My one wire alternator failed the other day. The parts store didn't have a replacement in stock. The parts guy showed me how to "jump" a three wire to make it self exciting. I installed it and started that motor and it was charging fine. Two days later the batteries are dead, I can't even open the hatch. Is this a coincidence or from "jumping" the alternator.

I think that jumping the leads caused the rotor to be excited all the time, even when the engine was off. No problem when you are running the engine, but when you switched off the engine it kept the circuit hot and draining power. This is, if you do not have a battery switch or did not turn it off when done.

I have a battery switch but I left it on. I can usaully leave it on for a few days at a time with no problem. You are confirming my theory though. I just don't understand how the parts guy thought this would be the same as a self exciting alternator.

Hot wiring an alternator like that definitely will drain your battery. I used to do that in a car because I was broke and didn't have the money to fix it at the time. It will kill your battery.

Rene

I'm assuming he told you to wire the output to both the sensor and excitation wires. If so, that's most likely the problem.
Gary

rdoactive,
you are correct. I already pulled the alternator, but if the parts shop doesn't have the one wire, I could just run the purple wire to the "F" post correct?

You need to run a jumper from the output to the sensor lead. The only problem is that the alternator wont compensate for voltage loss in the circuit.
And you can splice into the keyed ignition wire (usually purple) to the excitation lead. That's the way it's supposed to be wired.
Gary

You lost me,

What is the "sensor lead"?

A 3 wire alternator (Mando) will have 3 wires (duh, did I really just write that?).
The large post is the output with a large red or orange wire going to the starter.
There are 2 other smaller wires from a plug that act as "sensor" and "excitation".
The excitation is hooked to the keyed cicuit (purple wire), it tells the alternator it should be "on" when the ignition is on.
The other lead (sensor) goes to the boats permanent hot circuit (red or red purple wire). It senses the voltage after it drops, running through all the boats wiring and from the various items drawing current. That way, full voltage can be supplied by the alternator without dropping due to resistance in the wiring or load on it.
What kind of alternator do you have, Merc or aftermarket?
Gary

Aftermarket, Delco. The wire that goes to the starter is orange and the other two that are capped are purple, and red.