Now that I have your attention, there really is a mystery wire on the alternator. On my Volvo Penta D2 55, there is a red 12 gauge wire attached to the "D+" terminal on the alternator. There are also 2 wires on a spade connector there (one red/blue, one brown) that go into the big relay thingy on the port side of the motor. The red wire, which looks aftermarket, goes to a relay in the aft cabin (2-cabin model here) that appears to do nothing. The red wire attaches to the relay, and there are 2 black wires Siamesed and go to battery ground, I am pretty sure. There are no other wires on the relay. There are disconnected/cut off wires labeled "autopilot" in the area, but they are clearly not in use.
When the ignition key is off, the red wire has no voltage. When it is on, there is about 3.5 volts in the wire, and when running, the wire/stud shows system voltage - 13 volts or so. From my info search on the engine wiring, the two other wires go to the engine panel and input to the low voltage light. My feeble electrical knowledge says this is to excite the field of the alternator at low RPM.
Any ideas? I am planning to replace the alternator and want to make sure I don't remove/disconnect anything really important.
When the ignition key is off, the red wire has no voltage. When it is on, there is about 3.5 volts in the wire, and when running, the wire/stud shows system voltage - 13 volts or so. From my info search on the engine wiring, the two other wires go to the engine panel and input to the low voltage light. My feeble electrical knowledge says this is to excite the field of the alternator at low RPM.
Any ideas? I am planning to replace the alternator and want to make sure I don't remove/disconnect anything really important.