Use this quick check if the charging system isn't working and you don't know if the problem is in the alternator or the regulator/acu.
1. Attach the red lead of your VOM (voltage meter) to the field terminal on the alternator. Attach the black lead to ground.
2. Turn on the master switch and taxi/landiing lights. Do not start engine! The regulator/ACU should be trying to full-field the alternator, i.e. you should see 12 volts (on a 14v system) or 24 volts (on a 28v system)
3. Read the voltage on your VOM. If no voltage is present then the problem is in the regulator/ACU or wiring. If voltage is present then the problem is in the alternator.
If problem is in the alternator:
4. Touch a small screwdriver the the rear bearing housing on the alternator. If you have a good circuit through the field, you will feel a strong magnetic field. Possibly the stator is bad - swap out the alternator. If you don't feel a strong magnetic field then you don't have current flowing through the field - Possibly a brush problem. Brushes are relatively easy to replace.
Note: If your alternator uses lock tabs to secure the housing screws - order these along with the brushes.
