My primary alternator is original equipment TCM 60A alternator with 660 hours.  It should have been inspected at 500 hours but I deferred that.  This note suggests that the diodes get a check during that 500H inspection.  My secondary is a B&C 20A which was just replaced this year after 600+ hours after some windings failed.  I have had a lot of radio noise but nothing so bad as to take steps to fix it.  I had no idea that my primary was only partially functioning.  On this last trip from Calgary to Texas and onto Florida, my primary was noted to have Zero output while taxiing for takeoff at Midland TX.   I flew using the secondary to charge both busses for the two legs into Florida.  After some testing I ruled out the primary B&C regulator.  Today, I tested the field resistance (15 ohms, acceptable range should be 12-100 ohms) which was perfect.  The field is the rotor on this unit.  I decided to pull the housing and check brushes which were found to be almost perfect (90% remaining after 660 hours).

The stator was uniform in color and not shorted.   It pulls out easily for inspection.   The body of the alternator was not removed from the engine making this test very fast and easy.  A quick check revealed all six diodes (two each for the three phases) had failed.  Everything else was in excellent condition.  This unit is similar to the standard Delco Remy 10Si alternator and shares many parts.   The diode replacement from Aircraft Spruce is almost $200.  The new alternator is about $1400.   A repair with new rotor is about $500 including shipping and labor from a reputable shop I spoke with.  

The diode assembly was sourced at my local shop with a spare set of brushes for $20 total.  These are 50A diodes (50A x 6 diodes) which is a robust replacement satisfactory for even the 100A alternator.  I had to remove the B+ stud from the old diode assembly and press it into replacement diode package.  The re-assembly took about 20 minutes and it fired up with 10A charge on low engine idle.  This is much higher charge at low idle than I can ever recall.

This means that I've been flying for some time with less than 6 diodes.   I probably was flying on one or two phases and getting some radio noise from the failed diodes.   This may have been ongoing for years.  Testing diodes as part of a 500H inspection is simple to accomplish and guarantees some margin for partial failure.   I have photos I can markup and add for reference in a follow up email.  Here are the excellent contacts who helped today:

1) TCM technical support 888.826.5465 Provided technical guidance and emailed me the alternator service manual in PDF form (48p) without hesitation.
2) Bill at B&C Specialty, 316-283-8000, always helpful.  Provided testing procedure for regulator and the following reference for repair.
3) Aircraft Systems, Rockford IL, 815-399-0225 TCM alternator repair specialists
4) Rupp Auto, New Smyrna Beach, bench testing, parts and expert on 10Si alternators and repairs

It took about a day to get it done but the cost was right and I kept all my original parts and did not need to remove or ship the alternator.

Paul
Legacy, Spruce Creek.