Testing Alternator Output

[Armada]

Got a question for you sparky experts. What is the correct way to check the output of the 60 amp alternator? Pic shows the one I have.
The way I checked it was in vehicle, running at 1200rpms. First I disconnected the batterys (neg cable), removed the cover over the field and hot wires, then reconnected the batterys. I started it up and let it warm for a few minutes. Then I used my multimeter set on 'DC volts' and put one probe to ground and the other to the output lug (nothing attaches to it), which is located under the cover that was removed.
I'm getting 13.3 volts. Shouldn't it be showing 28vdc? Am I testing it wrong?
Thanks.

 

[acetomatoco]

With engine running and after looking at your in dash voltmeter and seeing it in the green zone at the white dot, put your voltmeter in the jumpstart plug or across both batteries and you should see 28 V more or less. You can adjust the voltage up or down a little by removing the allen plug on your alternator and using your insulated screwdriver to turn the screw. Were you using the right scale on your multimeter?

 

[cranetruck]

Like ACE says, test it while on the vehicle and mesure the voltage at the batteries, there is a direct connection. Also the bat/gen gauge is directly wired to the battery (although, the ground runs through the chassis).
The gauges on your deuce are pretty accurate when hooked up correctly (see image below).
I have to brag about the xm757 again, they insulated the dash from the chassis and ran a separate dedicated ground to it for max accurace.