Alternator diode checking can be done on the car with a DMM. The spec is no more than .5VAC with the engine running and no more than .5 milliamps on the output with engine off.
Bad diodes happen (more likely to go open than short) but they are something I have never actually seen in my 35+ years in thee shop. For me it was always full no output that caused me to replace alternators.
My point here is you are looking for the wrong value with engine off on the output, you should be looking for milliamps with the test you describe and looking for how much AC you have with engine on. Really you should be looking for voltage and amperage output at 1800 rpm or so, and if you see 80% of rated amperage output you are doing fine. What voltage are you seeing at the battery at 1800 rpm? if this is good go ahead and take a look at the AC component (remember less than .5VAC.)