Sounds like you might have put the Diode in the wrong alternator wire, or installed it backwards.
The diode needed to go in the 'Excite' wire, small, 'Brown', stiff.
The diode had a painted 'Stripe' on one end, that stripe needed to 'Face' the alternator.
Radio Shack p/n 276-1143 or p/n 276-1144 will work,
Make sure the 'Stripe' faces the alternator.
Here is a picture of the plug/harness rig I make up for my builds and as a 'Field Tester' for Delco Alternators...
When I use this, I usually remove the 'Brown' stiff resistor wire completely and splice this in instead.
The alternator plug & wiring pigtail can be found at any auto parts store for under $5.
The diodes are $1.59 a package at Radio Shack.
The Extra small crimp connectors are from Radio Shack also.