alternator diodes fried?

[cat30]

we were running an engine and the alternator belt was on, but the alternator was not hooked up to anything. did we fry the diodes? this is probably similar to turning the battery switch to off with the engine running?

[Don Moyer]

You probably did cause the diodes to fail. I recommend connecting the output of the alternator directly to the big battery terminal on the starter solenoid and then starting the engine. If you measure approximately 12 volts before and after you start, the diodes have probably failed. If you observe approximately 13.5 volts in the DC system after starting, you lucked out and the diodes are OK.

Don