continuous wire, but the blade in the pigtail looks pretty robust.

I had the same little snap clip on my alternator. It clipped into a mating part that was on a wire from coil +.

The whole easy relese clip was so corroded I cut it away and spliced, soldered, and heat shrunk the wire to the alternator to the wire from coil +. Yes I know, if I ever need to remove the alternator I'll need to un heat shrink, un solder, and un splice the wire. My choice.

After I removed the corroded clip the alternator seemed to find new life and start acting normally. Me being me I didn't have the presence of mind to take a before output voltage measurment. It's like when you get glasses: your vision degenerates slowly and then when you get glasses all of a sudden you realize how bad your eyes were. Voltage monitoring is a good idea.

TRUE GRIT