Measure resistance, using an ohmmeter, across "F" and "E" terminals for rotor coil resistance. Rotor coil circuit is normal if resistance is 4-5 ohms. If resistance is high, there is poor contact between brushes and commutator. If no continuity exists between "F" and "E" terminals, there is either an open rotor coil circuit, brush sticking or a broken lead wire. If resistance is low, it indicates a rotor coil layer short or grounded circuit.
NOTE:
The following test will not indicate an open state of the diodes. Tester will indicate continuity regardless of diode conditions if tester leads are connected to the terminals with polarity reversed.