Slick Nick Posted December 16 Report Posted December 16 After the recent talk of alternator failures due to field wires, I decided to give my own a close inspection. I have a Kelly Aerospace alternator on my 1991 MSE (28V) Everything looked good, I’ve got the main ground, the power wire, an AUX terminal (since I have a mechanical tach this is where my Hobbs meter is attached, through a 5A fuse.) I noticed the field terminals, F1 and F2 and compared them to the wiring diagram from the service manual. My diode wire was installed on the F1 terminal instead of F2. Is this correct? Looking at the diagram, F2 is connected to ground both through the diodes, and a straight ground wire. How does that make sense? Wouldn’t current be allowed to bypass the diodes and go straight to ground in this setup? I inspected my diode wire, and it’s actually got 2 diodes, soldered in opposite direction to one another, which made me scratch my head. I seem to remember that two diodes in series oriented opposite of one another, created some sort of a “bias” circuit in AC systems, the diodes were not the same. I should have taken a pic. So, I’ve got the field wire to F1, its shield to ground. (PA08A14) The diode wire from F1 to ground, a jumper wire from F2 to ground, my fused Hobbs wire (DM04A20) on the AUX terminal, the field wire shield to ground, the power wire shield to ground, and the grounding lug attached to the engine block through a skookum thick braided cable. Basically I’m trying to find out why the diode wire is on the F1 terminal instead of F2? It makes no sense to me for it to be on F2, but there are people much smarter than me on this forum so figured I’d ask. I’m not having any electrical issues, but want to make sure everything is wired as it should be. Perhaps it doesn’t even matter.
PT20J Posted December 16 Report Posted December 16 The F2 terminal should be grounded. The F1 terminal goes to the voltage regulator. The diode should be connected between F1 and ground with the cathode (the end with the band) connected to F1. (The Mooney schematic is incorrect). This is a flyback diode that absorbs EMF from the field when the current is suddenly removed which sometimes causes the field CB to pop when the ALT switch is turned off. I'm not sure why you would have two diodes. 1
Slick Nick Posted December 16 Author Report Posted December 16 16 minutes ago, PT20J said: The F2 terminal should be grounded. The F1 terminal goes to the voltage regulator. The diode should be connected between F1 and ground with the cathode (the end with the band) connected to F1. (The Mooney schematic is incorrect). This is a flyback diode that absorbs EMF from the field when the current is suddenly removed which sometimes causes the field CB to pop when the ALT switch is turned off. I'm not sure why you would have two diodes. Thanks Skip. Not sure why it’s got two diodes either, perhaps it was modified sometime in the past. Thanks for confirming the Mooney diagram is incorrect. I will have to check the IPC to see if I can find the specs of the diode. Edit: not listed in the IPC. I believe it’s part of the wiring harness. Is there a way to determine what size of diode should be used?
PT20J Posted December 16 Report Posted December 16 Some electrical components are listed in the IPC but the most reliable way to find them is to look at the end of the Service Manual for the electrical equipment chart that matches your serial number and schematic and locate the reference designator from the schematic in the chart to determine the component part number. In this case the reference designator is D1. 1
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