Diode Trio
Use a suitable testing tool which draws a one amp. load (maximum) at 12 volts.
NOTE:
An ohmmeter or test light will not shown diode breakdown caused by heat.
Unsolder wires at diode trio and attach tester to a 12 volt battery. Attach negative clip of tester to threaded stud terminal of diode trio and positive clip to one of diode trio terminals. Test lamp should light. Keep load on diode for 1-3 minutes and if light flickers or goes out, diode is defective. Test second and third diodes in same manner.